# Unveiling the bulge-disc structure, AGN feedback, and baryon landscape in a massive spiral galaxy with Mpc-scale radio jets

Joydeep Bagchi, Shankar Ray, Suraj Dhiwar, Mahadev B Pandge, Pratik Dabhade, Aaron J Barth, Luis C Ho, Mohammad S Mirakhor, Stephen A Walker, Nicole Nesvadba, et al.

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► To cite this version:

Joydeep Bagchi, Shankar Ray, Suraj Dhiwar, Mahadev B Pandge, Pratik Dabhade, et al. Unveiling the bulge-disc structure, AGN feedback, and baryon landscape in a massive spiral galaxy with Mpc-scale radio jets. Monthly Notices of the Royal Astronomical Society, 2025, 538 (3), pp.1628-1652. 10.1093/mnras/stf229. hal-05420565

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# Unveiling the bulge-disc structure, AGN feedback, and baryon landscape in a massive spiral galaxy with Mpc-scale radio jets

Joydeep Bagchi<sup>1,★</sup> Shankar Ray<sup>1,★</sup> Suraj Dhiwar<sup>2,3</sup> Mahadev B. Pandge<sup>1,4</sup> Pratik Dabhade<sup>1,5,6,7</sup> Aaron J. Barth<sup>1,8</sup> Luis C. Ho,<sup>9,10</sup> Mohammad S. Mirakhor<sup>1,11</sup> Stephen A. Walker,<sup>11</sup> Nicole Nesvadba,<sup>12</sup> Francoise Combes,<sup>13</sup> Andrew Fabian<sup>1,14</sup> and Joe Jacob<sup>15</sup>

Accepted 2025 January 22. Received 2024 December 1; in original form 2024 April 29

## ABSTRACT

We study the bulge-disc components and stellar mass distribution in the fast-rotating, highly massive spiral galaxy 2MASX J23453268−0449256, which is distinguished by extraordinary radio jets extending to Mpc scales. Using high-resolution multiwavelength Hubble Space Telescope (HST) observations and multiparameter panchromatic spectral energy distribution (SED) fitting, we derive estimates of key properties, such as the star formation rate, total baryonic mass in stars, and the characteristics of warm dust. Our findings, validated at a spatial resolution of approximately 100 pc, reveal a pseudo-bulge rather than a classical bulge, as well as a small nuclear bar and resonant ring, challenging traditional models of galaxy formation. Furthermore, the absence of tidal debris and the highly symmetric spiral arms within a rotationally supported stellar disc suggest a peaceful co-evolution of the galactic disc and its central supermassive black hole (SMBH). Notably, the galaxy exhibits suppressed star formation in its central region, which may be influenced by feedback from the central accreting SMBH, producing powerful radio jets. Detailed multiwavelength studies of potential star-forming gas show that while hot X-ray gas cools in the galaxy’s halo, new stars do not form in the centre, likely due to this feedback. This study raises important questions about the efficient fuelling and sustained collimated jet activity in J2345−0449, highlighting the need for a better understanding of the central black hole’s properties. The exceptional rarity of galaxies like 2MASX J23453268−0449256 presents intriguing challenges in uncovering the physical processes behind their unique characteristics.

**Key words:** galaxies: formation – galaxies: fundamental parameters – galaxies: individual: 2MASX J23453268−0449256 – galaxies: ISM – galaxies: spiral.

## 1 INTRODUCTION

Explaining the mechanism of galaxy formation in the Universe across cosmic eons in the framework of the widely accepted Concordance Lambda cold dark matter (ΛCDM) cosmology is one of the prime

goals of astronomy research. Since there may be several competing physical processes responsible for the observed galaxy properties, identifying and relating them to galaxy formation models is an exceptionally important goal. A major mystery is how galaxies convert their baryons into stars and why some galaxies are still busy forming new stars while others have virtually shut down their stellar factory. Observations show that the star formation rate (hereafter SFR) reached its peak at ‘cosmic high noon’ near redshift $z = 1-2$ when galaxies were rapidly building up their stellar masses by

© 2025 The Author(s).

Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

<footnote n="1"><sup>1</sup>Department of Physics and Electronics, CHRIST (Deemed to be University), Hosur Road, Bengaluru 560029, India</footnote>

<footnote n="2"><sup>2</sup>Inter-University Centre for Astronomy and Astrophysics (IUCAA), Post Bag 4, Ganeshkhind, Pune 411007, India</footnote>

<footnote n="3"><sup>3</sup>Department of Physics, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India</footnote>

<footnote n="4"><sup>4</sup>Department of Physics and Electronics, Dayanand Science College, Barshi Road, Latur 413512, India</footnote>

<footnote n="5"><sup>5</sup>Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, E-38205, La Laguna, Tenerife, Spain</footnote>

<footnote n="6"><sup>6</sup>Departamento de Astrofísica, Universidad de La Laguna (ULL), E-38206, Tenerife, Spain</footnote>

<footnote n="7"><sup>7</sup>Astrophysics Division, National Centre for Nuclear Research, Pasteura 7, PL-02-093 Warsaw, Poland</footnote>

<footnote n="8"><sup>8</sup>Department of Physics and Astronomy, 4129 Frederick Reines Hall, University of California, Irvine, CA 92697-4575, USA</footnote>

<footnote n="9"><sup>9</sup>Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China</footnote>

<footnote n="10"><sup>10</sup>Department of Astronomy, School of Physics, Peking University, Beijing 100871, China</footnote>

<footnote n="11"><sup>11</sup>Department of Physics and Astronomy, The University of Alabama in Huntsville, 301 Sparkman Drive, Huntsville, AL 35899, USA</footnote>

<footnote n="12"><sup>12</sup>Institut d'Astrophysique Spatiale, Centre Universitaire d'Orsay, F-91405 Orsay, France</footnote>

<footnote n="13"><sup>13</sup>LERMA, Observatoire de Paris, PSL Research University, College de France, CNRS, Sorbonne University, 75006 Paris, France</footnote>

<footnote n="14"><sup>14</sup>Institute of Astronomy, Cambridge University, Madingley Road, Cambridge CB3 0HA, UK</footnote>

<footnote n="15"><sup>15</sup>Department of Physics, Newman College, Idukki District, Thodupuzha 685585, Kerala, India</footnote>

<footnote>* E-mail: joydeep.bagchi@christuniversity.in (JB); shankar.ray@res.christuniversity.in (SR)</footnote>
converting cosmic baryons to stars and, subsequently, over the next few billions of years the SFR declined drastically. Eventually, by the present era, most of the massive galaxies have stopped growing (Brinchmann et al. 2004; Renzini & Peng 2015), having entered a life of apparent quiescence (Behroozi, Conroy & Wechsler 2010; Moster, Naab & White 2013; Behroozi et al. 2019; Dhiwar et al. 2023). Nevertheless, for reasons still unknown, in our local universe, some low-mass galaxies are still actively forming stars, while the majority of massive galaxies with halo masses $M_{\text{halo}} \gtrsim 10^{12} M_{\odot}$ display a strikingly low specific star formation rate (where the specific star formation rate, sSFR = SFR/stellar mass) compared to less massive galaxies (see reviews by Alexander & Hickox 2012 and Wechsler & Tinker 2018).

Why this is so and who the progenitors of these massive local galaxies were remain open questions. It is likely that their formation history may go as far back as $z \sim 6-10$ in redshift (about $\sim 0.9-0.5$ Gy after the birth of the Universe), in the era of re-ionization, when their ancestors started to grow in mass, as well as vigorously formed new stars from cosmic baryons, infalling, and condensing around their primordial dark matter (DM) haloes. Puzzlingly, JWST has uncovered some extremely massive (stellar mass $\gtrsim 10^{10} M_{\odot}$) galaxies existing even at very early epochs ($z \sim 7-9$, Labbé et al. 2023; Rowland et al. 2024). By the time the Universe was about three billion years old ($z \gtrsim 2$), half of the most massive galaxies were in place as compact proto-galaxies, having already exhausted their fuel for star formation. Of the remaining half, about 25 per cent of galaxies assembled before the peak of cosmic star formation, while the other 25 per cent formed later (Madau & Dickinson 2014). Observations reveal that at higher redshifts ($z \sim 4$) most of them were harbouring intense nuclear starbursts and possibly they ultimately grew into the most massive local galaxies seen today through mergers with other galaxies (Saha et al. 2021), and by means of gravitational accretion from cold gas filaments of the cosmic web (Wang et al. 2019). Understanding how this remarkable transformation happened and what physical factors were responsible for bringing about this change has been an enduring quest – which also brings forth some of the most widely debated key questions in galaxy formation (for reviews, see Vogelsberger et al. 2020; Somerville & Davé 2015).

In the above scenarios, the circumgalactic medium (hereafter CGM) of galaxies plays a decisive role in determining, at all cosmological epochs, how galaxies acquire, eject, and recycle their gas; which in turn are the most important physical ingredients in determining how the galaxies should evolve, how much star formation will take place, and eventually how much of the baryonic gas will be quenched (prevented from forming stars), or recycled back to the galaxy. Along with the unseen, gravitationally dominant DM, the CGM is an extremely dynamic environment composed of a complex mix of baryons existing in diverse phases of temperature, density and metal content, presenting an extremely important, but perhaps the least understood aspect of galaxy formation (for reviews, see Tumlinson, Peeples & Werk 2017; Donahue & Voit 2022). Among the many interesting and challenging questions that the CGM poses, one of the most perplexing ones has been the so-called missing baryons problem. Indeed, the term ‘missing baryons’ is a misnomer, but it emphasizes the fact that even in deep multi-wavelength observations of galaxies, the total detectable baryonic mass falls far short (by $\sim 50$ to 80 per cent) of the cosmic baryon fraction of $\approx 0.15-0.17$ as expected in $\Lambda$CDM cosmology based on big bang nucleo-synthesis and cosmic microwave background constraints (Dunkley et al. 2009; Komatsu et al. 2009). A number of theories and computer simulations have tried to explain where these baryons may reside and why only a small fraction of the baryons in

galactic haloes condense into stars, or why by the present era ($z=0$) the star formation efficiency (empirically defined as $f_* = M_* / M_{\text{halo}}^{\Omega_m/\Omega_h}$) reaches a peak of about 20–30 per cent for typical $L_*$ galaxies. Whereas for very massive galaxies (halo mass $\gtrsim M_h \sim 10^{12} M_{\odot}$), beyond the peak, the $f_*$ rapidly declines to only $f_* \sim 2-5$ per cent at $M_h \sim 10^{13} M_{\odot}$ (Behroozi et al. 2010; Moster et al. 2013; Schaye et al. 2015; Behroozi et al. 2019). It is likely that AGN (active galactic nucleus) feedback could be the main agency responsible for this steep decline in SFR of highly massive galaxies, but we need to understand if that is the only factor (Bluck et al. 2022). On the other hand, a similar decline in SFR is seen on the opposite, lower mass end of the peak ($\lesssim M_h \sim 10^{12} M_{\odot}$), which is mainly attributed to supernova feedback, as it is more effective in less massive galaxies.

The promising agents of AGN feedback are accreting super-massive black holes (SMBH) growing in the centre of galaxies (Magorrian et al. 1998) which occasionally turn into powerful engines (Lynden-Bell 1969) that can potentially heat and even expel the gas of their host galaxies, thereby affecting the star formation process (Di Matteo, Springel & Hernquist 2005; Somerville et al. 2008; McNamara & Nulsen 2012; Beckmann et al. 2017). It is now widely recognized that in our local Universe, such SMBHs of masses $\sim 10^6 - 10^{10} M_{\odot}$ lurk in the nuclei of almost all massive galaxies (see Kormendy & Ho 2013, for a review). Moreover, feedback from accreting BHs is likely to be more effective in most massive galaxies because a remarkably tight coupling exists between the mass of the black hole (BH) and the galaxy’s bulge mass or with its total stellar mass/luminosity, which further suggests that; (i) the mass of the central SMBH will be higher in massive galaxies and bulge-dominated galaxies, and so will be its energetic output and its impact on the surrounding media, (ii) the SFR and the final stellar mass of the galaxy has been somehow regulated by the growing BH over the cosmic eons (e.g. Magorrian et al. 1998; Gebhardt et al. 2000; Marconi & Hunt 2003; Häring & Rix 2004; Gültekin et al. 2009a; Sahu, Graham & Davis 2019).

The physical mechanism that links the SMBH mass to the galactic bulge and disc properties is still poorly understood, but AGN energy-driven feedback – either via the mechanical power of the narrow radio jets (Bridle & Perley 1984; Saikia 2022) or through a broad pressure-driven hot wind/outflow (and radiation flux) on the surrounding medium – plays a major role in regulating the growth of the BH and may affect the SFR and growth of the host galaxy. In recent years, we have learned that SMBHs are the essential ingredients shaping the lives of galaxies across cosmic time (reviewed in Kormendy & Ho 2013; Alexander & Hickox 2012). Nevertheless, there is little understanding of exactly how the BHs of AGN are fuelled and how the radio jets or the hot outflows deposit energy to their surroundings; the energy that is transferred to the different gas phases, and what are the physical conditions when significant positive or negative feedback on SFR is produced (see reviews by Fabian 2012; King & Pounds 2015). Despite numerous efforts to observe SMBH engines in the process of quenching star formation, conclusive evidence for such a process remains elusive, particularly in the nearby Universe (Martín-Navarro et al. 2018). However, the strong impact of AGN feedback via radio jets on the hot intracluster medium is observed directly through X-ray observations of the dark cavities and non-thermal plasma bubbles found around the central galaxies of cool-core galaxy clusters and groups in X-ray images (Bírzan et al. 2004).

Lastly, leading galaxy formation theories (e.g. White & Rees 1978; White & Frenk 1991; Fukugita & Peebles 2006; Sommer-Larsen 2006) postulate a warm-hot X-ray halo (or a ‘corona’ first mentioned by Spitzer 1956) containing a large fraction (at least 50 per cent) of cosmic baryons in highly massive galaxies. To validate galaxy
formation models and make a census of all cosmic baryons, sensitive X-ray or ultraviolet (UV) observations of circumgalactic haloes are necessary. It has been proved very challenging to detect the hot, baryon-filled coronae around, even in very luminous spirals and ellipticals with our present telescopes, due to sensitivity limitations. Nevertheless, in the past decade using the *Chandra* and *XMM-Newton* telescope, such coronae have been robustly detected in a small number of massive galaxies (e.g. Anderson & Bregman 2011; Dai et al. 2012; Bogdán et al. 2013; Walker, Bagchi & Fabian 2015; Anderson, Churazov & Bregman 2016; Bregman et al. 2018; Mirakhor et al. 2021), but independent constraints on their temperature, density, and metallicity from soft X-ray or UV spectroscopy are still lacking. These galaxies have provided compelling evidence for the existence of hot, low-metallicity atmospheres of gas that could originate with accretion from the IGM and subsequent heating to the virial temperature of the halo via accretion shocks. However, the fraction of baryons residing in the hot coronal phase and its dependence on stellar and halo mass and how they are affected by the AGN feedback of the central BH and the galactic star formation processes are not well determined and poorly understood (Donahue & Voit 2022).

Evidently, to further the field, it is necessary to understand how SMBHs have grown over cosmic time, how they accrete gas (both the rate of accretion, as well as the source of this gas) to create their relativistic jets and hot winds, and finally, their impact on the surrounding molecular gas and the star formation activity. The baryon budget and SFR are the two vital properties of galaxies that are significantly affected by AGN feedback which needs much better understanding. The quenching of star formation in massive galaxies is one of the core predictions of the AGN feedback hypothesis, yet it is also one of its aspects that are very little understood (Harrison 2017). Some previous studies have focused on gas removal in the host galaxies of very powerful AGN as the main feedback mode (Laha et al. 2021), however, in contradiction, some observations show that quasar-like outflows alone are not sufficient to understand AGN feedback. Even the most extreme outflows in very powerful AGN do not result in a significant reduction in SFR (e.g. Stanley et al. 2015), nor do many of the most frequent, medium to low-power AGN shows strong outflow signatures. Thus, in spite of the popularity of AGN feedback hypothesis, its central prediction – the direct physical link between the energy injection by the AGN and star formation – remains a subject of several contradictions, yet it provides many new, vibrant ideas for exploration (Morganti 2017).

Our prime focus is on a detailed exploration of active galaxies with massive BHs and jets, specifically examining their feedback impact on the interstellar medium (ISM) and the extended baryonic matter of the host galaxy. We are particularly intrigued by the prospect of identifying clear counterexamples – instances of powerful jets originating from an accreting SMBH formed under highly unusual conditions. One such remarkable case under scrutiny here involves an Mpc-scale radio galaxy hosted by a luminous, quiescent massive spiral disc without a classical bulge, exhibiting no major signs of disturbances. Investigating such scenarios allows for a direct test of AGN feedback. Moreover, within this galaxy, we aim to uncover the precise physical factors influencing the launch of relativistic jets in powerful radio galaxies (Bridle & Perley 1984), marking the first instance of such knowledge. This unique and rare galactic configuration serves as an exceptional testbed, offering valuable insights into the role of relativistic jets in the evolution of galaxies.

Hereafter, in this paper, we use redshift $z = 0.07557$ and $\Lambda$CDM cosmological parameters; $H_0 = 67.8$ km s$^{-1}$ Mpc$^{-1}$, $\Omega_m = 0.308$, and $\Omega_{\text{vac}} = 0.692$. This results in an image scale of 1.48 kpc arcsec$^{-1}$.

