# Probing the Shock Breakout Signal of SN 2024ggi from the Transformation of Early Flash Spectroscopy

Jujia Zhang, Luc Dessart, Xiaofeng Wang, Qian Zhai, Yi Yang, Liping Li, Han Lin, Giorgio Valerin, Yongzhi Cai, Zhen Guo, et al.

► To cite this version:

Jujia Zhang, Luc Dessart, Xiaofeng Wang, Qian Zhai, Yi Yang, et al. Probing the Shock Breakout Signal of SN 2024ggi from the Transformation of Early Flash Spectroscopy. The Astrophysical Journal Letters, 2024, 970 (1), pp.L18. 10.3847/2041-8213/ad5da4. hal-04634802

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# Probing the Shock Breakout Signal of SN 2024gg1 from the Transformation of Early Flash Spectroscopy

Jujia Zhang<sup>1,2,3</sup>①, Luc Dessart<sup>4</sup>①, Xiaofeng Wang<sup>5</sup>①, Qian Zhai<sup>1,3</sup>, Yi Yang<sup>5,6</sup>①, Liping Li<sup>1,2,3</sup>, Han Lin<sup>1,2,3</sup>, Giorgio Valerin<sup>7</sup>, Yongzhi Cai<sup>1,2,3</sup>, Zhen Guo<sup>8,9,10</sup>①, Lingzhi Wang<sup>11</sup>①, Zeyi Zhao<sup>1,2,3</sup>①, Zhenyu Wang<sup>1,2,3</sup>, and Shengyu Yan<sup>5</sup>①

<sup>1</sup>Yunnan Observatories (YNAO), Chinese Academy of Sciences (CAS), Kunming, 650216, People's Republic of China; jujia@ynao.ac.cn

<sup>2</sup>International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, People's Republic of China

<sup>3</sup>Key Laboratory for the Structure and Evolution of Celestial Objects, CAS, Kunming, 650216, People's Republic of China

<sup>4</sup>Institut d'Astrophysique de Paris, CNRS-Sorbonne Université, 98 bis boulevard Arago, F-75014 Paris, France

<sup>5</sup>Physics Department, Tsinghua University, Beijing, 100084, People's Republic of China

<sup>6</sup>Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA

<sup>7</sup>INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy

<sup>8</sup>Instituto de Física y Astronomía, Universidad de Valparaíso, ave. Gran Bretaña, 1111, Casilla 5030, Valparaíso, Chile

<sup>9</sup>Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK

<sup>10</sup>Millennium Institute of Astrophysics, Nuncio Monseñor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile

<sup>11</sup>Chinese Academy of Sciences South America Center for Astronomy (CASSACA), National Astronomical Observatories, CAS, Beijing, People's Republic of China

Received 2024 June 11; revised 2024 June 27; accepted 2024 June 27; published 2024 July 19

## Abstract

We present early-time, hour-to-day cadence spectroscopy of the nearby Type II supernova (SN II) 2024gg1, which was discovered at a phase when the SN shock had just emerged from the red supergiant (RSG) progenitor star. Over the first few days after the first light, SN 2024gg1 exhibited prominent narrow emission lines formed through intense and persistent photoionization of the nearby circumstellar material (CSM). In the first 63 hr, spectral lines of He, C, N, and O revealed a rapid rise in ionization as a result of the progressive sweeping up of the CSM by the shock. The duration of the IIn-like spectra indicates a dense and relatively confined CSM distribution extending up to ~4 × 10<sup>14</sup> cm. Spectral modeling reveals that a CSM mass-loss rate at this region exceeding 5 × 10<sup>-3</sup> M<sub>☉</sub> yr<sup>-1</sup> is required to reproduce low-ionization emissions, which dramatically exceeds that of an RSG. Analyzing the Hα emission shift implies the velocity of the unshocked outer CSM to be between 20 and 40 km s<sup>-1</sup>, matching the typical wind velocity of an RSG. The differences between the inner and outer layers of the CSM and an RSG progenitor highlight a complex mass-loss history before the explosion of SN 2024gg1.

Unified Astronomy Thesaurus concepts: Type II supernovae (1731); Supernovae (1668); Core-collapse supernovae (304)

Materials only available in the online version of record: data behind figure

## 1. Introduction

Type II supernovae (SNe), characterized by the presence of prominent hydrogen features in their spectra (Filippenko 1997; Gal-Yam 2017), are the death throes of most massive stars (e.g., red supergiants, RSGs, Heger et al. 2003). A fraction of SNe II exhibit interaction signatures with circumstellar material (CSM), characterized by narrow optical emission lines with broad electron-scattering wings classified as Type IIn SNe (Niemela et al. 1985; Schlegel 1990). Some of the diversity of SNe IIn lies in the duration of the interaction signatures, which can range from a few days (e.g., SN 2013fs; Yaron et al. 2017) to several weeks (e.g., SN 1998S; Leonard et al. 2000; Chugai 2001; Fassia et al. 2001; Shivvers et al. 2015; Dessart et al. 2016), months (e.g., SN 2010jl; Zhang et al. 2012; Fransson et al. 2014), and even a few years (e.g., SN 2015da; Tartaglia et al. 2020). This indicates a range in spatial scales and mass for the CSM around their progenitor stars.

