ngVLA will spatially pin-down mBHs and jets in optically-faint radio-loud galaxies
Kohei Ichikawa1,2 , Takuji Yamashita3 , Kohei Inayosh4 and WERGS team
1

Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan
2
Astronomical Institute, Graduate School of Science Tohoku University, 6-3 Aramaki, Aoba-ku, Sendai 980-8578, Japan
3
National Astoronomical Observatory of Japan, 2–21–1 Osawa,
Mitaka-shi, Tokyo 181–8588
4
Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, Chinak.ichikawa@astr.tohoku.ac.jp; takuji.yamashita@nao.ac.jp

Abstract
Understanding the mechanism on the formation of the massive black holes (mBHs), and growing process into
the super-massive black hole mass regime (MBH > 106 M⊙ ) is one of the most important studies in extragalactic
astronomy. Recent Subaru/Hyper Suprime-Cam (HSC) discovered a large fraction of the VLA/FIRST 1.4 GHz radiosources, including optically-faint radio galaxies down to gAB ∼ 26. This population covers a new parameter spaceof extremely radio-loud galaxies (ERGs) with radio-loudness parameter of log Rrest = log(f1.4GHz,rest /fg,rest ) > 4.
Combination of the IR studies revealed that ERGs exhibit a high specific black hole accretion rate, reaching theorder of the Eddington limit and some of them might reside in low-mass galaxies with log(M⋆ /M⊙ ) < 10. The
intrinsic radio-loudness (Rint ), defined by the ratio of jet power over bolometric radiation luminosity, is one orderof magnitude higher than that of radio quasars. This suggests that ERGs harbor a unique type of active galactic
nuclei (AGN) that show both powerful radiations and jets, and possibly in a slim disk phase. Thanks to the superb
sensitivity and resolution of the ngVLA, it will map the ∼ 10 pc scale radio image, separating the radio core and jet
structures of super-Eddington AGN as well as witnessing the effect of their feedback to the host galaxies up to z < 3.
Its supreme spatial resolution also enable us to pin down the location of the mBHs in the host galaxies, evaluating
the possibilities of wondering BH scenarios in those low stellar-mass ERGs.
Key words: quasars: supermassive balck holes —galaxies: active — galaxies: nuclei

1. Introduction

2. Radio and optical surveys as mBH finding machine

Understanding the properties of the massive black holes(mBHs) in the center of low-mass galaxies (M⋆ < 1010 M⊙ )and their growing path of mBHs into the SMBHs is important
to constrain the different mBH-seed formation/growing scenarios; direct collapse and/or super-Eddington accretion from themassive stars (Volonteri 2010). However, it is still unclear thathow much fraction mBHs exist (so called occupation fraction)in such low-mass galaxies (Greene et al. 2020).
This motivates BH hunters to search mBHs in the low-massgalaxies using multi-wavelength data; optical spectroscopy using broad Hα emission (e.g., Greene & Ho 2004; Greene & Ho
2007; Xiao et al. 2011), narrow emission line ratios (e.g., Barthet al. 2008; Reines et al. 2013), mid-IR (W 1 − W 2) colors
in WISE (e.g., Satyapal et al. 2014), X-ray observations (e.g.,
Mezcua et al. 2016; Chen et al. 2017; Kawamuro et al. 2019),
as well as through studies of the AGN flux variability based on
the properties that lower luminosity AGN have stronger variability amplitude (e.g., Morokuma et al. 2016; Baldassare etal. 2018; Kimura et al. 2020). On the other hand, radio surveys
have rarely been used for searching massive BHs in low-massgalaxies since most of the local radio galaxies are found in themost massive system with M⋆ ∼ 1012 M⊙ , which would not
host the mBHs considering the scaling relation (Kormendy &Ho 2013).