The luminosity distance to J2345–0449 in this model is 353.0 Mpc and the angular size distance is 305.2 Mpc. Magnitudes are quoted in the AB system.

## 2 2MASX J23453268–0449256: AN ENIGMATIC SPIRAL GALAXY

The focus of this paper is the galaxy 2MASX J23453268–0449256 (hereafter J2345–0449) at redshift 0.0755. J2345–0449 is known to be an extremely luminous ($M_r = -23.26$ and $M_K = -26.15$ mag), isolated spiral galaxy which also hosts one of the largest and most powerful radio jet known, clearly powered by the AGN residing at the centre of a spiral host galaxy and not an elliptical galaxy (Bagchi et al. 2014). GMRT (Giant Metrewave Radio Telescope) and VLA (Very Large Array) observations show that this giant radio source also exhibits a rarely observed ‘double-double’ radio morphology, containing a nested pair of smaller (active/young) inner radio lobes, and a second pair of extended (relic/old) diffuse outer radio lobes, with the jet axis (in projection) nearly orthogonal to the host galaxy’s stellar disc (Fig. 1). The bipolar radio emission of outer lobes can be traced up to ~ 1.63 Mpc (~ 19.1 arcmin) and inner lobes up to ~ 387.2 kpc (~ 4.52 arcmin), making it an example of the rare class of ‘giant radio galaxies’ (GRG), whose radio jets may extend upto ~ 1 Mpc or more (Dabhade et al. 2020a, b), far beyond the optical host. The inner ‘active’ radio lobes are of edge-brightened, Fanaroff–Riley class II morphology (Fanaroff & Riley 1974), energized by the collimated jets shot out from the central nucleus. The outer lobes have peculiar long, filamentary, and diffuse morphology, possibly decaying relics of past jet activity (Fig. 1).

These bright and enormous radio jets and lobes mark J2345–0449 as a unique source even among very massive spirals and the rapidly growing examples of GRGs in the local Universe, which themselves pose many important astrophysical questions (Dabhade, Saikia & Mahato 2023). Undoubtedly, this is a very unusual occurrence, since for reasons still unknown, such large-scale, powerful radio jets (typically on 100–1000 kpc scale) are almost exclusively launched from bulge-dominated elliptical galaxies hosting SMBHs (e.g. Matthews, Morgan & Schmidt 1964; McLure et al. 2004), whereas the host of J2345–0449 is a spiral galaxy (Fig. 2). Consequently, such counterintuitive AGNs challenge the standard paradigm of growth and co-evolution of massive BHs in bulges of galaxies (Magorrian et al. 1998; Gebhardt et al. 2000; Marconi & Hunt 2003; Häring & Rix 2004; Wu, Ho & Zhuang 2022) and therefore, they are also of profound interest as they may be the ideal astrophysical models of the central engines of radio galaxies and their feedback signatures on the stellar and gaseous matter of the galaxy.

Finding Mpc-scale jets originating from a spiral galaxy is highly unusual in itself, but the galaxy itself is equally remarkable. Its general properties were first discussed by Bagchi et al. (2014) and further investigated by Walker et al. (2015), Mirakhor et al. (2021), and Nesvadba et al. (2021). The *WISE* (Wide-field Infrared Survey Explorer) mid-infrared (IR) colours suggest that some mild star formation is still occurring within its disc. It is located in a sparsely populated galactic environment devoid of bright $L_*$ galaxies. Having $M_r = -23.26$ mag in SDSS (Sloan Digital Sky Survey) $r$ band and $M_{12} = -28.66$ in *WISE* 12 µm band, it lies at the very top of the luminosity function of red spirals. From Hα line studies, the galaxy’s systematic rotation speed is found to be very high, with maximum flat rotation speed reaching $v_{\text{rot}} = 429 \pm 26$ km s$^{-1}$ at $r = 15$ kpc radius (corrected for inclination angle $i \approx 59.7^\circ$), which is exceptionally large among all spirals. Assuming Newtonian dynamics, this rotation speed implies a huge mass of ~ $10^{13}$ M$_\odot$ within the virial radius of
<figure><img src="image_4.png" /></figure>

**Figure 1.** GMRT 323 MHz image of the giant radio source hosted by the spiral J2345−0449. The rms noise level is about 96 µJy beam⁻¹ and the 3σ surface brightness detection threshold is about 300 µJy beam⁻¹. The major and minor axes of the synthesized elliptical beam are 10.35 arcsec × 7.23 arcsec at PA 45.24°. The AGN core is detected at 5.5 mJy beam⁻¹ brightness. Noteworthy is the rare occurrence of two nested pairs of radio lobes. The extremely large extent of the emission is evident with the inner and outer radio lobe pairs extending over ~387 kpc (~4.52 arcmin) and ~1.6 Mpc (~19.1 arcmin), respectively. The white dotted ellipse represents an outline of the optical galaxy zoomed by a factor of ~4 for clarity. The scale bar represents 500 kpc. The inset image shows zoomed-in details of the inner radio-double observed with VLA at 4.8 GHz and 19.8 arcsec × 13.3 arcsec restoring beam at PA 178.5° (lower left corner). Contour levels are (−0.1, 0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 mJy beam⁻¹). The scale bar represents 50 kpc. The ellipse represents a zoomed outline of the optical galaxy. The inner double has an edge-brightened FR-II (Fanaroff & Riley 1974) morphology, being fed by jets, but the outer lobes have a peculiar filamentary and diffuse structure, possibly decaying relics of jet activity that had ceased millions of years ago.

$R_{200} = 450$ kpc, thereby making J2345−0449 one of the largest and most massive galaxies known in our local Universe. Bagchi et al. (2014) found that the galaxy’s central velocity dispersion to be $\sigma_{*} = 351 \pm 25$ km s⁻¹, much higher than normal Milky Way-like spirals and close to the highest velocity dispersions found among nearby E and S0 galaxies (van den Bosch 2016).

As the mechanism of radio-mode suppression of star formation in massive galaxies is still unknown and there could be several ways to suppress star formation in early-type galaxies, a powerful jet emanating from a massive spiral disc offers an important alternate probe of jet-related feedback. J2345−0449 is one such galaxy, with its nested pair of extended radio lobes and a smaller scale restarted radio source that is interacting with a molecular disc, injecting turbulence and affecting the SFR. Moreover, the large repository of warm/hot baryons discovered in the CGM of the galaxy and the cold molecular gas content in the inner disc further highlight its baryonic budget and feedback properties. Nesvadba et al. (2021) detected signatures of AGN feedback on the inner CO gas ring and suggested possible suppression of star formation on kpc scales at the centre. From VLT/MUSE (Multi Unit Spectroscopic Explorer) spectroscopy, Drevet Mulard et al. (2023) concluded that more than 93 per cent of the stellar mass is very old, having formed $\gtrsim 10$ Gy ago in the disc; they detected only a few young star-forming (SF) regions that formed $\lesssim 11$ My ago. Walker et al. (2015) and Mirakhor

et al. (2021), using *Chandra* and *XMM-Newton* data, discovered the galaxy’s extended soft X-ray (electron temperature $T_{e} \sim 0.6$ keV or $6.9 \times 10^{6}$ K) corona/halo of low-metallicity ($Z \sim 0.1Z_{\odot}$) gas, which extends at least to $r \approx 160$ kpc, making J2345−0449 one of just a few spirals for which such large, extended, massive ionized gaseous haloes have been directly detected via X-ray imaging. Extrapolating the gas mass profile out to the virial radius ($R_{vir} \sim 450$ kpc) and accounting for the stellar and molecular gas masses of the disc, the baryon mass fraction was found to be $0.121 \pm 0.043$, close to the universal baryon fraction expected in LCDM cosmology (Komatsu et al. 2009; Walker et al. 2015; Mirakhor et al. 2021).

Ground-based (SDSS and Canada-France-Hawaii Telescope) photometry showed that J2345−0449 may harbour a flattened pseudo-bulge rather than a typical ellipsoidal classical bulge. The galaxy does not contain a bright central AGN point source at optical wavelengths, and lacks obvious signs of mergers or interaction with any massive neighbouring galaxy (Bagchi et al. 2014). Such rare, isolated, and ultramassive galaxies ($M_{200} \gtrsim 10^{13}$ M$_{\odot}$; Bagchi et al. 2014, Walker et al. 2015), located at the upper end of the galactic mass spectrum, are highly valuable laboratories to understand more general astrophysical mechanisms, allowing us to isolate specific processes that are challenging to study in the dominant galaxy population due to the co-existence of various potentially competing mechanisms producing similar effects. Consequently, J2345−0449
<figure><img src="image_5.png" /><figcaption>**Figure 2.** A tricolour image of J2345−0449, constructed using the *HST* WFC3 UVIS (F438WB-band and F814WI-band) and IR channel (F160WH-band) filter images. The component images have been superposed with individual intensities adjusted to obtain a natural colour balance. Dark, winding dust lanes and compact, star forming regions are visible in the outer parts of the galaxy disc. Image size is ~ 50 × 50 arcsec and north is up and east is to the left.</figcaption></figure>

becomes a distinctive laboratory for investigating crucial aspects of bulge-disc stars, relativistic jet formation, AGN feedback, and the co-evolution of SMBHs and galaxies. These aspects, often difficult to probe in early-type galaxies, come to the forefront in this exceptional astronomical specimen.

In this paper, we perform a detailed multiwavelength, high-resolution study of J2345−0449 with data from the *Hubble Space Telescope (HST)*.<fnref n="1" /> From optical and near-IR imaging, made with the Wide Field Planetary Camera 3 (WFC3), providing a high spatial resolution of ~0.05 to 0.1 arcsec FWHM (full width at half-maximum, or ~ 50 − 100 pc), we quantify the SFR and interstellar dust properties in the disc. Furthermore, we unambiguously resolve the stellar and dust content within the bulge and disc components and determine the bulge-disc morphological and photometric parameters accurately, thereby vastly improving on the previous ground-based studies. We also estimate the SFR in the disc and explore its implications in synergy with some of the results from our previous investigations of the cold molecular gas content and its dynamics

observed via CO lines, and the properties of an extended soft X-ray halo/corona detected in *Chandra* and deep *XMM-Newton* observations.

We also present a spectral energy distribution (SED) model of J2345−0449 using all available observations across the wavelength range 0.1 to 500 µm. We decompose the SED into star formation, ISM dust, and AGN components. The SED fitting has been done using two different software packages; CIGALE and MAGPHYS (Multi-wavelength Analysis of Galaxy Physical Properties), for cross-checking, obtaining several key ISM and stellar parameters of the galaxy, such as its global SFR, stellar mass, dust mass, dust temperature and dust luminosity, gas mass, and the AGN fraction.

Finally, we examine the location of J2345-0449 on the star formation main-sequence and other well-established scaling relations, with an aim to determine how this exceptional galaxy compares with other ordinary galaxies. We also obtain a detailed census of baryons in the disc and the circumgalactic halo and derive estimates of the overall star formation efficiency and the total baryon fraction. Our studies enable probing the environment of the central BH powering the powerful radio jets and investigating the feedback action of jets on the stellar disc and the surrounding circumgalactic environment.

This multiwavelength data set and the inferences constitute our first steps towards understanding the origin and evolution of this complex and enigmatic galaxy.

<footnote n="1">¹Based on observations made with the NASA/ESA *HST*, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with program no. 14091.</footnote>
<table><caption>**Table 1.** The central wavelength listed is the 'pivot wavelength', a source-independent measure of the characteristic wavelength of the filter, and the bandwidth is the 'passband rectangular width', the integral with respect to the wavelength of the throughput across the filter passband divided by the maximum throughput, as defined in the WFC3 Instrument Handbook.</caption><thead><tr><th>Filter</th><th>Channel</th><th>Central wavelength (Å)</th><th>Bandwidth (Å)</th><th>Exp. time (s)</th></tr></thead><tbody><tr><td>F438W</td><td>UVIS</td><td>4326.5</td><td>197.30</td><td>3 × 830</td></tr><tr><td>F814W</td><td>UVIS</td><td>8029.5</td><td>663.33</td><td>3 × 510</td></tr><tr><td>F160W</td><td>IR</td><td>15369.0</td><td>2683.0</td><td>3 × 199</td></tr></tbody></table>

## 3 DESCRIPTION OF HST OBSERVATIONS

Under HST program GO-14091 (PI: Barth) the spiral galaxy J2345–0449 was observed with WFC3 camera in three filter bands taken over two orbits on 2016 September 17. A series of three exposures were taken in each filter band. Table 1 lists the exposure times in each filter. These exposures were offset by fractional-pixel shifts relative to each other using a three-point box dither pattern. The final images were constructed by combining these exposures, which allowed for the removal of cosmic-ray hits and hot pixels. In the UVIS (UV/Visible) channel, we used the F438W (B band) and F814W (I band) filters. The UVIS channel CCD has a pixel scale of 0.04 arcsec pixel⁻¹. At a distance of 323 Mpc, a single pixel on UVIS subtends about 56 pc. The point spread functions (PSFs) have FWHM 0.090 arcsec in the F814W and 0.074 arcsec in F438W filters, respectively. Since J2345–0449 has effective radius of approximately 1 kpc, WFC3 provides excellent spatial resolution for a detailed study of its internal structure. In the IR channel, we used the wide-band, F160W (H-band) filter. The WFC3/IR detector array has somewhat coarser resolution than the UVIS, with a pixel scale of 0.13 arcsec pixel⁻¹ and PSF FWHM of 0.241 arcsec.

## 4 HST DATA ANALYSIS, PHOTOMETRIC, AND SED MODEL FITTING

For reduction of the HST data we have followed the same procedure as described in Carson et al. (2015). The standard HST calibration pipeline was used for flat fielding, bias correction, and dark current subtraction. In order to reject cosmic-ray hits and bad pixels and to correct for the geometric distortion introduced by the optics of HST and WFC3, the three offset exposures for each filter were combined into a final image using the PYRAF/STDSAS task AstroDrizzle, using a square kernel with `pixfrac=1`, exposure time weighting, and sky subtraction turned off. In addition, as part of the AstroDrizzle process, we re-sampled the IR images onto the UVIS grid, resulting in a uniform pixel size of 0.04 arcsec in all three bands.

The resulting WFC3 images provide the first detailed view of the J2345–0449 host galaxy. We created a tri-colour image of spiral galaxy J2345–0449 by superposing the HST WFC3 UVIS (F438WB and F814WI bands) and IR (F160WH-band) monochrome images (Fig. 2). The colour image shows blue SF regions along the outer spiral arms, flocculent dust lane structure throughout much of the disc region, and a smoother and less dusty bulge region surrounding the nucleus.

### 4.1 Surface brightness profile fitting

We used a composite of the Sérsic (1963, 1968) and exponential functions to model the galaxy’s light profile, where the former models

the bulge and the latter models the disc. In addition, to account for any point-like excess light at the centre, an additional PSF model was used. A 2D model of the surface brightness profile was fit for each image using GALFIT version 3.0.4 (Peng et al. 2002, 2010). For the image in each band, we specified the model surface brightness component profile to fit the image (e.g. Sérsic, exponential, and PSF). We supplied initial estimates for the model parameters (e.g. the position of the centre of the profile, total magnitude, and effective radius). For simplicity, we did not attempt to fit non-axisymmetric features like spiral arms, dust lanes, bars, or resonant rings in our models, and such features should then be visible in the residual images.

In the fitting procedure, a model image based on the above components is produced and then convolved with the corresponding WFC3 PSF. Then, the $\chi^2$ between the HST input image and the model image is computed, and minimization of $\chi^2$ is achieved, yielding the best model parameters, using the Levenberg-Marquardt downhill-gradient algorithm. To properly weight each pixel in the $\chi^2$ calculation, we used the sigma image generated internally by GALFIT based on each input image. To check whether the final results correspond to a global minimum, we repeated the fit using different starting values for the model parameters. We found that the final results are stable against different initial conditions.

The Sérsic (1963, 1968) profile is defined as

$$ \Sigma(r) = \Sigma_e \exp \left[ -\kappa \left( \left( \frac{r}{r_e} \right)^{1/n} - 1 \right) \right]. \quad (1) $$

Here, $\Sigma(r)$ and $\Sigma_e$ are the surface brightness at radius $r$ and $r_e$ respectively, $r_e$ is the effective (half-light) radius, $n$ is the Sérsic index, and $\kappa$ is a dependent variable coupled to $n$ such that $r_e$ encloses half of total brightness (Ciotti & Bertin 1999).

The exponential profile is defined as

$$ \Sigma(r) = \Sigma_0 \exp \left( -\frac{r}{r_s} \right). \quad (2) $$

Here, $\Sigma_0 = \Sigma(r)|_{r=0}$, and $r_s$ is the exponential scale radius.

For each filter, simulated images of the HST/WFC3 PSF were generated using version 7.5 of the TINYTIM software package (Krist, Hook & Stoehr 2011), which creates a model HST PSF based on the instrument, detector chip, detector chip position, focus and filter used in the observations. As the data have three exposures for each filter with subpixel offsets, we produced three versions of each PSF on a subsampled grid with subpixel offsets, using the same three-point box dither pattern as the HST/WFC3 exposures to ensure that the model PSFs were processed in the same way as the HST images.