These cataclysmic events provide a unique window into the final stages of stellar evolution, especially the mass-loss history and the environment surrounding the progenitor stars. The

mass-loss rates deduced from SN observations often differ from those derived from studies of RSGs in their quiescent phases, underscoring the gaps in our understanding of late-stage stellar evolution.

For example, the variety of SNe II with short-lived IIn-like spectral features, as witnessed in SNe like SN 2013fs (Dessart et al. 2017, referred to as D17 hereafter; Yaron et al. 2017), SN 2014G (Terreran et al. 2016), SN 2017ahn (Tartaglia et al. 2021), SN 2018zd (Zhang et al. 2020; Hiramatsu et al. 2021), SN 2020pni (Terreran et al. 2022), SN 2020tlf (Jacobson-Galán et al. 2022), and the recent SN 2023ixf (e.g., Bostroem et al. 2023; Hiramatsu et al. 2023; Smith et al. 2023; Zhang et al. 2023; Zimmerman et al. 2024), reveals the diversity of CSM structures and the mass-loss histories of that subset of SNe and their associated massive star progenitors. Quantitative studies show that over 30% of SNe II exhibit these transient IIn-like features (e.g., Bruch et al. 2021, 2023; Jacobson-Galán et al. 2024a), suggesting that such mass-loss events are common in massive stars about to explode, with certain SNe displaying intermediate properties between regular SNe II and strongly interacting SNe IIn.

Moreover, the early IIn-like spectra resulting from ionizations of CSM by shock photons provide a way to investigate the initial shock breakout (SBO) signal of SN explosion. The earliest emission of electromagnetic radiation from an SN

<figure><img src="image_3.png" /></figure>

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Spectral evolution of SN 2024ggi. Dashed gray lines indicate the rest-frame wavelengths of spectral features, while dotted blue lines represent a blueshift of -8000 km s⁻¹. Artifacts resulting from the incomplete removal of telluric absorption and skyline emissions are identified by dashed-dotted green lines, with each marked with an Earth symbol indicating the terrestrial origin. Details of the observations, including dates and conditions, are given in Table A1. The spectra are provided in machine-readable format as Data behind the Figure and can be downloaded.
(The data used to create this figure are available in the online article.)</figcaption></figure>

explosion is associated with SBO, a brief yet brilliant event signifying the transition from an opaque to a transparent state as the expanding shock wave reaches the stellar surface (Waxman & Katz 2017). Once the shock approaches the progenitor (within an optical depth of $\tau \sim 10–30$), radiation begins to leak from the shock, initiating the ionization of the cool atmosphere and environment of the RSG.

The ionization process, occurring on timescales from seconds to tens of minutes, is marked by a UV flash (Waxman & Katz 2017). It is followed by UV and optical emissions from the cooling envelope. However, if there is an optically thick CSM or dust shell, the SBO might occur within this shell, and the shock emission will be reddened and prolonged, as seen in SN 2023ixf (Bostroem et al. 2023; Li et al. 2024; Zimmerman et al. 2024).

In this Letter, we present the spectroscopic observations of a nearby SN II in NGC 3621, which provides another chance to detect the SBO signals. SN 2024ggi was discovered by the Asteroid Terrestrial-impact Last Alert System (Chen et al. 2024a; Tonry et al. 2024) on 2024 April 11.14 (UTC dates are used throughout this Letter) in the nearby galaxy NGC 3621, located at a distance of $D = 7.0 \pm 0.2$ Mpc (based on the averaged Cepheid distance derived by different period–luminosity relations presented in Kanbur et al. 2003). The archive images taken with the Hubble Space Telescope about 20 yr before the explosion reveal an RSG progenitor with a temperature $T_* = 3290_{-27}^{+19}$ K and radius $R_* = 887_{-51}^{+60} R_\odot$ (Xiang et al. 2024). With the first-light date at MJD = 60411.03 ± 0.05 determined through high-cadence

photometric observations (S. Yan et al. 2024, in preparation), SN 20024ggi stands out as an SN II with extremely early photometric and spectroscopic observations (Tonry et al. 2024; Zhai et al. 2024).

## 2. Spectral Observation

### 2.1. Classification

Utilizing the Li-Jiang 2.4 m telescope (LJT; Fan et al. 2015) equipped with the Yunnan Faint Object Spectrograph and Camera (YFOSC; Wang et al. 2019), we obtained a classification spectrum for SN 2024ggi at ~13.9 hr after its explosion (Zhai et al. 2024). This classification spectrum was predominantly characterized by narrow emission lines of H, He, and CNO elements, as seen in Figure 1. These features, also known as flash features (Gal-Yam et al. 2014), are commonly observed in SNe IIn (e.g., SN 1998S; Leonard et al. 2000). They are generated by recombinations of the surrounding dense CSM that is photoionized by radiation from the embedded shock.