Recent Subaru/Hyper Supreme Cam (HSC) strategic survey
has discovered > 3000 optically-faint counterparts of the radio bright VLA/FIRST radio continuum surveys, at z ∼ 0–3(Yamashita et al. 2018), which is named as WERGS project.
The WERGS survey has improved the optical counterpart fraction rate of the FIRST sources from < 30% (Ivezić et al.
2002) to > 60% (Yamashita et al. 2018). Figure 2 showsthat WERGS sample covers very optically-faint RGs down
to gAB ∼ 26 and a wide range of radio-loudness parameter
Rrest = f1.4GHz /fg band (e.g., Ivezić et al. 2002), opening thenew parameter space of radio galaxies: extremely radio-loudgalaxies (ERGs) with log Rrest > 4. Such ERGs were undetectable in the previous SDSS surveys because of their shallow
sensitivity down to only gAB ∼ 22, suggesting a unique population we have never explored. Combining the multi-wavelength
data covering the optical, IR, and radio band for the ∼ 103
WERGS sample, Toba et al. 2019 derived IR-based physical parameters such as star-formation rate (SFR; from far-IR),stellar-mass (M⋆ ; from near-IR), as well as the AGN luminosity (LAGN,bol ; from mid-IR).
Based on these derived quantities, Ichikawa et al. 2021 summarized interesting properties of the ERGs. The first important
one is that all ERG are in the star-forming or starburst phasereaching specific star-formation rate of sSFR ∼ 10−8 yr−1 ,suggesting that ERGs are in a rapid stellar-mass assembly
phase with mass doubling times of ∼ 100 Myr.

2[Vol. ,

Fig. 1. logRrest versus rest g band magnitude of the WERGS sample.
The all WERGS sample (gray dots), the WERGS sample with IR detections (blue points) and the ERGs with IR detections (the sample withlog Rrest > 4; orange stars) are shown. Rrest is defined as a ratio of
the rest-frame 1.4 GHz flux density and the g-band one. Subaru/HSC
survey discovered large number of very optically-faint (g > 22), andextremely radio-loud galaxies with log Rrest > 4. The figure is takenfrom Yamashita et al. 2018 and Ichikawa et al. 2021.