Each model PSF was convolved with the appropriate charge diffusion kernel in order to account for the effect of electrons leaking into neighbouring pixels on the CCD. The PSFs were then combined and resampled onto a final grid with a pixel size of 0.04 arcsec in all bands using AstroDrizzle. The resulting simulated model PSFs are shown in Fig. 3.

### 4.2 Bulge-disc decomposition and best-fitting photometry

We used a 1200 × 1200 pixel (48.0 arcsec × 48.0 arcsec), 1800 × 1800 pixel (72.0 arcsec × 72.0 arcsec), and 1800 × 1800 pixel (72.0 arcsec × 72.0 arcsec) fitting region centred on the brightest pixel in the H, B, and I bands, respectively. We experimented with different combinations of model components available in GALFIT to fit the WFC3 images (see Figs 4, 5, 6, 7, 8 and 9 for the results). Model A represents our simplest model, including a Sérsic bulge and
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Model PSFs in each *HST* filter used in GALFIT models. The FWHM of PSF is given in arcseconds at lower left corner of each image.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Model A. Grey-scale images and GALFIT modelling of the inner 25 arcsec × 25 arcsec of spiral galaxy J2345 in all three *HST*/WFC3 filters, shown with a log scale. Top row: *H* band, middle row: *I* band, and bottom row: *B* band. Left image panels show the *HST* input image of the galaxy, middle image panels show the best-fitting GALFIT model, and right panels show the residual image. Spiral arms were not included in the GALFIT model.</figcaption></figure>

an exponential disc but no nuclear PSF component. The best-fitting models in each filter and the residual images are shown in Fig. 4. Interestingly, in the model A residual image shown in Fig. 10 one can see evidence for a small nuclear bar on 0.5 arcsec (~ 700 pc) scales, as well as spiral-shaped dust lanes winding inwards, and a large resonant ring surrounding the nuclear bar. In addition, a dark disc-shaped region surrounding the nuclear bar is more prominently visible in the *H*-band image. We further discuss the implications of these structures below.

From Fig. 4, it is evident that model A with one Sérsic bulge plus an exponential disc is promising but not sufficient to fully describe the innermost light profile seen in the target galaxy. There is an excess, compact light core and a stellar halo and ring seen at the

central region of the galaxy which was not accounted for by this simple two-component model. This excess light was not seen in previous photometric decompositions of this galaxy using ground-based imaging data (Bagchi et al. 2014), but only visible here due to the exceptionally high spatial resolution of *HST*.

Next, in model B, we added a nuclear PSF function to the set of components used in model A. In this case, the fit resulted in an improved $\chi^2$, but even then the residual images show some excess light and halo around the central AGN core region (Fig. 6, right panels).

Finally, we tried a more complex model C with additional subcomponents. We used a combination of two Sérsic functions, which we label as an inner Sérsic1 and another outer Sérsic2, plus
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Model A. Left to right: *HST F160W*, *F814W*, and *F438W* filter radial profiles of the surface brightness (mag arcsec⁻²) with GALFIT model (solid blue line) and *HST/WFC3* image data (black squares) as a function of radius (kpc). The data represent measurements along elliptical annuli of constant ellipticity and position angle as obtained from the best-fitting parameters. These radial profiles were obtained using the elliptical isophote fitting routine *Ellipse*. The parameters of fitting are tabulated in Table 2.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Model B. Grey-scale images and GALFIT modelling of the inner 25 arcsec × 25 arcsec of spiral galaxy J2345 in all three *HST/WFC3* filters, shown with a log scale. Top row: *H* band, middle row: *I* band, and bottom row: *B* band. Left image panels show the *HST* image, middle image panels show the best-fitting GALFIT model, and right panels show the residual image. Spiral arms were not included in the GALFIT model.</figcaption></figure>

one exponential function and one PSF function to fully account for the excess light at the centre. This composite model provides improved fits to the *F160W*, *F438W*, and *F814W* images (Fig. 8) resulting in the smallest $\chi^2$. The best-fitting parameters for models A, B, and C and their respective $\chi^2$ values are tabulated in Table 2.

We also subtracted the best-fitting PSF, Sérsic, and exponential disc models from the observed images and searched for any residual bar-like stellar component in the galaxy. In addition, we looked for the possible presence of double or multiple nuclei as a sign of mergers. As can be seen in the input data and the residual images, no evidence of multiple nuclei is found in any of the *HST* bands, but a central nuclear bar and a stellar ring are seen (Fig. 10).

## 4.3 Obtaining radial surface brightness profiles

We fit elliptical isophotes (ellipses of constant surface brightness) to both the *HST/WFC3* images and the PSF-convolved GALFIT model subcomponents using the PYRAF/STSDAS routine *Ellipse* (Freudling 1993), which is based on methods detailed by Jedrzewiński (1987). We also performed isophotal fits to the model PSFs in order to display their radial surface brightness profiles. Plots of the *F160W*, *F814W*, and *F438W* filter radial profiles of the surface brightness for spiral J2345-0449 and residuals of fits are shown in Figs 5, 7 and 9, showing the best-fitting profiles corresponding to models A, B, and C. We caution that while 1D radial profile plots are useful for visualization of the model fits, they do not account for
<table><caption>**Table 2.** The best-fitting bulge and disc photometric parameters for J2345–0449. The parameters obtained in models A, B, and C are listed here. The error in each parameter as well as the global minimum value of $\chi^2$ achieved in each model is also listed. For each model, the corresponding input images, best-fitting model images, and the residual images in all three bands are shown in Figs 4, 6, and 8, while their 1D radial profiles are shown in Figs 5, 7, and 9.</caption><thead><tr><th></th><th></th><th>F160W (H band)</th><th>F438W (B band)</th><th>F814W (I band)</th></tr></thead><tbody><tr><td rowspan="10">Model A<br>(Sérsic + exponential disc)</td><td>Sérsic mag</td><td>13.92<sup>+0.05</sup><sub>-0.05</sub></td><td>18.95<sup>+0.07</sup><sub>-0.07</sub></td><td>16.04<sup>+0.08</sup><sub>-0.08</sub></td></tr><tr><td>Sérsic r<sub>e</sub> (kpc)</td><td>2.50<sup>+0.04</sup><sub>-0.04</sub></td><td>1.07<sup>+0.03</sup><sub>-0.03</sub></td><td>1.35<sup>+0.08</sup><sub>-0.08</sub></td></tr><tr><td>Sérsic n</td><td>2.69<sup>+0.03</sup><sub>-0.03</sub></td><td>1.11<sup>+0.01</sup><sub>-0.01</sub></td><td>1.67<sup>+0.06</sup><sub>-0.06</sub></td></tr><tr><td>Sérsic b/a</td><td>0.61<sup>+0.02</sup><sub>-0.01</sub></td><td>0.61<sup>+0.05</sup><sub>-0.05</sub></td><td>0.60<sup>+0.02</sup><sub>-0.01</sub></td></tr><tr><td>Sérsic pa</td><td>89.66<sup>+0.02</sup><sub>-0.02</sub></td><td>87.75<sup>+0.02</sup><sub>-0.02</sub></td><td>90.87<sup>+0.02</sup><sub>-0.02</sub></td></tr><tr><td>Expdisc mag</td><td>13.16<sup>+0.08</sup><sub>-0.08</sub></td><td>16.44<sup>+0.10</sup><sub>-0.13</sub></td><td>14.67<sup>+0.08</sup><sub>-0.07</sub></td></tr><tr><td>Expdisc scale radius r<sub>s</sub> (kpc)</td><td>6.94<sup>+0.07</sup><sub>-0.07</sub></td><td>8.19<sup>+0.03</sup><sub>-0.03</sub></td><td>6.86<sup>+0.18</sup><sub>-0.13</sub></td></tr><tr><td>Expdisc b/a</td><td>0.52<sup>+0.02</sup><sub>-0.01</sub></td><td>0.61<sup>+0.05</sup><sub>-0.05</sub></td><td>0.60<sup>+0.02</sup><sub>-0.01</sub></td></tr><tr><td>Expdisc pa</td><td>96.03<sup>+0.03</sup><sub>-0.03</sub></td><td>94.58<sup>+0.02</sup><sub>-0.02</sub></td><td>95.03<sup>+0.06</sup><sub>-0.06</sub></td></tr><tr><td>Reduced χ<sup>2</sup></td><td>0.76</td><td>1.34</td><td>1.89</td></tr><tr><td rowspan="10">Model B<br>(Sérsic + exponential disc + PSF)</td><td>Sérsic mag</td><td>14.28<sup>+0.05</sup><sub>-0.05</sub></td><td>18.97<sup>+0.09</sup><sub>-0.09</sub></td><td>16.09<sup>+0.03</sup><sub>-0.03</sub></td></tr><tr><td>Sérsic r<sub>e</sub> (kpc)</td><td>1.37<sup>+0.03</sup><sub>-0.02</sub></td><td>1.08<sup>+0.08</sup><sub>-0.06</sub></td><td>1.32<sup>+0.08</sup><sub>-0.08</sub></td></tr><tr><td>Sérsic n</td><td>1.62<sup>+0.02</sup><sub>-0.03</sub></td><td>1.03<sup>+0.01</sup><sub>-0.01</sub></td><td>1.49<sup>+0.08</sup><sub>-0.08</sub></td></tr><tr><td>Sérsic b/a</td><td>0.61<sup>+0.02</sup><sub>-0.02</sub></td><td>0.60<sup>+0.07</sup><sub>-0.07</sub></td><td>0.60<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>Sérsic pa</td><td>89.71<sup>+0.02</sup><sub>-0.02</sub></td><td>88.71<sup>+0.10</sup><sub>-0.10</sub></td><td>90.93<sup>+0.03</sup><sub>-0.03</sub></td></tr><tr><td>Expdisc mag</td><td>13.05<sup>+0.07</sup><sub>-0.07</sub></td><td>16.44<sup>+0.04</sup><sub>-0.04</sub></td><td>14.66<sup>+0.03</sup><sub>-0.03</sub></td></tr><tr><td>Expdisc scale radius r<sub>s</sub> (kpc)</td><td>6.71<sup>+0.04</sup><sub>-0.04</sub></td><td>8.18<sup>+0.04</sup><sub>-0.04</sub></td><td>6.77<sup>+0.07</sup><sub>-0.07</sub></td></tr><tr><td>Expdisc b/a</td><td>0.53<sup>+0.01</sup><sub>-0.01</sub></td><td>0.53<sup>+0.36</sup><sub>-0.03</sub></td><td>0.54<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>Expdisc pa</td><td>95.34<sup>+0.06</sup><sub>-0.06</sub></td><td>95.03<sup>+0.06</sup><sub>-0.06</sub></td><td>94.93<sup>+0.02</sup><sub>-0.02</sub></td></tr><tr><td>PSF mag</td><td>18.22<sup>+0.01</sup><sub>-0.01</sub></td><td>24.33<sup>+0.05</sup><sub>-0.05</sub></td><td>21.25<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>Reduced χ<sup>2</sup></td><td>0.97</td><td>1.326</td><td>1.442</td></tr><tr><td rowspan="10">Model C<br>(Sérsic1 + Sérsic2 +<br>Exponential disc + PSF)</td><td>Sérsic 1 mag</td><td>15.09<sup>+0.03</sup><sub>-0.03</sub></td><td>19.68<sup>+0.01</sup><sub>-0.01</sub></td><td>16.74<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>Sérsic 1 r<sub>e</sub> (kpc)</td><td>1.58<sup>+0.02</sup><sub>-0.02</sub></td><td>1.66<sup>+0.56</sup><sub>-0.56</sub></td><td>1.75<sup>+0.04</sup><sub>-0.04</sub></td></tr><tr><td>Sérsic 1 n</td><td>0.66<sup>+0.01</sup><sub>-0.01</sub></td><td>0.30<sup>+0.01</sup><sub>-0.01</sub></td><td>0.53<sup>+0.08</sup><sub>-0.08</sub></td></tr><tr><td>Sérsic 1 b/a</td><td>0.55<sup>+0.02</sup><sub>-0.02</sub></td><td>0.48<sup>+0.07</sup><sub>-0.07</sub></td><td>0.54<sup>+0.03</sup><sub>-0.03</sub></td></tr><tr><td>Sérsic 1 pa</td><td>96.94<sup>+0.02</sup><sub>-0.02</sub></td><td>92.01<sup>+0.10</sup><sub>-0.10</sub></td><td>94.53<sup>+0.04</sup><sub>-0.04</sub></td></tr><tr><td>Sérsic 2 mag</td><td>15.21<sup>+0.03</sup><sub>-0.03</sub></td><td>19.84<sup>+0.01</sup><sub>-0.01</sub></td><td>17.22<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>Sérsic 2 r<sub>e</sub> (kpc)</td><td>0.71<sup>+0.53</sup><sub>-0.05</sub></td><td>0.61<sup>+0.02</sup><sub>-0.02</sub></td><td>0.58<sup>+0.07</sup><sub>-0.07</sub></td></tr><tr><td>Sérsic 2 n</td><td>2.45<sup>+0.03</sup><sub>-0.03</sub></td><td>0.92<sup>+0.01</sup><sub>-0.01</sub></td><td>1.33<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>Sérsic 2 b/a</td><td>0.62<sup>+0.02</sup><sub>-0.02</sub></td><td>0.70<sup>+0.07</sup><sub>-0.07</sub></td><td>0.68<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>Sérsic 2 pa</td><td>80.45<sup>+0.14</sup><sub>-0.14</sub></td><td>78.61<sup>+0.10</sup><sub>-0.10</sub></td><td>82.35<sup>+0.09</sup><sub>-0.09</sub></td></tr><tr><td>Expdisc mag</td><td>13.04<sup>+0.04</sup><sub>-0.04</sub></td><td>16.43<sup>+0.05</sup><sub>-0.05</sub></td><td>14.65<sup>+0.05</sup><sub>-0.05</sub></td></tr><tr><td>Expdisc scale radius r<sub>s</sub> (kpc)</td><td>6.00<sup>+0.06</sup><sub>-0.06</sub></td><td>8.11<sup>+0.03</sup><sub>-0.03</sub></td><td>6.47<sup>+0.08</sup><sub>-0.08</sub></td></tr><tr><td>Expdisc b/a</td><td>0.54<sup>+0.02</sup><sub>-0.02</sub></td><td>0.70<sup>+0.07</sup><sub>-0.07</sub></td><td>0.68<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>Expdisc pa</td><td>94.57<sup>+0.02</sup><sub>-0.02</sub></td><td>88.61<sup>+0.10</sup><sub>-0.10</sub></td><td>82.35<sup>+0.09</sup><sub>-0.09</sub></td></tr><tr><td>PSF mag</td><td>23.17<sup>+0.02</sup><sub>-0.02</sub></td><td>24.72<sup>+0.03</sup><sub>-0.03</sub></td><td>22.22<sup>+0.02</sup><sub>-0.02</sub></td></tr><tr><td>Reduced χ<sup>2</sup></td><td>0.85</td><td>1.314</td><td>1.392</td></tr></tbody></table>

non-axisymmetric features such as a stellar bar which are best seen in the 2D images. We further discuss the non-axisymmetric nuclear bar structure below.

## 4.4 A small-scale nuclear bar and a resonant ring

Figs 4 and 6 reveal in the residual panels a clear nuclear bar in the innermost arcsecond. This bar is surrounded by a bright ring, forming a circumnuclear disc. The later is then embedded in a

two-armed spiral on larger scales, which is somewhat flocculent, but still appears to be part of a coherent density wave. There are clearly two large-scale spiral arms in the galaxy disc, which look coherent over 180° (see Fig. 2). The details of the nuclear bar and ring are better seen in the residual images of the innermost region, as shown in Fig. 10. Bars typically have a radial profile characterized by a Ferrers function or a Sérsic function with $n = 0.5$. We note that one of the inner components of a double Sérsic (plus one exponential function) GALFIT model C indeed has inner Sérsic $n \approx 0.5$ (Table 2);
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Model B. Left to right: HST F160W, F814W, and F438W filter radial profiles of the surface brightness (mag arcsec⁻²) with GALFIT model (solid blue line) and HST/WFC3 image data (black squares) as a function of radius (kpc). The data represent measurements along elliptical annuli of constant ellipticity and position angle as obtained from the best-fitting parameters. These radial profiles were obtained using the elliptical isophote fitting routine *Ellipse*. The parameters of fitting are tabulated in Table 2.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 8.** Model C. Grey-scale images and GALFIT modelling of the inner 25 arcsec × 25 arcsec of spiral galaxy J2345 in all three HST/WFC3 filters, shown with a log scale. Top row: *H* band, middle row: *I* band, and bottom row: *B* band. Left image panels show the HST image, middle image panels show the best-fitting GALFIT model, and right panels show the residual image. Spiral arms were not included in the GALFIT model.</figcaption></figure>

and the residuals seem to have removed the bar-like feature seen visually in the other models that did not include this extra component (Fig. 8). We point out that Sérsic profiles with low indices ($n \sim 0.5$) behave similarly to the Ferrers functions (Peng et al. 2010). In GALFIT model C above, we have achieved the best-fitting model with two components Sérsic1 and Sérsic2 plus an exponential disc, and a PSF function. We obtained a good fit with a small Sérsic1 index in all three bands i.e. $n = 0.66$ (H band), 0.30 (B band), and 0.53 (I band). So, we may say that the inner Sérsic component is behaving similar to a Ferrers bar function which seems to fit the nuclear bar component quite well.

The nuclear bar, oriented at PA ~ 18°, has a ~ 0.8 arcsec diameter in projection. Since it is located near the minor axis, its intrinsic diameter is about 1.6 arcsec, or 2.4 kpc. This nuclear ring might be

at the origin of the slight velocity residual from disc rotation along a similar PA in the MUSE Hα data cube (fig. 12 of Drevet Mulard et al. 2023). The nuclear bar is encircled by a nuclear stellar ring, of mean diameter of ~ 3 arcsec, or 4.4 kpc. The ring is clearly seen in contrast to the darker regions east and west of the nuclear bar. These regions appear dark even in the H-band residual image, indicating that they represent a deficit of starlight (relative to the fitted model) rather than being highly dust-obscured regions.

Bars are dynamical structures in galaxies that can generate a variety of resonant phenomena, including Lindblad Resonances (LR), due to their gravitational influence on the surrounding stellar and gas populations. LRs are locations within a galactic disc where the rotational frequency of stars or gas matches the frequency of the gravitational potential induced by the rotating bar. These resonances
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Model C. Left to right: HST F160W, F814W, and F438W filter radial profiles of the surface brightness (mag arcsec⁻²) with GALFIT model (solid blue line) and HST/WFC3 image data (black squares) as a function of radius (kpc). The data represent measurements along elliptical annuli of constant ellipticity and position angle as obtained from the best-fitting parameters. These radial profiles were obtained using the elliptical isophote fitting routine *Ellipse*. The parameters of fitting are tabulated in Table 2.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 10.** Grey-scale image of the innermost nuclear region of the galaxy showing the residual of the difference between the input *HST* image and the best-fitting model image (here, we show the GALFIT model A). The left panel shows the *H*-band and the right one shows the *I*-band image. A small nuclear bar and inner resonant ring of stars are indicated by arrows. Note the *I*-band image is more affected by dust extinction, particularly in the empty region surrounding the nuclear bar.</figcaption></figure>

occur at specific distances from the centre of the galaxy and are associated with the spiral density waves generated by the bar. Overall, the presence of a galactic bar can significantly influence the dynamics of stars and gas within a galaxy, and LRs play a central role in mediating these interactions. Studying the connection between bars and resonances is essential for understanding the formation and evolution of galactic structures.

From what has been learned with numerical simulations of embedded bar decoupling, a nuclear bar forms usually inside the resonant ring of a primary bar. The ring is then both the inner LR (ILR) of the primary bar, and the UHR (ultra-harmonic) resonance near the corotation of the nuclear bar (e.g. Buta & Combes 1996). The common resonance occurs at the decoupling of a nuclear bar, embedded in a primary bar, to avoid chaos in stellar orbits, and ensures stability of both bars (Friedli & Martinet 1993).

The exact position of the circumnuclear rings with respect to the resonances in the discs of the galaxies is an open problem. Such rings indicate the approximate region of the ILR. It is not easy to determine precisely a single radius for a ring. Moreover, stellar orbits may be elongated and not circular. When there exist two ILR, an outer and another inner one, the ring is observed in between these two (Schmidt et al. 2019). When there is only one ILR, the ring might be at a slightly smaller radius of the ILR radius, defined approximately through the first-order epicyclic theory, as shown through numerical simulations (e.g. Buta & Combes 1996). The resonance can only be defined as a range of radii, in a non-axisymmetric potential.

There is no primary bar seen at the present epoch in the J2345−0449 galaxy, but it might have been destroyed in the past by the gas infall to the centre (e.g. Bournaud & Combes 2002). Indeed, there has been recent AGN activity in the galaxy, as evidenced by
the giant radio jets. A primary bar, supported by a secondary bar, is the most efficient dynamical mechanism to drive the gas to the centre, and to fuel the AGN (e.g. Combes et al. 2014). The presence of a nuclear bar can affect how the material is funnelled toward the SMBH, influencing its feeding rate and the resulting activity, such as the emission of powerful jets of particles and radiation (for a review, see Storchi-Bergmann & Schnorr-Müller 2019).

This scenario implies that in the past a primary bar likely existed with an ILR at ~ 2.2 kpc radius, which gave birth to the embedded nuclear bar, and then dissolved. When gas has been driven inward because of torques from the bar (due to strong tangential forces), the gas then torques the bar (from the equality of action and reaction), and gives its angular momentum to the bar. This can destroy the primary bar, as has been examined in models by Bournaud & Combes (2002) and Bournaud, Jog & Combes (2005).

We propose that the AGN in J2345–0449 has been highly active in the past, and that it has consumed and/or ejected the majority of the molecular gas in the central region of the host. There is no longer cold gas left in the very centre, but the disappeared primary bar might have played a major role in driving the gas to the centre. Today we observe a large molecular ring, with CO emission (Fig. 14) at a mean radius of 6.6 arcsec, or 10 kpc (Nesvadba et al. 2021). This neutral gas ring could correspond to the OLR (outer LR) of the past primary bar.

As we discuss in Section 5.1, the central structure in this galaxy appears to be that of a pseudo-bulge, which is often the result of secular evolutionary processes within a barred galaxy. The presence of a pseudo-bulge implies that there existed a primary bar in the past, with a ring at its ILR. This also fits with the position of the galaxy in the green valley (Salim et al. 2016), after a star formation episode corresponding to the primary bar driving the gas inwards.

## 4.5 Spectral energy distribution models

For the SED fitting and to investigate the various physical parameters of J2345–0449, such as its stellar mass, SFR, dust mass, dust temperature, dust opacity, dust luminosity, and AGN contribution, we have made use of the publicly available SED fitting code MAGPHYS V 2.0 developed by da Cunha et al. (2010). The input data span a broad wavelength range incorporating GALEX in the UV, SDSS in the optical, 2MASS (Two Micron All Sky Survey) in the near-IR, WISE in the mid-IR, and Herschel in the far-IR. The best-fitting parameters and SED model fit to the photometric data points are tabulated in Table 3 and shown in Fig. 11, respectively. In this figure, the black line is the best-fitting SED model and the blue line is the unabsorbed optical stellar model. The bottom panel shows the residuals of the difference between the best fit and the observed data points. The fit provides normalized likelihood distribution of the best-fitting physical parameters which are given in Table 4 and shown in Fig. 11. These parameters are as follows: (1) fraction of total dust luminosity contributed by dust in the ambient (diffuse) ISM $f_μ$; (2) average V-band dust attenuation, $τ_V$; (3) total effective V-band absorption optical depth of the dust seen by young stars inside birth clouds $μτ_V$; (4) stellar mass, $log(M_*/M_⊙)$; (5) specific star formation rate, $log(sSFR/yr^{-1})$; (6) star formation rate, $log(SFR/M_⊙yr^{-1})$; (7) total dust luminosity, $log(L_{dust}/L_⊙)$; (8) total dust mass, $log(M_{dust}/M_⊙)$; (9) equilibrium temperature of cold dust in the ambient ISM, $T_{C}^{ISM}/K$; (10) equilibrium temperature of warm dust in the ambient ISM, $T_{W}^{ISM}/K$; (11) fractional contribution by cold dust to the total infrared emission, $\xi_{C}^{tot}$; and (12) fractional contribution by warm dust in the total infrared emission, $\xi_{W}^{tot}$.

The goodness of the fit can be judged from the $χ^2 = 5.76$ with reduced $χ^2/ν = 0.98$ for $ν = 6$ degrees of freedom (18 data

<table><caption>**Table 3.** Observed multiwavelength data used in SED fitting. Radiative flux in different passbands (effective wavelength $λ_{eff}$) is shown which is used for the SED fitting by CIGALE and MAGPHYS. The data are taken from NED and CDS (Lasker et al. 2008; Bianchi et al. 2011; Ahumada et al. 2020; Herschel Point Source Catalogue Working Group et al. 2020).</caption><thead><tr><th>Telescope and channel/detector</th><th>Wavelength (λ<sub>eff</sub>) (μm)</th><th>Flux (mJy)</th><th>Error (mJy)</th></tr></thead><tbody><tr><td>GALEX/FUV</td><td>0.15386</td><td>0.078</td><td>0.001</td></tr><tr><td>GALEX/NUV</td><td>0.23157</td><td>0.127</td><td>0.002</td></tr><tr><td>SDSS/<i>u</i></td><td>0.3551</td><td>0.52</td><td>0.02</td></tr><tr><td>SDSS/<i>g</i></td><td>0.4686</td><td>3.53</td><td>0.20</td></tr><tr><td>SDSS/<i>r</i></td><td>0.6165</td><td>5.55</td><td>0.21</td></tr><tr><td>SDSS/<i>i</i></td><td>0.7481</td><td>7.31</td><td>0.17</td></tr><tr><td>SDSS/<i>z</i></td><td>0.8931</td><td>8.41</td><td>0.62</td></tr><tr><td>2MASS/<i>J</i></td><td>1.2500</td><td>3.34</td><td>0.52</td></tr><tr><td>2MASS/<i>H</i></td><td>1.6500</td><td>4.16</td><td>0.89</td></tr><tr><td>2MASS/<i>K</i></td><td>2.17</td><td>4.31</td><td>0.67</td></tr><tr><td>WISE/<i>W1</i></td><td>3.4</td><td>5.29</td><td>0.22</td></tr><tr><td>WISE/<i>W2</i></td><td>4.6</td><td>3.22</td><td>0.13</td></tr><tr><td>WISE/<i>W3</i></td><td>12</td><td>7.87</td><td>0.51</td></tr><tr><td>WISE/<i>W4</i></td><td>24</td><td>11.71</td><td>2.77</td></tr><tr><td>HERSCHEL/SPIRE</td><td>250</td><td>352.7</td><td>17.8</td></tr><tr><td>HERSCHEL/SPIRE</td><td>350</td><td>196.7</td><td>19.5</td></tr><tr><td>HERSCHEL/SPIRE</td><td>500</td><td>77.3</td><td>21.4</td></tr></tbody></table>

points and 12 fitted parameters), which indicates an acceptable fit. The best-fitting results of MAGPHYS are shown in Table 4. The error bars represent $1σ$ confidence intervals taken between the 16th–84th percentile range of the likelihood distributions of each parameter.

# 5 RESULTS AND DISCUSSION

## 5.1 On the bulge and disc morphology and implications

Our high-resolution bulge–disc modelling of HST multiwavelength images reveal that the Sérsic index of the bulge is $n ≈ 1$, which implies that the galactic bulge is a pseudo-bulge having a disc-like exponential light profile rather than a classical bulge (see Table 2) which appear as thick oblate spheroids. However, in the H band we find that the Sérsic index of the bulge is slightly larger (range of $n ≈ 1.6–2.7$ in models A, B, and C). This may result from the fact that in the near-IR, the dominant population consists of old stars which have a larger scale height in the bulge compared to bluer stars in the disc. We note that, while pseudo-bulges generally have $n < 2$ and classical bulges have $n > 2$, the division is not perfectly bimodal and there is some degree of overlap between the two classes (Fisher & Drory 2008; Weinzirl et al. 2009). The Sérsic index $n = 1$ represents a pure exponential disc-like profile whereas $n = 4$ corresponds to the de Vaucouleurs elliptical galaxy-like profile. While this is rather a simplified division, more observations, and analysis are needed to investigate their physical details (Graham 2016). Several previous observations have found that bulges may exhibit fairly complex composite structures with a mix of spherical/ellipsoidal, disc-like, or boxy/bar-like components, potentially showing a wide array of possible bulge morphologies. Nevertheless, pseudo-bulges are always systematically flatter than classical bulges and appear more disc-like in both their morphology and shape (Fisher & Drory 2008).

We find that the pseudo-bulge scale length $r_e$ is much smaller than the disc $r_d$, such that $r_d ≈ 7 × r_e$, (Table 2), as one expects for pseudo-bulges. To quantify the relative light distribution in the bulge and disc components, the bulge (B), disc (D), and total (T) luminosity were derived along with B/D and B/T measurements indicating bulge
<figure><img src="image_14.png" /><figcaption>**Figure 11.** Upper figure: the rest-frame SED of J2345−0449, with observed photometric data points (red points) from the UV to the far-IR. The black line is the best-fitting MAGPHYS model ($\chi^2 = 5.76$ and reduced $\chi^2 = 0.98$). The blue line is the non-absorbed stellar model. The bottom panel shows the normalized residuals between the best-fitting model and the observed data points. Central and lower panels: the normalized likelihood distributions of a few best-fitting physical parameters for J2345−0449, derived from MAGPHYS modelling. These are (1) a fraction of total dust luminosity contributed by dust in the ambient (diffuse) ISM $f_\mu$; (2) average V-band dust attenuation, $\tau_V$; (3) total effective V-band absorption optical depth of the dust seen by young stars inside birth clouds $\mu\tau_V$; (4) stellar mass, $\log(M_{\text{stars}}/M_{\odot})$; (5) specific star formation rate, $\log(\text{sSFR})\text{yr}^{-1}$; (6) star formation rate, $\log(\text{SFR}/M_{\odot}\text{yr}^{-1})$; (7) total dust luminosity, $\log(L_{\text{dust}}/L_{\odot})$; (8) total dust mass, $\log(M_{\text{dust}}/M_{\odot})$; (9) equilibrium temperature of cold dust in the ambient ISM, $T_{\text{C}}^{\text{ISM}}/\text{K}$; (10) equilibrium temperature of warm dust in the ambient ISM, $T_{\text{W}}^{\text{ISM}}/\text{K}$; (11) fractional contribution by cold dust in the total infrared emission, $\xi_{\text{C}}^{\text{tot}}$; and (12) fractional contribution by warm dust in the total infrared emission, $\xi_{\text{W}}^{\text{tot}}$.</figcaption></figure>

<table><caption>**Table 4.** SED best-fitting parameters from MAGPHYS. These parameters are explained in the text and shown in Fig. 11 where their likelihood distribution functions are also plotted.</caption><thead><tr><th>Best-fitting parameters</th><th>Value</th></tr></thead><tbody><tr><td>sSFR (yr<sup>-1</sup>)</td><td>(9.55<sup>+2.45</sup><sub>-0.00</sub>) &times; 10<sup>-12</sup></td></tr><tr><td>M<sub>*</sub> (M<sub>&odot;</sub>)</td><td>(2.88<sup>+0.00</sup><sub>-0.79</sub>) &times; 10<sup>11</sup></td></tr><tr><td>L<sub>dust</sub> (L<sub>&odot;</sub>)</td><td>(7.94<sup>+0.19</sup><sub>-1.91</sub>) &times; 10<sup>10</sup></td></tr><tr><td>M<sub>dust</sub> (M<sub>&odot;</sub>)</td><td>(1.21<sup>+0.44</sup><sub>-0.21</sub>) &times; 10<sup>8</sup></td></tr><tr><td>SFR (M<sub>&odot;</sub>yr<sup>-1</sup>)</td><td>2.74<sup>+0.03</sup><sub>-0.33</sub></td></tr><tr><td>&tau;<sub>V</sub></td><td>2.48<sup>+0.00</sup><sub>-0.93</sub></td></tr><tr><td>&mu;&tau;<sub>V</sub></td><td>0.37<sup>+0.00</sup><sub>-0.08</sub></td></tr><tr><td>f<sub>&mu;</sub></td><td>0.71<sup>+0.05</sup><sub>-0.03</sub></td></tr><tr><td>T<sub>W</sub><sup>BC</sup> (K)</td><td>50.74<sup>+7.87</sup><sub>-7.83</sub></td></tr><tr><td>T<sub>C</sub><sup>ISM</sup> (K)</td><td>20.41<sup>+1.03</sup><sub>-2.05</sub></td></tr><tr><td>&xi;<sub>C</sub><sup>tot</sup></td><td>0.58<sup>+0.06</sup><sub>-0.05</sub></td></tr><tr><td>&xi;<sub>W</sub><sup>tot</sup></td><td>0.27<sup>+0.06</sup><sub>-0.06</sub></td></tr></tbody></table>

<table><caption>**Table 5.** Here, we show the bulge and disc luminosity along with $B/D$ and $B/T$ measurements indicating bulge to disc and bulge to total (bulge + disc) luminosity ratios. It can be clearly seen that our object has a 'pseudo-bulge' with only ~20 per cent contribution to the total intrinsic light in the I and H bands and only ~8.5 per cent contribution in B band.</caption><thead><tr><th>Band</th><th>L<sub>bulge</sub><br>(L<sub>&odot;</sub>)</th><th>L<sub>disc</sub><br>(L<sub>&odot;</sub>)</th><th>B/D</th><th>B/T</th></tr></thead><tbody><tr><td>F438W</td><td>4.10 &times; 10<sup>9</sup></td><td>4.48 &times; 10<sup>10</sup></td><td>0.093</td><td>0.085</td></tr><tr><td>F814W</td><td>2.65 &times; 10<sup>10</sup></td><td>1.11 &times; 10<sup>11</sup></td><td>0.240</td><td>0.193</td></tr><tr><td>F160W</td><td>1.50 &times; 10<sup>11</sup></td><td>5.24 &times; 10<sup>11</sup></td><td>0.287</td><td>0.223</td></tr></tbody></table>

to disc and bulge to total (bulge + disc) luminosity ratios. It can be seen in Table 5 that the galaxy has a B/T ratio of ~22 per cent in the H band and only ~8.5 per cent in the B band. Similarly, about 14–18 per cent of the total SDSS g-band or r-band luminosity is contributed by the bulge. These numbers again suggest that this galaxy has a pseudo-bulge rather than a classical bulge component
made up of an older stellar population. Galaxies with classical bulges generally have a much more centrally peaked light profile, containing a higher fraction of total light, and their Sérsic index is larger (n = 2 – 6) than pseudo-bulges (Fisher & Drory 2008; Weinzirl et al. 2009; Fisher & Drory 2011; Ray et al. 2022).