The second important feature is visualized in the left panelof Figure 3 that IR detected ERGs are in a rapid BH accretion
phase with high specific black hole accretion rate (sBHAR)
with the expected Eddington ratio ⟨log λEdd ⟩ ≈ −0.4, and some
ERGs may be experiencing a super-Eddington phase. Besides,
their stellar mass is relatively small, including the low-massgalaxies with M⋆ < 1010 M⊙ .
The third one is illustrated in the right panel of Figure 3
that sources with higher Rrest tend to have both higher
sBHAR (which is conventionally defined as sBHAR =−1
LAGN,bol /M⋆ erg s−1 M⊙
and thus it is a good indicatorof λEdd ; see Mullaney et al. 2012) and higher intrinsic radioloudness Rint , which is defined by the Rint = Ljet /LAGN,bol ;an energy balance of jet power over bolometric radiation luminosity. This paints a different picture of radio galaxies compared to conventionally known local radio galaxies with low
λEdd . ERGs represent a population of unique radio galaxies
characterized by both high λEdd and high radio power.
3. Revealing Jet Properties of super-Eddington accretion:
Test of slim-disk model
ERGs contains a population whose accretion disk state is ina possible super-Eddington phase associating with the radio jet(Ichikawa et al. 2021). One possible origin of this high production efficiency of radiation and jets is that the accretion diskof ERGs is actually in the “radiatively inefficient” state, but
the physical origin of radiative inefficiency is different fromthe disks in the local radio galaxies. For example, the ERGs
may be undergoing more rapid mass accretion (so called slim
disk model; Abramowicz et al. 1988). Recent radiation hydrodynamical simulations suggest that when the mass accretion
rate significantly exceeds the Eddington rate, radiation is effectively trapped within the accreting matter and advected to thecentral BH before escaping by radiative diffusion. As a result,the emergent radiation luminosity is saturated at the order ofLEdd (i.e., ηrad <∼ 0.1 at ≫ ṀEdd ) and the accretion flow turns
into a radiatively inefficient state even with a super-Eddington
accretion rate (e.g., Ohsuga et al. 2005; Inayoshi et al. 2016).
This phase is considered to be a key process of the BH seed
growth in the high-z (z > 6) universe to describe the alreadyknown massive high-z SMBHs (e.g,. Bañados et al. 2018), and
ERGs are promising low-z candidates of such an interesting
population.
The ngVLA will provide us the first observational test on thephysics of such slim disk model by observing possibly superEddington phase ERGs. Figure 4 shows the expected resolution by ngVLA. This shows the resolution could reach downto ∼ 1 mas, which corresponds to < 10 pc in physical scale at
0.5 < z < 3, that covers most of our targets sources. Since ourtarget is the radio core and jet lobe, relatively higher frequency
band at 2.4 GHz and 16 GHz would be the most suitable band
sets, which is not suffered by the self-absorption of the core.
With this superb resolution, ngVLA will spatially resolve thecore and jet, and will constrain the radio core flux for the firsttime for the AGN with super-Eddington phase. Obtaining theradio core flux by ngVLA, and combining the AGN luminosities, the intrinsic radio-loudness Rint and the jet efficiency ηjetcan be obtained without the contamination of the radio lobe.
4. AGN feedback in low-mass (log M⋆ /M⊙ < 10) ERGs
Another important study of ERGs to be explored by ngVLA
is on the effectiveness of AGN feedback in the low-mass galaxies. In recent years, the theoretical study of AGN feedback has
been extended to low-mass galaxies (Silk 2017). This was motivated mainly by the increasing number of intriguing observational results showing signs of AGN feedback in low-massgalaxies (e.g., Mezcua et al. 2019). The reported jet power ofLjet > 1044 erg s−1 in ERGs is equivalent to that produced
from radio quasars and local radio AGN residing in massive
galaxies (Best et al. 2005). On the other hand, low stellarmass ERGs are still in a starburst phase, indicative of a substantial gas reservoir, and lacking clear signatures of negative
AGN feedback.
To investigate whether such a negative feedback from thejet is effective in the host galaxies, it is necessary to obtainhigh spatial resolution radio images that resolve the host galaxy
down to scales of < 10 kpc (Reines et al. 2020). The spatialcompactness of radio sources from VLA/FIRST with a spatial
resolution of ∼ 5 arcsec only provide poor upper-bounds of theradio jet size of < 40 kpc at z ∼ 1. ngVLA will revolutionize this thanks to its supreme spatial resolution, which reaches
down to < 100 pc at 2.4 GHz, and < 10 pc at 16 GHz. This over
one-order-of-magnitude better resolution enables us to directlycompare the jet location and the gas reservoir in the image, andto investigate the effect of the jet on the host galaxy.
5. Pinning Down the radio core: Test of wondering BHs
in ERGs
Recently, Reines et al. 2020 employed the radio-survey approach to search for massive BHs by using VLA/FIRST and
conducting VLA high spatial resolution follow-up observations

No. ]

3−1
Fig. 2. (Left) The relation between sBHAR and M⋆ . The sBHAR is conventionally defined as sBHAR = LAGN,bol /M⋆ erg s−1 M⊙, an indicatorof Eddington ratio λEdd . The two dashed lines are the expected Eddington limits λEdd = 1 and = 0.01 using the scaling relation of (Kormendy & Ho2013). The orange shaded area represents the region of super-Eddington phase. (Right) The relation between Ljet /LAGN,bol = Rint and sBHAR. The
yellow shaded area represents the corresponding λEdd = 1 region assuming the scaling relation of (Kormendy & Ho 2013). The symbols are same as in
Figure 1, but NRGs are shown with color gradation based on Rrest in the range of 1 < log Rrest < 4. Both figures are taken from Ichikawa et al. 2021.

can be spatially separated from the jet lobes. Another advantage of the ngVLA observation is that it can reach sensitivities
in a short (<1 hr1 ) exposures and contamination from X-ray
binaries would be negligible considering its luminous radio luminosity for ERGs. Furthermore, with its long baselines, contamination from star formation can be minimised by selecting
only point-like radio emission. Therefore, large-scale statistical studies of ERGs would provide insights into the frequency
and environment of wandering BHs as a function of redshift
and stellar-mass.