Here, we consider the (B/T) or (B/D) ratio an important parameter (notably both are strongly colour dependent) of galaxy structure that is also well correlated with several other key physical quantities relevant in studies of galaxy evolution, such as the morphology, kinematics, stellar mass, and SFR. The B/T is also directly related to the fraction of total stellar mass contained in the bulge, which in turn tightly correlates with the mass of the central SMBH of the galaxy (Kormendy & Ho 2013).

These bulge–disc decomposition results reinforce the conclusions of our previous ground-based study (Bagchi et al. 2014) where we concluded that J2345–0449 possibly features a pseudo-bulge, whose small Sérsic index and scale radius, and low bulge-to-total light ratio make it similar to pseudo-bulges observed in other galactic systems (e.g. Gadotti 2009). Recent recognition of a high fraction (~60 per cent) of nearby spiral galaxies having no kinematically ‘hot’ central bulge challenges the conventional ideas of hierarchical galaxy formation; i.e. how have so many bulgeless pure disc galaxies come to exist, despite the galaxy mergers known to occur (Fisher & Drory 2008; Weinzirl et al. 2009; Fisher & Drory 2011)? While classical bulges are possible products of galaxy mergers, pseudo-bulges seem to be still evolving, more likely as a result of their host galaxies’ slow internal (secular) evolution, which involves disc instabilities and angular momentum transfer.

At present very little is known about the formation mechanism and evolutionary path of such extremely massive disc galaxies, as they are found rarely, and moreover, not known to eject powerful relativistic jets extending to megaparsec scales, like those found in J2345–0449. Based on its structural and photometric properties discussed above, here we propose that the cosmic co-evolution of the galactic disc of J2345–0449 and the central SMBH driving the radio jets may have been primarily driven by external cold gas accretion and internal ‘secular’ processes (Kormendy 2013) instead of recent major merger events. This is suggested by its fast-rotating exponential disc-like pseudo-bulge, the absence of close galactic neighbours or signatures of tidal debris (e.g. stellar streams, plumes, or shells), and its stable, rotation-supported massive stellar disc showing well-formed spiral arms. In a recent HST study of a sample of spiral host galaxies showing double-lobed radio jets, Wu et al. (2022) found that they have exceptionally weak bulges, as judged by the low global concentrations, small global Sérsic indices, and low bulge-to-total light ratios (median B/T = 0.13). With a median Sérsic index of 1.4 and low effective surface brightnesses, they are consistent with harbouring pseudo-bulges.

In the late stages, dynamically, a major merger of galaxies will inevitably create a prominent classical bulge and may exert a strong perturbation of the central BH's spin axis as it may be torqued by the gravitational influence of a second BH nearby, and eventually both will merge into a single stable BH of a larger mass, emitting nano-Hertz frequency gravitational waves (Agazie et al. 2023; EPTA Collaboration 2023; Reardon et al. 2023; Xu et al. 2023) during inspiral. On the contrary, in J2345–0449, a close alignment (within 10°) of the inner and outer radio lobe pairs, and lack of violent merger signs, implies that the BH's spin axis, which in equilibrium orients itself orthogonal (either parallel or antiparallel) to the inner accretion disc spin direction because of the Lense–Thirring frame dragging effect (Bardeen & Petterson 1975), has remained stable (non-precessing), at least between the two last episodes of jet triggering

on time-scales of ~10^8 yr. Thus, the jets are possibly emitted from a stable gyroscope, such as a rapidly spinning massive object, and present theories provide a framework of jet formation via a magnetohydrodynamic plasma mechanism (Blandford & Znajek 1977; MacDonald & Thorne 1982; Penna, Narayan & Sadowski 2013). All this suggests that the galactic disc and the central BH might have had a quiet evolution together, without upheavals of major mergers.

Recently Izquierdo-Villalba et al. (2019) studied the evolution of pseudo-bulges from the catalogues of merger-trees obtained by the Millennium (Springel et al. 2005) and Millennium II (Boylan-Kolchin et al. 2009) N-body simulations, using the ‘L-Galaxies’ semi-analytical model. They find that at early epochs ($z \approx 1$) pseudo-bulges are the main type of galaxies at high stellar masses $M_* > 10^{10.5} M_\odot$, while classical bulges and ellipticals are the main ones for $M_* < 10^{10.5} M_\odot$. While at low masses, there is little evolution between $z = 1$ and 0, at high masses by $z = 0$, bulged ellipticals are now the dominant galaxy population, and pseudo-bulges have declined in number. This results from the hierarchical growth of structures: pseudo-bulges hosted in the most massive galaxies at high $z$ are subsequently destroyed by major mergers which turn galaxies into pure bulges. However, since massive galaxies of $M_* > 10^{10.5} M_\odot$ only makeup ~4 per cent of bright galaxies in the local volume, pseudo-bulge galaxies and those with no bulge are the dominant type of bright galaxy by number in the local Universe (Fisher & Drory 2008; Weinzirl et al. 2009; Fisher & Drory 2011). Contrary to the above findings, we find that J2345–0449 is a rare example of a highly massive galaxy ($M_* > 2 \times 10^{11} M_\odot$) that apparently has managed to avoid the destruction of its pseudo-bulge by violent mergers, as it might have evolved predominantly through secular, disc-driven processes – the same conclusion that we arrived at from other arguments.

Using an optical spectrum obtained from the IUCAA (Inter-University Centre for Astronomy and Astrophysics) telescope, Bagchi et al. (2014) measured the stellar velocity dispersion $\sigma_*$ of the central region of the bulge from fitting the Mg b (λ5175 Å) lines, obtaining $\sigma_* = 379(\pm25)$ km s$^{-1}$ with the slit oriented along the galaxy major axis and $\sigma_* = 351(\pm25)$ km s$^{-1}$ along the minor axis. Both values are spatially averaged over an extraction window of size 2.35 kpc centred on the galactic nucleus. This implies an extremely large bulge dynamical mass of $M_{\text{bulge}} = 1.07 - 1.20(\pm0.14) \times 10^{11} M_\odot$, roughly within one scale radius $r_e \approx 1.2$ kpc (we discuss the implications of this large bulge dispersion velocity and mass concentration below). The massive stellar and gaseous disc of J2345–0449 is clearly rotationally supported, with $(v_{\text{rot}}/\sigma_*)^2 \approx 1.5$, where $v_{\text{rot}}$ is the rotational speed and $\sigma_*$ is velocity dispersion of stars) and therefore, the galaxy disc should be dynamically stable against gravitational instability.

## 5.2 Stellar mass, SFR, dust, and gas properties in the disc

Structurally, J2345–0449 shows a prominent disc of stars, and an ISM containing warm dust along with a coplanar rotating cold, molecular gas disc in the equatorial plane. Based on our SED model fitting results (Tables 4 and 6), we can examine the properties of the ISM and dust components.

Rowlands et al. (2012) carried out a study of dust properties and star formation histories of the local submillimetre-detected galaxies, with a mixture of early-type and passive spirals. The passive spirals are defined as early-type galaxies with dust abundance and low SFR. They found the mean dust mass is $\log M_{\text{dust}} = 7.74 M_\odot$ with $M_{\text{dust}}$ in the range $10^5 - 10^8 M_\odot$ for early-type galaxies, and for passive spirals the mean $\log M_{\text{dust}} = 7.47$ with $M_{\text{dust}}$ in the range
<table><caption>**Table 6.** SED best-fitting parameters obtained from CIGALE with varying AGN fraction.</caption><thead><tr><th>Fitted parameter</th><th>AGN<sub>frac</sub> = 0 per cent</th><th>AGN<sub>frac</sub> = 10 per cent</th><th>AGN<sub>frac</sub> = 20 per cent</th></tr></thead><tbody><tr><td><i>M</i><sup>*</sup><sub>old</sub> (M<sub>☉</sub>)</td><td>1.53 × 10<sup>11</sup></td><td>1.51 × 10<sup>11</sup></td><td>2.30 × 10<sup>11</sup></td></tr><tr><td><i>M</i><sup>*</sup><sub>young</sub> (M<sub>☉</sub>)</td><td>2.28 × 10<sup>7</sup></td><td>2.16 × 10<sup>7</sup></td><td>1.7 × 10<sup>7</sup></td></tr><tr><td><i>L</i><sub>dust</sub> (L<sub>☉</sub>)</td><td>3.36 × 10<sup>10</sup></td><td>3.40 × 10<sup>10</sup></td><td>3.00 × 10<sup>10</sup></td></tr><tr><td>SFR (M<sub>☉</sub>yr<sup>-1</sup>)</td><td>2.38</td><td>2.19</td><td>1.88</td></tr><tr><td>τ<sub>V</sub></td><td>1.53</td><td>1.53</td><td>1.71</td></tr><tr><td>χ<sup>2</sup></td><td>5.10</td><td>5.63</td><td>5.86</td></tr></tbody></table>

$10^6 - 10^8 M_{☉}$. From SED fitting, we find $log M_{dust} = 8.1 M_{☉}$ in J2345–0449 (Table 4), consistent with the results from the sample of Rowlands et al. (2012), with the dust mass for J2345–0449 falling at the high-mass end of passive spirals. The small ratio of dust to stellar mass $M_{dust}/M_{\star} \approx 6.3 \times 10^{-4}$ is typical of passive galaxies (Orellana et al. 2017), while the ratio of molecular gas mass (the H$_2$ mass is $1.62 \times 10^{10} M_{☉}$) to stellar mass is 0.085 indicating a large, mostly quiescent (non-SF) repository of potential SF material.

The estimated integrated stellar mass, $M_{\star}$, of the galaxy is $2.88(+0, -0.79) \times 10^{11} M_{☉}$ from the MAGPHYS SED model and $1.53(\pm0.5) \times 10^{11} M_{☉}$ from the CIGALE model. They are consistent with each other within their respective uncertainties. We may take the stellar mass of the galaxy as the mean of two values at $M_{\star} = 2.2(\pm0.7) \times 10^{11} M_{☉}$. In an earlier paper, Nesvadba et al. (2021) quoted a value of $M_{\star} = 3.6 \times 10^{11} M_{☉}$ based on the 2MASS J-band absolute magnitude. This value is higher than our SED fitted values, but we note it is dependent on the assumed star formation history and the mass-to-light ratio in the J band.

The global SFR computed via two SED fitting models, MAGPHYS and CIGALE, gives consistent results. The SFR and sSFR from MAGPHYS are $SFR_{mag} = 2.45 M_{☉}yr^{-1}$ and $sSFR_{mag} = 1.09 \times 10^{-11} yr^{-1}$, while from CIGALE they are $SFR_{cig} = 2.38 M_{☉}yr^{-1}$ and $sSFR_{cig} = 1.55 \times 10^{-11} yr^{-1}$. In previous works, Nesvadba et al. (2021) measured an SFR of $SFR_J = 3.6 M_{☉}yr^{-1}$ based on the J-band magnitude and Walker et al. (2015) obtained $SFR_{FUV} = 1.6 M_{☉}yr^{-1}$ based on the GALEX far-UV (FUV) flux, bracketing our value. These are global SFRs from the integrated flux values. Drevet Mulard et al. (2023) have examined the SFR surface density and observed properties of individual SF regions in the disc based on VLT/MUSE spectroscopic observations.

J2345–0449 shows mild ongoing SF activity across the galactic disc, traced by Hα and [N II] emission lines. It is also strongly detected by the Galaxy Evolution Explorer in the near (2316 Å) and FUV (1539 Å) bands, giving evidence for formation of OB stars during the past ~100 Myr. Interestingly, UV or Hα signatures of very recent (<100 Myr old) stars are absent closer to the nucleus, which shows only stellar populations older than ~1 Gyr in the bulge, while our HST imaging (Fig. 2) indicates that most of the ongoing star formation in blue stellar nurseries is occurring in the outer disc (see also Drevet Mulard et al. 2023). This requires some physical process that can shut down star formation in the central pseudo-bulge region, possibly a consequence of heating and expulsion of the SF medium by the energetic, episodic feedback of the central AGN, as revealed by two pairs of radio jets and lobes.

## 5.3 Contribution of the active galactic nucleus to SED and the accretion state of the black hole

Vigorous star formation produces significant UV emission, and the IR luminosity of dust is often interpreted as being directly proportional to the absorbed fraction of the energy from star formation. In addition,

it is possible that a significant radiative flux from the AGN may contribute to the SED as well as drive an outflow. Active nuclei can emit strongly in UV, and if embedded in a dusty torus, may also contribute to the IR emission by heating dust in the torus and clouds surrounding the AGN. In addition, an older stellar population can also heat dust that is present in the ISM of a galaxy, which can add to the total FIR emission. Fitting model to the SED can provide insights into these processes.

In order to investigate all such complex astrophysical processes we used the CIGALE V 0.9,<fnref n="2" /> a PYTHON-based SED fitting code (Noll et al. 2009; Boquien et al. 2019) to constrain the fractional contribution of AGN radiation to the total IR luminosity, represented by the AGN-fraction parameter. In addition, CIGALE also simultaneously provides several other parameters of the ISM dust and star formation activity in the galaxy, similar to MAGPHYS. We have carried out SED fitting taking fiducial AGN fractions of 0 per cent, 10 per cent, and 20 per cent. The AGN fraction is defined as the AGN contribution to the total IR luminosity in the ~5–1000 µm band. In CIGALE, the AGN fraction explicitly takes into account three emission components through a radiative transfer model: the primary source surrounded by a dusty torus, the scattered emission by dust, and the reprocessed thermal dust emission (more details can be found in Boquien et al. 2019).

The results of SED fitting with varying AGN contributions are shown in Table 6. Varying the AGN fraction did not significantly (at > 99 per cent confidence level) improve the reduced $\chi^2$ of the fit, on the basis of a Fisher-test. Therefore, SED fitting reveals no evidence of a radiatively efficient, accretion-driven energy source in the galaxy’s SED (either obscured or unobscured). In contrast, high-excitation radio galaxies (HERGs) are fuelled at high Eddington rate onto a radiatively efficient standard accretion disc by cold gas, perhaps brought in through mergers and interactions, processes which are clearly absent in this galaxy. Rather, the AGN in J2345–0449 can be classified as a low-excitation radio-loud AGN (LERG; e.g. Best & Heckman 2012; Heckman & Best 2014) possibly powered by a radiatively inefficient, low Eddington ratio, hot accretion flow of gas known as Bondi–Hoyle type accretion (Heckman & Best 2014; Hardcastle & Croston 2020, and references therein). In terms of astrophysical process, the gaseous fuel for the LERGs is possibly derived from the hot X-ray emitting haloes surrounding massive galaxies, groups, and clusters, as part of a radio-AGN feedback loop. Consistent with this picture, indeed an extended (≥ 160 kpc scale) soft X-ray emitting halo of warm/hot baryonic matter has been robustly detected in this galaxy (Walker et al. 2015; Mirakhor et al. 2021), whose implications are further discussed in next sections.

Furthermore, in order to better constrain the bolometric luminosity of the AGN we make use of results from Bagchi et al. (2014) and the optical spectrum of the galaxy’s central region taken with the

<footnote n="2">²https://cigale.lam.fr/</footnote>
<figure><img src="image_15.png" /><figcaption>**Figure 12.** Optical spectrum of J2345–0449 taken with IFOSC (IUCAA Faint Object Spectrograph and Camera) on IUCAA 2-m telescope, centred on the nucleus. We used two IFOSC grisms, grism nos 7 and 8, in combination with a 1.5 arcsec slit, yielding a wavelength coverage of 3800–6840 and 5800–8350 Å and spectral resolutions of 300 and 240 kms⁻¹, respectively (Bagchi et al. 2014, for more details). The absorption lines are marked by blue dot-dashed lines and the expected positions of the emission lines are marked with magenta dashed lines. The red end of the spectrum is contaminated by telluric features. A telluric reference spectrum is shown in green at the bottom. Note the absence of any prominent strong emission lines. Weak Hα and [N II] emission lines can be seen.</figcaption></figure>

IUCAA 2-m telescope (Fig. 12). The emission-line spectrum is typical of low-luminosity AGN (LLAGN) and LINER-type galaxies with weak Hα emission (Tadhunter et al. 1998; Buttiglione et al. 2010; Best & Heckman 2012). From VLT/MUSE spectroscopy, Drevet Mulard et al. (2023) also concluded the same. The spectrum of the J2345–0449 core is characteristic of an old stellar population. The [O III]λλ4949 Å + 5007 Å emission doublet is not detected in our spectrum. We use the empirical relationship of Heckman et al. (2004) to constrain the bolometric AGN luminosity by setting $L_{bol;AGN} = 3500 \times L_{[OIII]\lambda5007}$. With the upper limit of $L_{[OIII]\lambda5007} < 1 \times 10^{40}$ erg s⁻¹ given by Bagchi et al. (2014), this would correspond to $L_{bol;AGN} = 3.5 \times 10^{43}$ erg s⁻¹ and an Eddington ratio of $\log(\lambda_{Edd}) = \log(L_{bol;AGN}/L_{Edd}) = -(2.6-3.6)$ for the BH mass range $10^{(8-9)}$ M☉. Drevet Mulard et al. (2023) estimated $L_{bol;AGN} = 3.3 \times 10^{43}$ erg s⁻¹ from [O III]λλ4949 Å + 5007 Å luminosity, quite close to our estimate. However, both values should be considered upper limits because the fluxes are dominated by the old stellar populations around the nucleus, which are the dominant gas heating source in the nuclear regions of J2345–0449 (Drevet Mulard et al. 2023). In addition, the absence of broad Balmer emission lines in our slit spectrum (mainly the Hβ line) implies that J2345–0449 is not a broad-lined radio galaxy (BLRG or a type-1 AGN). Nevertheless, it is evidently extremely radio-loud for a spiral galaxy, which as a rule, does not create such large-scale radio jets. The integrated flux density and radio luminosity of J2345–0449 at 1.4 GHz are estimated at $S_{1.4} = 180.60 \pm 20.0$ mJy and $L_{1.4} = 2.5(\pm 0.3) \times 10^{31}$ erg s⁻¹ Hz⁻¹, respectively, which are typical of a low/medium-powered radio galaxy.

This bolometric luminosity is also indicative of the current rate of mass accretion and growth rate of the BH over time. The current rate of growth of the BH (ζ) is governed by the mass accretion rate on the BH and the resulting bolometric luminosity,

$$ \frac{\zeta}{M_{\odot} \text{yr}^{-1}} = \frac{(1 - \epsilon) L_{\text{bol;AGN}}}{\epsilon c^2}, \qquad (3) $$

where c is the speed of light and ϵ is the conversion efficiency of the accreted mass to radiative energy. The value of ϵ is not precisely known, but some previous works (e.g. Carraro et al. 2020; D'Eugenio et al. 2021) find ϵ = 0.1, which for J2345–0449 gives an upper limit to the accretion rate of ζ < 5.5 × 10⁻³ M☉yr⁻¹, assuming a lower limit of $L_{bol;AGN}$ as described above. At this fairly low accretion rate in the present epoch, assembling even a lower mass BH of 5 × 10⁷ M☉ would take about a *Hubble* time, and much longer for a more massive BH. This implies that either the primordial ‘seed’ BH was very massive initially or the mass accretion rate was much higher in the past.

This is a promising scenario for J2345–0449 because its AGN is presently in a low-excitation state (LERG/LLAGN), showing extremely low radiative luminosity, but with a very high kinetic luminosity in jetted outflows (a ratio of jet power to bolometric luminosity > 6), strongly reminiscent of a hot advection-dominated accretion flow or radiatively inefficient accretion flow (ADAF-RIAF, Narayan & Yi 1994; Yuan & Narayan 2014). In this accretion mode, because of rapid advection, compared to the radiative cooling time, the radiative efficiency of accretion flow is in general lower than that of a standard thin accretion disc. This model also provides an
important clue as to how the radiatively inefficient, Mpc-scale kinetic radio jets in J2345−0449 might be launched via a low Eddington rate accretion state, as observed in local radio-loud galaxies (Ho 2002). Recently, in an extensive study of Mpc-scale GRGs, Dabhade et al. (2020b) came to the conclusion that almost every one of the GRGs in their sample is fuelled by an AGN with a low value of Eddington ratio ($\lambda_{Edd} < 0.01$), conforming to the low accretion rate, radiative inefficient ADAF/RIAF model. Interestingly, in a parallel study, Mahato et al. (2022) showed that the average accretion rate of giant radio quasars (GRQs) ($\lambda_{Edd}$) is roughly about 10 times higher than GRGs (radio jets of both extend to similar Mpc scales, but GRQ jets have more kinetic power than GRGs), while their central BHs are about equally massive ($M_{BH} \sim 10^8–10^9 M_{\odot}$). This implies that powerful Mpc-scale radio jets can be launched even at high rates of accretion ($\lambda_{Edd} \gtrsim 0.1$), and the mass of the BH is not a main deciding factor. Recently, Breiding et al. (2024) found strong evidence for jet activity in radio-loud quasars triggered by mergers. A corollary of this result is that the launch of radio jets needs some additional favourable or necessary conditions, such as a fast spin of the BH and interaction of the accretion disc with strong magnetic fields near the BH, possibly launched via the Blandford–Znajek (BZ) or Blandford–Payne (BP) type mechanism (Blandford & Znajek 1977; MacDonald & Thorne 1982; Penna et al. 2013). In the BZ model, dynamically important horizon-scale magnetic fields near spinning Kerr BHs naturally produce powerful electromagnetic outflows (Event Horizon Telescope Collaboration 2021, 2024) which may source the Mpc-scale jets. These important studies provide fundamental insight into the accretion disc-relativistic jet coupling process in radio galaxies and quasars.

In J2345−0449, the inner lobes show an FR-II morphology, clearly being fed by active jets, while the outer lobes have a peculiar, filamentary, and diffuse structure, possibly decaying relics of previous jet activity that ceased millions of years ago. This makes the outer lobes excellent tracers of the final stages of radio jet activity (Fig. 1). The kinetic power of the radio jets is estimated at $L_{jet;kinetic} \approx 2 \times 10^{44} \text{erg s}^{-1}$ from the GMRT 323 MHz flux (Bagchi et al. 2014). As we have applied no correction for the (unknown) energy loss in the expansion of the outer radio lobes, this is likely to be a lower limit. This jet power is much higher than $L_{bol;AGN}$ and therefore, any radiation-driven feedback impact of the AGN, for example, a radiation pressure driven outflow, akin to a quasar, would be at least about a factor of 10 less energetic than the kinetic power carried by the radio jets.

Thus, in this intriguing and rare spiral galaxy, we are witnessing a radio-mechanical (sometimes also called the maintenance mode) energetic outflow in action wherein the AGN, despite its low Eddington accretion rate and low radiative output, manages to produce large mechanical energy carried by the energetic jets which may couple efficiently to the circumgalactic atmospheric gas (e.g. Churazov et al. 2002). In the absence of such feedback heating, the hot X-ray emitting intra-luster gas in many clusters, groups, and elliptical galaxies would cool and rapidly form stars, building much larger galaxies than are seen (Fabian 2012; McNamara & Nulsen 2012). Intriguingly, J2345−0449 neither originates in an elliptical galaxy (but rather in a spiral) nor is embedded in a rich galaxy cluster or a group environment, and the working surface of the farthest ends of the radio jets (the termination shocks) clearly extends well beyond the galaxy’s virial radius (~450 kpc). At this region, they do not encounter a dense thermal plasma to inflate X-ray dark non-thermal cavities or bubbles, unlike the cluster central brightest cluster galaxies (BCGs) of elliptical morphology. Nevertheless, the radio-mechanical impact of its radio jets on the circumgalactic ambient medium is

<figure><img src="image_16.png" /><figcaption>**Figure 13.** The variation of the sSFR (=$SFR/M_*$) of galaxies as a function of their stellar masses $M_*$ is shown. The dotted black line is the SFMS for redshift $z \sim 0$ (Elbaz et al. 2007). The graph is divided into three parts, showing the actively SF region ($sSFR > 10^{-10.5} \text{yr}^{-1}$, blue), the green valley region ($10^{-11.5} < sSFR < 10^{-10.5} \text{yr}^{-1}$, green), and the quiescent region ($sSFR < 10^{-11.5} \text{yr}^{-1}$, pink-red; see Salim et al. 2016). Miscellaneous galaxy samples spanning a broad range of SFR are also plotted: (a) the highly SF galaxies from Hoopes et al. (2007, pink), (b) super-spirals, lenticulars, and post-mergers from Ogle et al. (2019, green), (c) a sample of galaxies provided by Kalinova et al. (2021) including galaxies that are SF (dark blue), mixed (orange), quiescent nuclear-ring (bright red), nearly retired (brown), and fully retired (dark red) are also shown for comparison. The position of galaxy J2345−0449 is well within the ‘green valley’ region. Other than this object, the objects shown with star symbols are six other highly massive galaxies namely, UGC 12591 (black), NGC 1030 (brown), NGC 1961 (grey), NGC 4501 (blue), NGC 5635 (purple), and NGC 266 (cyan), from Ray et al. (2022, 2024).</figcaption></figure>

inevitable. This could be preventing the cooling of its extended, hot gaseous atmosphere strongly detected in deep X-ray observations (Mirakhor et al. 2021), thereby possibly partially quenching new star formation activity in the disc. As we show below, compared to the radiation pressure-dominated quasar-mode feedback, the radio-mode mechanical feedback in this galaxy is possibly keeping it in a state of semi-quiescence (in maintenance mode) rather than fully quenching the star formation in the disc. Nevertheless, in the early growth phase of the central SMBH, when growing via super-Eddington or Eddington-rate accretion, it may have exerted a strong feedback effect via a hot wind on the molecular/atomic gas within the host and reduced the SFR on these very small scales. In contrast, UGC 12591 is one such extreme mass, highly bulge-dominated hybrid galaxies reported by Ray et al. (2022) which has no visible powerful radio jets observed now, yet it has reached a state of virtual quiescence showing an extremely low SFR, illustrated in Fig. 13 along with J2345−0449 and other galaxies for comparison.

## 5.4 An accreting supermassive black hole growing in a bulgeless environment

At present, very little is known of the growth and AGN activity of SMBHs in bulgeless spiral galaxies. Our HST results above firmly establish J2345−0449 to be a disc-dominated spiral with unusual
characteristics. Despite lacking a classical bulge, this exceptional spiral galaxy hosts a pseudo-bulge and possibly a massive, mass accreting (radio-loud) central BH of minimal mass $M_{BH} > 10^8 M_{\odot}$, which is needed to launch the large Mpc-scale relativistic jets at low- to medium-mass accretion rates (Dabhade et al. 2020b; Mahato et al. 2022).

Here, we attempt to estimate the mass of the central BH using several established scaling relations. Our present data do not provide strong constraints on $M_{BH}$, in part because J2345-0449 lacks a classical bulge, whereas the BH mass is more easily obtained from the tight $M_{BH} - \sigma_{*}$ correlation in bulge-dominated systems (Marconi & Hunt 2003; Häring & Rix 2004; Gültekin et al. 2009a). For this reason, the demographics of BHs in pure disc galaxies are poorly determined. However, there is growing evidence that a classical bulge is not essential for nurturing a massive BH and some pseudo-bulge systems may also host fairly massive BHs of masses $10^3 - 10^7 M_{\odot}$ (Kormendy, Bender & Cornell 2011; Shankar et al. 2012). Therefore, these BHs must somehow attain masses substantially larger than the values implied by their bulge properties, possibly via a disc-driven, non-merger growth route. Possibly J2345-0449 is one such system, being an extreme member of such a population.

The central stellar velocity dispersion value on ~1 arcsec scales in J2345-0449 is $\sigma_{*} = 351 \pm 25$ km s$^{-1}$. This is much higher than normal Milky Way-like spirals and close to the highest velocity dispersion found among nearby E and S0 galaxies. It is also significantly larger than that known for the majority of bulgeless disc galaxies on such a spatial scale (e.g. Hu 2008; Shankar et al. 2012; Graham 2016), and is comparable to values found in very massive elliptical galaxies (e.g. Marconi & Hunt 2003; Häring & Rix 2004; Gültekin et al. 2009a). This strongly hints at a large central concentration of mass, including a putative SMBH in this overall massive galaxy.

Using the dynamical mass of the pseudo-bulge $1.07(\pm0.14) \times 10^{11} M_{\odot}$, within one scale radius ($r_{e} = 1.25$ kpc), we calculate the putative BH mass $M_{BH} = 2.54(\pm0.48) \times 10^8 M_{\odot}$ using the BH mass versus bulge mass correlation found by Marconi & Hunt (Marconi & Hunt 2003). This value is comparable to another estimate of $M_{BH} = 3.88(\pm0.40) \times 10^8 M_{\odot}$ obtained from the $M_{BH} - \sigma_{*}$ correlation of Hu (2008), applicable to pseudo-bulges. Another estimate using the Fundamental Plane relation of BH activity of Gültekin et al. (2009b) gives the BH mass $M_{BH} = 5(\pm0.5) \times 10^8 M_{\odot}$ using the 5 GHz radio luminosity $L_{R} = 45(\pm5) \times 10^{38}$ erg s$^{-1}$ and 2–10 keV X-ray luminosity $L_{X} = 9.9(\pm1.5) \times 10^{40}$ erg s$^{-1}$ (Bagchi et al. 2014). However, if we use the $M_{BH} - \sigma_{*}$ relation of Gültekin et al. (2009a), which is calibrated using a mixed sample of both ellipticals and spirals, with dynamically detected central BH masses and has an intrinsic rms scatter of $\epsilon = 0.44 \pm 0.06$ dex, we obtain the BH mass $M_{BH} = 1.43 \times 10^9 M_{\odot}$. Finally, using the K-band $M_{BH} - L_{K}$ correlation of Graham (2007), we obtain $M_{BH} = 1.22 \times 10^9 M_{\odot}$. Both these latter BH masses are close to each other, yet are significantly larger than the previous mass estimates. It is necessary to verify these rather large BH mass values via more direct and robust methods. If confirmed it would imply that J2345-0449 hosts an SMBH as massive as those found in giant elliptical galaxies ($M_{BH} \sim 10^{8-9} M_{\odot}$), which is not expected at all for normal disc galaxies, and which would pose a tension and raise various interesting questions about the origin and growth history of an accreting BH of such a high mass in a spiral host galaxy.

We note that some previous works have suggested (e.g. Hu 2008; Graham et al. 2011; Shankar et al. 2012; Graham 2016) that pseudo-bulge and barred galaxies may host BHs that are about 3–10 times less massive than pure ellipticals with the same $\sigma_{*}$. It has also

been proposed that their BH masses do not correlate well with the properties of galaxy discs or their pseudo-bulges (Kormendy et al. 2011), and hence the growth process of massive BHs in flattened galactic discs lacking prominent bulges is still puzzling and therefore a subject of ongoing deeper investigations. Interestingly, Kormendy et al. (2011) and Kormendy & Ho (2013) find that there is no convincing evidence of co-evolution of SMBHs and the disc component of galaxies, implying that feedback from BH accretion does not affect disc evolution or its stellar content. They propose that feedback from star formation is much more important in disc-dominated systems than radio-mode feedback. On the contrary, in J2345-0449, a predominantly disc-dominated system, we find that the SFR is very small (significantly quenched) for its extremely large stellar and molecular gas masses, and it clearly hosts an SMBH launching powerful radio jets, which is located in the nucleus of a (classical) bulge-less disc system, contrary to the above idea. This makes J2345-0449 a unique and exceptional galactic laboratory where powerful AGN-energized radio jets seem to be directly regulating the star formation activity in the surrounding disc.

## 5.5 Molecular gas, star formation rate, and comparison with other galaxies

As many of the properties of this galaxy are rather extreme, like its huge mass, fast rotation speed, and unusual ‘double-double’ episodic relativistic radio jets extending up to Mpc scale, it would be very informative to investigate their relationship to the stellar and baryonic mass contents of the galaxy and contrast that to other local less massive galaxies. The raw fuel for star formation in galaxies is the cold molecular gas (e.g. Kennicutt 1998; Larson 2003), and understanding galaxy evolution requires understanding the various physical processes that are responsible for the accretion and depletion of this gas in different environments existing within the galaxy. The SF molecular clouds are predominantly made up of molecular hydrogen. As $H_2$ lacks a permanent dipole moment, it is difficult to directly observe at low gas temperatures. Instead highly abundant CO is a good molecular gas tracer in the ISM. The Doppler shift in the frequency of any such molecular line tracers provides valuable information on kinematics and gas flows.

To compare the estimated SFR state of J2345-0449 with other disc galaxies we consider the largest sample of local SDSS galaxies on the SFR-$M_{*}$ plane (Brinchmann et al. 2004). The galaxies that lie on the star formation main sequence (SFMS) are the active SF ones, and those that have a significant offset from the SFMS are the passive or quenched galaxies. To avoid the selection biases in the normalization, slope, and shape of the SFMS, we use an ‘objective definition’ of the SFMS defined by Renzini & Peng (2015) from the SDSS galaxy sample (Brinchmann et al. 2004) that does not rely on a pre-selection of SF or passive galaxies at all.