Physical scale [pc]

102

101
2.4 GHz (10 mas)
16 GHz (1 mas)
0.0

0.5

1.0

1.5

Redshift

2.0

2.5

3.0

Fig. 3. Achievable resolution with ngVLA. Our main target bands are
2.4 GHz and 16 GHz, whose best resolution is at 10 mas (< 100 pc)
and 1 mas (< 10 pc), respectively.

of local dwarf galaxies at z < 0.1, and found more than 10 candidates. One remarkable result was that most radio cores were
not located at the center of the host, but off-nucleus, a possible signature of a previous merger. If the detection of an offnucleus core is indeed the consequence of a galaxy merger, it
indicates that the orbital decay of a black hole through dynamical friction is inefficient in this BH mass range, resulting in a
population of BHs that fail to reach the galactic center withina Hubble time, since major mergers in dwarf galaxies becomesrare at z < 3 (e.g., Fitts et al. 2018), which is called a “sinking problem”. Currently there are many theoretical implication
regarding the existence of wandering BHs (e.g., Comerford &Greene 2014; Tremmel et al. 2018).
Considering that some ERGs are low-mass galaxies, they areimportant targets hosting such wondering BHs. High spatial
resolution radio imaging by ngVLA can pin down the locations of such wandering BHs up to z ∼ 3 since the radio core

6. Strategy of Sample construction of ERGs in the era ofngVLA
The small number density of ERGs (∼ 1 source deg−2 ) andtheir optical faintness of iAB ∼ 25 suggest that ERGs were
easily missed in previous optical surveys of radio sources.
Ichikawa et al. 2021 showed that ERGs can be discovered
with wide (> 100 deg2 ) and deep (iAB <∼ 26) optical followup observations of radio sources. The number of known ERGs
will increase in near future, once the Subaru/SSP survey iscomplete, covering an area of ∼ 103 deg2 . The upcoming
Subaru/Prime Focus Spectrograph (Takada et al. 2014) willconduct extensive optical and near-IR spectroscopic follow-up
observations in the footprint of the HSC-SSP field, providing
spec-z information of ERGs with iAB <∼ 24. The forthcoming
LSST survey (Ivezić et al. 2019) will cover half of the sky, andit will also increase the number of sources by another orderof magnitude, resulting in ∼ 104 sources. A significant fraction of these ERGs will also be expected to reside in low-massgalaxies. In the radio survey side, VLA/FIRST gives the most1

Since WERGS sample is selected from VLA/FIRST sources, their
1.4 GHz radio emission is very bright, with > 1 mJy. The ngVLA 1 hr
integration time could reach the flux limit of 0.5 µJy/beam at 2.4 GHz
and 0.26 µJy/beam at 16 GHz, respectively. If we assume a typical flux
density of fν = 5 mJy with the spectral index of α = −0.7 with fν ∝ ν α ,S/N= 28 (2.4 GHz) and S/N= 14 (16 GHz), which is enough for ourstudy.

4
important population since our targets are high radio-loudness
values of logRrest > 4. In addition, the ongoing VLASS (Lacyet al. 2020), which will achieve a sensitivity down to 0.1 mJy
at 2–4 GHz, giving an order of magnitude deeper radio observations than VLA/FIRST. This would increase the sample inaddition to the traditional VLA/FIRST survey. Therefore, thecombination of wide area (either shallow or deep) radio anddeep optical imaging and spectroscopic surveys will give us astatistically large number of massive BH candidates in the eraof ngVLA.
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