From Renzini & Peng (2015), the fit to the SFMS ridge line is given by f

$$ \log \left( \frac{\text{SFR}}{\text{M}_{\odot}\text{yr}^{-1}} \right) = (0.76 \pm 0.01) \log \left( \frac{M_{*}}{\text{M}_{\odot}} \right) - (7.64 \pm 0.02) \quad (4) $$

Therefore, for our estimated $M_{*} = 1.9 (\pm 0.5) \times 10^{11} M_{\odot}$, the global SFR for J2345-0449, if it were to lie on the $z=0$ SFMS, should be $\sim 8.6 M_{\odot}yr^{-1}$ and $sSFR = 4.5 \times 10^{-11}yr^{-1}$ as against estimated SFR (2.5–2.4) $M_{\odot}yr^{-1}$ and $sSFR$ of (1.1–1.6) $\times 10^{-11}yr^{-1}$. The range of values shown above spans the range between the MAGPHYS and CIGALE SED fitting estimates. Therefore we find that, that J2345-0449 shows a globally integrated SF quenching by about a factor of four when compared to the $z=0$ SFMS of Renzini &
<figure><img src="image_17.png" /><figcaption>**Figure 14.** Left panel shows the gas kinematic map of the $^{12}$CO(J = 1–0) molecular line transition observed by ALMA in the central region, from Nesvadba et al. (2021). Colours represent the gas velocity relative to the kinematic centre as shown by the colour bar at the bottom in km s$^{-1}$. The kinematic centre, located at $\alpha = 23h 45m 32.708s$ and $\delta = -04^\circ 49^\circ 25.42^\circ$, is represented by a cross. This position is < 0.7 arcsec from the compact 3.6 mJy beam$^{-1}$ (about 10$\sigma$ above noise) radio source detected in the VLA FIRST survey at 1.4 GHz as well as the 5 mJy beam$^{-1}$ point source in the GMRT 323 MHz map. No $^{12}$CO(J = 1–0) emission was detected above a flux limit of rms = 332 $\mu$Jy beam$^{-1}$ in the central 2.8 × 1.5 arcsec region, corresponding to a size of 4.2 kpc × 2.2 kpc (projected). This CO-depleted region is shown by a dotted ellipse superposed on the CO rotation map (left) and on the HSTI-band image (right). The diameter of the CO-depleted zone shown is about two times the effective diameter of the stellar bulge in the HSTB- and I-band images. This corresponds to $2 \times (2r_e) = 2 \times (2.2 - 2.6)$ kpc.</figcaption></figure>

Peng (2015). We infer that although the global SFR and sSFR for J2345−0449 are significantly below the well-established main sequence, its SFR is well above the fully quenched (spheroidal) galaxies, indicating a drastic reduction, yet not a full termination of star formation. These plots also reveal a clear bimodality in global and resolved realizations of the SF properties of galaxies around the main sequence (Bluck et al. 2022).

In Fig. 13, we show the sSFR of galaxies against $M_*$ When compared to the $z=0$ main sequence of UV-bright star forming galaxies of Hoopes et al. (2007), J2345−0449 is placed anomalously low in the region of partially quenched the ‘green valley’ galaxies. The graph is divided into three parts, showing an actively SF region (sSFR > $10^{-10.5}$ yr$^{-1}$), a green valley region ($10^{-11.5} < $ sSFR < $10^{-10.5}$ yr$^{-1}$), and a quiescent region (sSFR < $10^{-11.5}$ yr$^{-1}$, see Salim et al. 2016). Very interestingly, within the ‘green valley’ region, we can also locate a few other most massive local galaxies (NGC 1030, NGC 1961, NGC 4501, NGC 5635, and NGC 266) of comparably high stellar masses ($M_* \gtrsim 10^{11}$ M$_\odot$). The position of J2345−0449 is in stark contrast with the UV luminous galaxies of Hoopes et al. (2007) which show far more recent SF activity for the same mass range. On the other hand, an extraordinarily massive and fast-rotating hybrid spiral UGC 12591 is located in the lowermost region of highly quiescent ‘fully quenched’ galaxies showing the lowest specific sSFR of all (Ray et al. 2022). The fairly low sSFR of all these very massive galaxies ($M_* > $ few × $10^{11}$ M$_\odot$) located in sparse environments shows that perhaps these galaxies’ high masses and local environments are responsible for quenching of star formation. Presently, very few detailed studies of the cold molecular gas content of galaxies falling below the main sequence of star formation are available, and a general perception is that these systems are devoid of gas. However, a detailed study of their molecular gas content is a key element for understanding the reason behind their remarkably low SFR.

What about local SFRs in the disc of J2345−0449? From resolved $^{12}$CO(J = 1–0) line (rest frequency, 115.271 GHz) observations with ALMA (Atacama Large Millimeter/submillimeter Array), Nesvadba et al. (2021) and Drevet Mulard et al. (2023) showed that J2345−0449 falls well below the standard Kennicutt–Schmidt relationship. The local SFRs are strikingly lower, by factors of 50–75, than what is expected from the standard Kennicutt–Schmidt main-sequence relation (Kennicutt 1998) at the given gas mass surface densities, in ordinary SF galaxies with similar stellar and gas mass surface densities. Significant offsets towards lower SFR densities are also found from the Silk–Elmegreen law and the extended Schmidt law when taking into account the resolved stellar mass surface density in the disc (Nesvadba et al. 2021).

Previous ALMA CO observations fully resolved the molecular gas distribution and kinematics, and thus it is possible to distinguish between rotating discs, galaxy mergers, or gas inflows and outflows in J2345−0449. The $^{12}$CO(J = 1–0) line imaging and GALEX UV images both show a low-rate SF disc beneath the disc-like pseudo-bulge. CO emission is sensitive to relatively low-density molecular gas, making it an excellent tracer of the bulk molecular material in SF regions. It is particularly useful for mapping the overall distribution and dynamics of molecular gas in galaxies. In J2345−0449, CO molecular gas is located in an annular ring as shown in Fig. 14. The dimensions of the molecular ring are $r_{in} = 4.2$ kpc and $r_{out} = 11.8$ kpc (projected) at the inner and outer (truncated) edges respectively, and using the deprojected rotation velocities, Keplerian values $v_{rot,in} = 366$ km s$^{-1}$ and $v_{rot,out} = 378$ km s$^{-1}$ are measured at these radii, respectively. The kinematic centre of the CO molecular gas ring is located at position (J2000) RA= 23h45m32.708s, Dec. = −04d49m25.42s, coincident with the faint (S$_{115GHz}$ = 1.3 ± 0.34 mJy) radio continuum point source, presumably the AGN. This position is also within 0.5 arcsec of the radio core detected on the GMRT 323 MHz and VLA FIRST (Faint Images of the Radio Sky
at Twenty-centimeters) 1.4 GHz maps. The position angle of the CO annular ring on the sky is 94 ± 2 deg, and the inclination angle i = 59 ± 2 deg, the same values previously also found for the stellar component by Bagchi et al. (2014). This is another indication that most of the molecular gas resides in the disc, and as measured from the CO line Doppler mapping, rotates largely unperturbed within the gravitational potential of the host galaxy. Moreover, no kinematic signature of rotation is detected along the minor axis, which is clear evidence for a planar, rotating molecular disc (see Fig. 14).

Next, we investigate the spatial correlation of the CO emission
ring and the central pseudo-bulge component. As shown in Fig. 14,
no ¹²CO(J=1-0) line emission is detected in the central 2.8 × 1.5
arcsec from the nucleus, corresponding to a diameter of 4.2 kpc
× 2.2 kpc (projected) at a flux limit of rms = 332 µJy beam⁻¹.
Summing the CO line flux over all regions where CO line FWHM
> 70 km s⁻¹ shows that a total of 1.6 × 10¹⁰M☉ of cold gas mass
is being stirred up, possibly by interactions with the compact radio
nucleus (AGN) and the inner jet, amounting to about 75 per cent
of the total molecular gas mass in the ring (Nesvadba et al. 2021).
The spatial extent of the CO non-emission/depleted zone (Fig. 14) is
about two times the effective diameter of the stellar bulge observed in
the HSTB- and I-band images, which is 2 × (2r_c) = 2 × (2.2 - 2.6)
kpc, close to the bulge size measured by Bagchi et al. (2014) from
SDSS data. Therefore, we conclude that the cold molecular gas is
only associated with the inner disc of J2345−0449, while it clearly
avoids the central pseudo-bulge where the AGN is located at the
very centre. This reduced recent star formation in the innermost
regions of the galaxy could be a manifestation of the growth of the
bulge itself. Alternatively, this could be a consequence of inside–
out quenching of star formation as a result of supernova and/or
AGN feedback which could inject energy to gravitationally unbind
SF gas from the galaxy’s centre (Nelson et al. 2021); a process
most effective in massive galaxies M_★ > 10¹⁰.⁵M☉ hosting SMBHs
of masses > 10⁸.²M☉ accreting at low Eddington rates (Terrazas
et al. 2020). In J2345−0449 we may be seeing observational signs
of a similar AGN/SN feedback process at work, but more detailed
investigations of cold molecular gas in the future will provide a more
definite answer.

Recently, Dabhade et al. (2020c) reported observations of ¹²CO(J = 1-0) and ¹²CO(J = 2-1) transition emission lines from the repository of cold molecular gas present in this and other GRGs using the IRAM (Institut de radioastronomie millimétrique) 30-m telescope. Their global CO abundance analysis (from single-dish observations) suggests that quenching does not require the total removal or depletion of molecular gas, as many quenching models have previously assumed. Moreover, cold molecular gas in the central region may comprise the reservoir for fuelling the central SMBH. In J2345-0449, the molecular gas (H₂) mass was estimated at M<sub>H2</sub> = 1.62 × 10<sup>10</sup>M<sub>☉</sub>, much smaller than the galaxy's stellar mass, yet this is a significant repository of H₂ which can seed new stars. On the other hand, the galaxy has a very long global molecular gas depletion time t<sub>dep</sub> = M<sub>H2</sub>/SFR = 6.6 Gy, for M<sub>H2</sub> = 1.62 × 10<sup>10</sup> M<sub>☉</sub> and SFR = 2.45 M<sub>☉</sub>y<sup>-1</sup>. This seems an extreme value even for spirals, and it is the highest of all GRGs studied so far (Dabhade et al. 2020c). But what is peculiar about this is whereas this galaxy contains a massive reservoir of cold molecular gas and dust (M<sub>dust</sub> > 10<sup>8.1</sup> M<sub>☉</sub>, Table 4), it is currently forming new stars at a minimal rate. The exact cause of it is still unclear but a good possibility is that this could be related to suppression of SF due to strong AGN feedback (Nesvadba et al. 2021).

5.6 The baryon landscape, dark matter halo, and the ‘missing baryon’ budget

How did such ultra-massive galaxies come to be? What were the progenitors of these giants in the early Universe? Recent detection of a hidden (in UV to near-IR bands) population of massive galaxies at z > 3 detected by ALMA in sub-mm bands (Wang et al. 2019) has provided new insights about the origin of these elusive galaxies. Such dust-laden galaxies contribute a total SFR density 10 times larger than that of equivalently massive UV-bright galaxies at z > 3. Forming within the most massive DM haloes of mass ~ 10¹³M☉, they probably are the progenitors of the present-day massive galaxies. Moreover, JWST has recently found some extremely massive (stellar mass ~ 10¹⁰ M☉) galaxies existing even at very early epochs (z ~ 7 – 9; Labbé et al. 2023). The IR and sub-mm detection of an unexpectedly large population of massive, evolved galaxies in the early Universe suggests that our present theories of massive galaxy formation may require substantial revision. With JWST, Carnall et al. (2023) found a similarly massive galaxy, which formed most of its stellar mass in a remarkably short interval of ~ 200 Myr before this galaxy quenched its star formation activity at z ≈ 6.5, possibly via AGN feedback when the Universe was about 800 Myr old. They suggest that such galaxies are both likely descendants of the highest-z sub-mm galaxies and quasars, and probably the progenitors for the dense, ancient cores of the most massive local galaxies.

Much can be learned by exploring the warm/hot gaseous coronae
around massive disc galaxies lacking prominent bulges. Contrary
to the bulged systems, these galaxies are likely to be much more
direct tracers of the galaxy formation process governed by ‘secular’
(isolated) disc-based mechanisms, since they have not undergone
major mergers (Kormendy & Kennicutt 2004) which drastically
alters them. On the other hand, due to their extreme stellar and
halo mass and possible AGN feedback of the central SMBHs, the
baryons in these galaxies may reside in unusual physical states that
are strongly affected by these processes and may participate in much
less star formation in the disc. Moreover, since the hot halo mass
and thermal X-ray emission of baryons both scale with galaxy mass,
it is highly informative to observe deeply the most massive spiral
galaxies in soft X-rays to achieve a detectable signal.

Indeed, in J2345−0449 *XMM–Newton* has robustly detected a giant halo of diffuse, soft X-ray emission gas (*T* < 1 keV) which surrounds the galaxy and extends out to a minimum radius of 160 kpc (Mirakhor et al. 2021), which is roughly 35 per cent of the virial radius and ~ 20 times the exponential disc half-light radius. *Chandra* observations have also detected the halo, but over a smaller spatial extent due to the reduced detection sensitivity of the telescope (Walker et al. 2015). The baryonic mass of the hot gas in the halo is very high, about three times the total stellar mass of the galaxy (Table 7). This remarkable detection is currently limited by the detection capability of X-ray telescopes, yet it is the largest repository of a hot baryonic gaseous atmosphere observed in any single galaxy so far. Interestingly, the X-ray emitting gaseous halo is not spherically distributed but forms a flattened pancake elongated parallel to the host galaxy’s stellar disc (Mirakhor et al. 2021).

What could be the reason for this elongated shape? The 3D shape
of DM haloes around galaxies and galaxy groups or clusters is
a frontier topic of ongoing research, but theoretical models and
simulations (e.g. Chua et al. 2019; Emami et al. 2021; Baptista
et al. 2023) and limited observations (Buote et al. 2002) suggest
that they are not perfectly spherical but assume triaxial spheroidal
geometry, with long axis preferably oriented parallel to the longer
<table><caption>**Table 7.** In this table, we specify the mass of various baryonic components and the DM mass of our target galaxy J2345–0449. The stellar mass and dust mass are from this work, second estimate of stellar mass is from Walker et al. (2015), molecular hydrogen mass is provided by Dabhade et al. (2020c), and the hot X-ray halo gas mass and the total halo mass (DM + baryons) is provided by Mirakhor et al. (2021). The estimated total halo mass within $r_{200}$, $M_{DM+bar}$ is affected by systematic and statistical uncertainties, originating mainly from the unexplored properties of the hot halo beyond a radius of 160 kpc (see Mirakhor et al. 2021).</caption><thead><tr><th>Parameter</th><th>Symbol</th><th>Value (in M<sub>☉</sub>)</th></tr></thead><tbody><tr><td>Stellar mass (this work)</td><td>M<sub>*</sub></td><td>2.88<sup>+0.00</sup><sub>-0.79</sub> × 10<sup>11</sup></td></tr><tr><td>Stellar mass (Walker et al. 2015)</td><td>M<sub>*</sub></td><td>4.60<sup>+0.1</sup><sub>-0.1</sub> × 10<sup>11</sup></td></tr><tr><td>Molecular hydrogen mass</td><td>M<sub>H2</sub></td><td>1.62 × 10<sup>10</sup></td></tr><tr><td>Total gas mass (including He)</td><td>M<sub>gas</sub></td><td>2.20 × 10<sup>10</sup></td></tr><tr><td>Dust mass (this work)</td><td>M<sub>dust</sub></td><td>1.21<sup>+0.44</sup><sub>-0.21</sub> × 10<sup>8</sup></td></tr><tr><td>Hot halo gas mass upto 160 kpc radius</td><td>M<sub>hot,gas</sub><sup>160</sup></td><td>1.15<sup>+0.22</sup><sub>-0.24</sub> × 10<sup>11</sup></td></tr><tr><td>Hot halo gas mass upto virial radius</td><td>M<sub>hot,gas</sub><sup>200</sup></td><td>8.25<sup>+1.62</sup><sub>-1.77</sub> × 10<sup>11</sup></td></tr><tr><td>Total baryonic mass upto virial radius</td><td>M<sub>baryon</sub><sup>200</sup></td><td>1.13<sup>+xx</sup><sub>-xx</sub> × 10<sup>12</sup></td></tr><tr><td>Total halo mass (DM + baryons) upto virial radius (450 kpc)</td><td>M<sub>DM+bar</sub></td><td>1.07<sup>+0.09</sup><sub>-0.09</sub> × 10<sup>13</sup></td></tr><tr><td>Baryon fraction (this work)</td><td>f<sub>b</sub></td><td>0.11<sup>+0.04</sup><sub>-0.04</sub></td></tr><tr><td>Baryon fraction (Mirakhor et al. 2021)</td><td>f<sub>b</sub></td><td>0.121<sup>+0.043</sup><sub>-0.043</sub></td></tr></tbody></table>

axis of the central galaxy or group. This is a consequence of the fact that the gravitational force that shapes DM haloes is not perfectly symmetrical, which governs their 3D equilibrium figures in a rotating frame, much like a Jacobi ellipsoid. A Jacobi ellipsoid is a triaxial ellipsoid that is in hydrostatic equilibrium under the influence of its own gravity and rotation. This means that the pressure inside the ellipsoid is everywhere equal to the gravitational force acting on a small volume of fluid (Chandrasekhar 1969). Since the X-ray halo gas is essentially collisionless and likely to be bound in the gravitational potential of DM, it too will acquire a similar triaxial shape, if not disturbed by major mergers and tidal forces. This is a simple model but this makes the X-ray haloes around massive galaxies remarkably accurate tracers of DM as the DM halo shape is a result of initial density field anisotropy and the subsequent halo assembly history (cold filament accretion and mergers), and the galaxy halo shape statistics can be used to test and validate the $\Delta$CDM structure formation paradigm. Future deep X-ray observations and theoretical modelling will reveal if the X-ray halo around J2345–0449 could be shaped by this mechanism, directly related to its currently unknown formation history.

Alternatively, Walker et al. (2015) suggest that the pancake-like shape of the X-ray halo in J2345–0449 is possibly due to feedback by the energetic radio jets which might have disrupted or expelled hot gas from an extended region ($\approx$ 100 kpc) above and below the disc plane. Although still unproven, if confirmed by further observations and detailed modelling, this will be an extremely unusual scenario for a spiral galaxy environment as radio jet-mode feedback is commonly seen in AGN hosted by BCG at the centres of galaxy clusters with a cooling core, but is extremely rare to observe in an isolated spiral galaxy like J2345–0449 (Fabian 2012; McNamara & Nulsen 2012).

A third possibility is that the hot X-ray halo gas could have been lifted and heated up by hot stellar winds ejected by multiple supernovae from a higher SFR in the past, which has now been suppressed due to either the AGN feedback or a morphological quenching by the massive DM halo (Martig et al. 2009). Such a

massive DM halo is also needed to stabilize the extremely fast-rotating stellar disc. On the other hand, we find that a massive stellar spheroid/bulge formed by mergers, which is most effective in morphological quenching, is definitely not present in J2345–0449, but an extended ($>$ 160 kpc radius), hot X-ray halo is observable (Walker et al. 2015; Mirakhor et al. 2021).

It would prove very informative if the X-ray halo in J2345–0449 is the signpost of initially cold gas that has fallen into the halo of the massive galaxy and has been shock heated and virialized during galaxy structure formation. This can be shown as follows:

(1) The gas temperature of the X-ray halo is at the right order of magnitude for being close to the virial temperature of the binding gravitational potential:

$$T_{\text{vir}} = 4 \times 10^5 \times \left( \frac{v_{\text{circ}}}{100 \text{ km s}^{-1}} \right)^2 \text{K}, \quad (5)$$

which comes to $6.4 \times 10^6$ K (or 0.55 keV) for J2345–0449 for a circular rotation speed of $v_{\text{circ}} \sim 400\text{kms}^{-1}$. This is about the same temperature as what Chandra and XMM have measured in the hot halo region (Walker et al. 2015; Mirakhor et al. 2021).

(2) If not a coincidence, this implies that gas in X-ray emitting halo is in viral equilibrium with a massive DM halo of total mass few $\times 10^{13} \text{M}_\odot$ as suggested by Dai et al. (2012). This, in turn, implies the halo gas is possibly shock-heated after having been accreted from cold filaments of the cosmic web beyond the virial radius at $\gtrsim 500$ kpc, rather than being recycled from the disc. The thermal and metal line cooling time of the quasi-hydrostatic X-ray halo gas is large, in the range 2.8 – 6.3 Gyr for material within the cooling radius, and has an effective cooling rate of $0.15 – 0.21 \text{M}_\odot \text{yr}^{-1}$ (Fukugita & Peebles 2006; Dai et al. 2012; Kelly, Jenkins & Frenk 2021). The average estimated cooling time is much larger than the free-fall time. Therefore, it would require more than a Hubble time to build the entire stellar mass of a galaxy by condensation of cooling gas from the halo, thus disfavouring such a possibility. The dynamical state of the circumgalactic gas will be revealed by future sensitive spectroscopic observations from the detection of the ‘warm hot intergalactic medium’ in the soft X-ray lines at $\sim 10^6$ K, and via the detection of even cooler gas through the UV/EUV lines of metals (Nicastro et al. 2021; Simionescu et al. 2021).

The XMM–Newton detection of an extended, luminous, soft X-ray halo around the galaxy J2345–0449 is a prominent signpost of the so-called missing baryons in such massive galactic haloes. All prominent galaxy formation models predict such gaseous circumgalactic haloes (White & Rees 1978; White & Frenk 1991; Fukugita & Peebles 2006; Sommer-Larsen 2006). The entire baryonic mass contained in the X-ray halo of J2345–0449 implies a baryon fraction near the cosmic average $\sim 0.15 – 0.17$ (Dunkley et al. 2009; Komatsu et al. 2009), which is very high for a galaxy. In J2345–0449, the baryon to DM mass density ratio appears to approach the cosmic value if the baryonic mass is extended up to the virial radius where the DM mass dominates. Such a massive DM halo is also needed to gravitationally stabilize the extremely fast rotating disc. This finding does not support the possibility that in most massive galaxies in the Universe, the baryon shortfall is due to the fact that the majority of the cosmic baryons never ‘condensed’ onto the DM halo and thus are invisible. On the contrary, in J2345–0449, we find that these baryons actually may reside in the circumgalactic halo.

In Table 7, we list the masses of the different baryonic components in the galaxy disc, including the stellar mass, the mass of the cold molecular ($H_2$ and He) gas, and the interstellar dust mass. As typically done in many studies, we assume that the total disc gas mass $M_{\text{gas}}$ is the sum of the atomic and molecular gas masses, corrected for the
<figure><img src="image_18.png" /><figcaption>**Figure 15.** This plot shows the baryon fraction in relation to the baryonic mass of different populations of galaxies, galaxy groups, and galaxy clusters with the virial temperature $T_{vir}$ (K) as the dependent variable. Non-starburst field galaxies are selected from Li, Crain & Wang (2014), spirals from Li et al. (2018), galaxy groups from Sun et al. (2009), and galaxy clusters from Vikhlinin et al. (2006). On top of these, we plot a handful of fast-rotating massive galaxies from Bogdán et al. (2013), Anderson et al. (2016), and Ray et al. (2022, 2024). The Milky Way galaxy (Miller & Bregman 2015) is also shown for reference. The dotted line indicates the broken empirical power-law model of Dai et al. (2010) which is given as $f_b = [0.14(v_{rot}/440)]^{1.6}/[(1 + (v_{rot}/440)^2)]^{1.5/2}$. For J2345-0449, we have used the baryon masses given in Table 7. The cosmological baryon fraction is shown with a shaded grey region of $f_b = 0.17 - 0.19$ reflecting the uncertainty range of current measurements.</figcaption></figure>

Helium contribution by multiplying by a factor of 1.36, i.e. $M_{gas} = 1.36 \times [M_{H_2} + M_{HI}]$. We have neglected the atomic hydrogen mass $M_{HI}$, as it is not measured and usually is a small fraction of total gas mass. In addition, we also list the estimated mass of the hot X-ray halo gas, as detected in XMM-Newton observations, and the model-fitted total halo mass (DM plus baryons) up to the virial radius, both as determined by Mirakhor et al. (2021).

Combining all contributions to the total baryon mass, we have determined a total baryon fraction of $f_b = 0.11^{+0.04}_{-0.04}$, close to the estimate $f_b = 0.121^{+0.043}_{-0.043}$ reported by Mirakhor et al. (2021). Both these estimates are close to but lower than the cosmological baryon fraction of $0.15 - 0.17$ (Dunkley et al. 2009; Komatsu et al. 2009). This is shown in Fig. 15 where the galaxy J2345-0449 with a baryon fraction of $\sim 0.11^{+0.04}_{-0.04}$ (within the error bar of Mirakhor et al. 2021) who provided $f_b$ as $\sim 0.121 \pm 0.043$ up to the virial radius), falls above the nearby region of the broken power-law model surrounded by a few other highly massive spiral galaxies and galaxy groups. Note that, amongst the labelled massive spiral galaxies in the graph, we have X-ray detections in the galactic halo for the objects J2345-0449, NGC 1961, NGC 6753, NGC 266, and UGC 12591, and in effect their hot gas masses up to the virial radius is also included in the total baryon fraction.

To further explore the implications of the above findings, we examine the galactic star formation efficiency, $f_*$, which can be expressed as,

$$f_* = \frac{M_* \Omega_m}{M_{\text{halo}} \Omega_b}, \qquad (6)$$

with $f_* = 1$ if all baryons are converted to stars (Posti, Fraternali & Marasco 2019). Now for J2345-0449, if we take the virial mass $M_{halo} = 1.07 \times 10^{13} M_{\odot}$ as calculated in Walker et al. (2015) and Mirakhor et al. (2021) and our estimated mean stellar mass $M_* \approx (2.2 \pm 0.1) \times 10^{11} M_{\odot}$, we obtain $f_* \sim 0.12 \pm 0.01$, taking

the universal cosmic baryon fraction $\Omega_b/\Omega_m = 0.174$. This shows that J2345-0449 is able to convert only about 10 per cent of all available cosmic baryons to stars. Another indicator of the remarkably low efficiency of star formation in this galaxy is its very large gas depletion time, given by $t_{dep} = M_{gas}/\text{SFR} \sim 10$ Gy, taking $M_{gas} = 2.2 \times 10^{10} M_{\odot}$ and SFR $\sim 2 M_{\odot} \text{yr}^{-1}$. On the contrary, the sample of the most massive spirals with $M_* = 1 - 3 \times 10^{11} M_{\odot}$ of Posti et al. (2019) yields $f_* \approx 0.3 - 1$ which shows their larger efficiency of star formation and their stellar baryon fraction approaching the cosmological baryon fraction. Although a very interesting result, it is still too early to tell if these unusual massive spirals of Posti et al. (2019) have experienced the same evolutionary and star formation history as the majority of normal spirals. On the contrary, J2345-0449 appears to be very different because of its very low $f_*$, a large repository of potentially viable molecular gas, and because most of the ‘missing’ cosmic baryons in this galaxy appear to reside within the DM halo inside the virial radius in a phase of warm/hot low temperature ($\le 1$ keV) gas, but unable to turn into stars.

These baryons may enable galaxies to continue forming stars steadily for long periods of time and account for the elusive ‘missing baryons’ in galaxies in the local Universe. For some of the most massive galaxies in the local Universe, like J2345-0449, UGC 12591, NGC 1961, etc., we find just the opposite. This finding is broadly consistent with the constraints from abundance matching techniques to determine the typical galaxy stellar mass at a given halo mass. The simulations show that peak efficiency of $f_*$ is always fairly low, by $z=0$ reaching a peak of about $20-30$ per cent for typical $L_*$ galaxies with halo mass $M_{200} \sim 10^{12} M_{\odot}$. Beyond the peak $f_*$ declines rapidly to $f_* \sim 2-5$ per cent for ultra-high mass $M_{200} \sim 10^{13} M_{\odot}$ (Behroozi et al. 2010, 2019; Moster et al. 2013). It is believed that AGN feedback is one of the main ingredients required to bring this decline in SFR (e.g. Di Matteo et al. 2005; Beckmann et al.
2017) and a cut off in the total stellar mass. On the contrary, from UV observation with multi wavelength space observatory ASTROSAT (Tandon et al. 2017), Ray et al. (2024) found that some of the local galaxies of this large halo mass do not show the drastic decline in $f_*$ as expected from simulations.

Next, we compare the 0.5–2 keV band X-ray luminosity ($L_X$) of all known massive spiral galaxies with detected soft X-ray haloes. We find that at $L_X = 4.0 \pm 0.5 \times 10^{41} \text{erg s}^{-1}$ the luminosity of J2345–0449 is highest (Mirakhor et al. 2021), about 10 times that of UGC 12591 (Dai et al. 2012), NGC 6753 (Bogdán et al. 2013; Bregman et al. 2018), and significantly higher than that of NGC 1961 (Anderson et al. 2016). The metal abundance of J2345–0449 halo is low at $\sim 0.1 Z_{\odot}$, similar to those found for UGC 12591, NGC 1961, and NGC 6753. This metallicity is consistent with baryon deficit, low-metallicity coronal gas in the halo of massive galaxies shown in EAGLE hydro simulations (Schaye et al. 2015; Kelly et al. 2021). Although the halo mass of UGC 12591, NGC 1961, NGC 6753, and J2345–0449 are similar, the higher X-ray luminosity and the larger extent of the J2345–0449 corona (about 160 kpc, or 35 per cent of $r_{200}$) require better understanding. We point out that J2345–0449 is the only one of those four galaxies that host large-scale radio jets. Therefore, one may infer it is a consequence of powerful Mpc-scale radio jets and past supernovae activity in this galaxy, heating and expelling the inner baryons up to a larger radius and thus preventing gas cooling in the halo; although a very suggestive possibility, more observational proof is needed to robustly demonstrate this scenario.

# 6 SUMMARY AND CONCLUSIONS

In the present work, our research endeavour was directed toward examining the bulge and disc components and stellar mass distribution in the fast-rotating, highly massive spiral galaxy J2345–0449 showing unusual radio jets extending up to Mpc scale. Using the high-resolution multiband HST observations and multiparameter panchromatic SED fitting we obtained estimates of the SFR, the total baryonic mass contained in stars and derived the warm dust properties, which were then connected to the cold molecular gas distribution based on previous ALMA and IRAM CO spectral line observations.

The HST imaging confirms earlier conclusions based on ground-based imaging data that this spiral galaxy features a pseudo-bulge rather than a classical bulge. A search for non-axisymmetric structures in the AGN region revealed evidence of a small nuclear bar on 0.5 arcsec ($\sim 700$ pc) scales, surrounded by a bright ring, forming a circumnuclear disc. This is consistent with the expectation that pseudo-bulges evolve from bars secularly. These results have profound implications for how this galaxy formed and how the growth of its mass accreting SMBH happened in the past. Classical bulges form via more violent processes such as hierarchical merging, whereas the pseudo-bulges seem to be still evolving, more likely as a result of their host galaxies’ slow internal evolution, which involves disc instabilities and angular momentum transfer. One important conclusion from this study is that the galactic disc and its SMBH in J2345–0449 may have evolved together quietly (secularly), and have not experienced the upheaval of recent major merger events. This possibility is strengthened by the absence of any tidal debris (such as tails, shells, or faint plumes of stars), the location of the galaxy in an isolated galactic environment having no nearby massive galactic neighbours, and the fact that the galaxy has highly symmetric spiral arms within a rotationally supported stellar disc and an inner, highly organized Keplerian disc of cold molecular gas.

Our detailed multiwavelength study of potential SF gas reveals that, while hot gas cools down in the massive extended halo of this galaxy, new stars do not form in the centre because of possible feedback from the central accreting SMBH with its powerful radio jets, which heat up the gas again or push it beyond the galaxy’s reach. For most of the galaxies like J2345–0449 and a few others observed, the activity of the BH seems to have put an end to or drastically reduced star formation but has not yet succeeded in clearing them of all their cold gas.

Much more detailed follow-up work is required and will be taken up in later studies. Future studies will be directed to understanding better how such extremely massive, rotationally supported disc galaxies have formed and are evolving, as they depend on non-linear baryonic physics and complex interactions with the DM haloes. Obviously, the most striking property that sets J2345–0449 apart from nearly all other spiral galaxies is its large mass and unusual radio loudness, resulting in the highly efficient ejection of relativistic jets on Mpc scales, possibly resulting from an unusual growth of an SMBH in a bulgeless galaxy. The extreme rarity of such galaxies implies that whatever physical process had created such huge radio jets in J2345–0449 must be very difficult to realize and maintain for long periods of time in most other spiral/disc galaxies. Thus, an important question is: what could be responsible for the efficient fuelling and sustained collimated jet ejection activity in J2345–0449? The answer requires very detailed knowledge of the properties of its central BH engine, i.e. its mass, spin, and the nature of its accretion flow, namely the mass accretion rate and the magnetic field topology of the inner regions of the accretion disc, most of which is lacking at the present moment.

In summary, our study sheds light on the unique characteristics of J2345–0449 and also raises new questions about the interplay between galactic structure, star formation, and the activity of the central SMBH. Future investigations will likely focus on obtaining more detailed information to unravel the complexities of this intriguing galactic system.

# ACKNOWLEDGEMENTS

JB acknowledges the support from the Department of Physics and Electronics, CHRIST (Deemed to be University), Bangalore. SR gratefully acknowledges the support from IUCAA under the student visiting program and funding by the Indian Space Research Organisation (ISRO) under the 'AstroSat Data Utilization' project. SR is also supported by the Department of Physics and Electronics, CHRIST (Deemed to be University), Bangalore. SD acknowledges support from the Indian Space Research Organisation (ISRO) funding under project PAO/REF/CP167. MBP gratefully acknowledges the support from the Science and Engineering Research Board (SERB), New Delhi, under the SERB 'SERB_CRG' funding under CRG/2023/003463. LCH was supported by the National Science Foundation of China (11721303, 11991052, 12011540375, and 12233001), the National Key R&D Program of China (2022YFF0503401), and the China Manned Space Project (CMS-CSST-2021-A04, CMS-CSST-2021-A06). Support for HST program 14091 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This research has made use of NASA's Astrophysics Data System, and of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We have used images and
results from SDSS and funding for SDSS has been provided by the Alfred P. Sloan Foundation, the participating institutions, the National Science Foundation, and the U.S. Department of Energy's Office of Science. We thank Ned Wright's Javascript Cosmology Calculator for several cosmological calculations (Wright 2006).

## DATA AVAILABILITY

The HST data used in this paper are publicly available at the Hubble Legacy Archive (https://hla.stsci.edu) and the Mikulski Archive for Space Telescopes (https://archive.stsci.edu). All the retrieved data used in the analysis are tabulated in Tables 1 and 3 which are publicly available. The GALAFIT code used in photometric bulge-disc modelling and SED fitting routines MAGPHYS and CIGALE are publicly available at https://usersobs.carnegiescience.edu/peng/work/galfit/galfit.html, http://www.iap.fr/magphys/, and https://cigale.lam.fr. For any further help, the authors can be contacted.

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