Interestingly, in the initial-phase spectrum, SN 2024ggi exhibits a significant difference from other SNe II-P/CSM events like SN 2013fs, SN 2018zd, and SN 2023ixf. As presented in Figure 2(a), the classification spectrum of SN 2024ggi lacks the strong emission features of high-ionization states. It showed lines of He I, N III, and C III rather than those of He II, N V, and C IV or the highly ionized O V $\lambda\lambda5576, 5598$ and N V $\lambda\lambda4604, 4620$ lines that appeared in SN 2013fs.
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Spectral comparison between SN 2024ggi and the representative SNe II. (a) Flash spectroscopy of SN 2024ggi, compared with those of SN 1998S (Leonard et al. 2000; Fassia et al. 2001; Shivvers et al. 2015), SN 2013fs (Yaron et al. 2017), SN 2018zd (Zhang et al. 2020), and SN 2023ixf (Zhang et al. 2023). (b) Postflash spectra of SN 2024ggi, compared with those from SN 1998S, SN 1999em (Hamuy et al. 2001; Leonard et al. 2002; Dessart & Hillier 2006), SN 2018zd, and SN 2023ixf. The extinction correction was applied to each spectrum for which the extinction of SN 2024ggi is derived in Appendix A.2, while the extinction values of the comparisons are adopted from the lectures. Dashed gray lines represent the rest-frame wavelengths of spectral features, while the dotted blue lines represent a blueshift of -8000 km s⁻¹ in velocity space.</figcaption></figure>

In particular, SN 2024ggi and SN 2013fs exhibit two opposite spectral line morphologies in the 4600–4700 Å region. The spectra of SN 2013fs at $t \le 10$ hr (where *t* denotes time after the first light) show a narrow He II λ4686 emission line, while the nearby N III λλ4630, 4641 doublets appear flatter without prominent narrow-line components. Instead, there is a small bump consisting of N V λλ4604, 4620. However, the classification spectrum of SN

2024ggi reveals narrow emission lines of N III λλ4630, 4641, while the position corresponding to He II λ4686 appears flat. These two SNe belong to those with the earliest discoveries among the sample of SNe II-P with CSM. Spectroscopic data from other samples, usually obtained one day after the SBO, did not show similar phenomena, suggesting a high dependency of early-time spectra on the exact postbreakout epoch and the CSM properties.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** EW evolution of the ionization line in SN 2024ggi at $t < 3$ days. C IV 5806 is the abbreviation of C IV λλ5801, 5811.</figcaption></figure>

## 2.2. High-cadence Sampling within the Initial 72 hr

Given the observed IIn-like features in the classification spectrum and a relatively lower ionization state revealed by these features, we initiated an hour-cadence observation campaign with the LJT. A total of four spectra were acquired within 2.4 hr after the identification, which allowed us to monitor possible variations in the ionization state for this young SN II.

From the second spectrum, SN 2024ggi exhibited narrow He II λ4686 emission that was absent before. It is possible that, in the first spectrum, the CSM has not been shocked enough to generate sufficient heat for emitting a significant amount of He II, except for the deep CSM near the shock, where the material could be rushing. The subsequent two spectra did not show significant morphological evolution. Further analysis of the equivalent widths (EWs) of the main spectral lines, as shown in Figure 3, reveals that within the 1.5 hr from $t \sim 14.7$ to $t \sim 16.2$ hr, the strength of the He II λ4686 and He I λ6678 lines remained almost unchanged. In contrast, the C III λ5696 and He I λ7065 lines became gradually weak.

On the second day after explosion, an observation relay across different time zones was conducted with LJT, Telescopio Nazionale Galileo (TNG), and Very Large Telescope Unit 1 (VLT UT1), which allowed continuous monitoring of the rapid evolution of SN 2024ggi. In particular, to examine the structure of narrow spectral lines, we utilized the cross-dispersion capability of YFOSC with a resolution of 3500. This resolution had proven its effectiveness in our previous research of SN 2023ixf, enabling us to discern the intricate structure of $H\alpha$ and provide a more precise constraint on its broadening (Zhang et al. 2023).

Nonetheless, in the case of SN 2024ggi, the $H\alpha$ line had already undergone significant broadening in the spectra taken at $t \sim 38.7$ hr, with a full width at half-maximum (FHWM) of $\sim 700 \text{ km s}^{-1}$, exceeding the instrumental FWHM (i.e., $\sim 85 \text{ km s}^{-1}$). Concurrently, as the narrow line broadened swiftly, its EW declined steeply at $t \gtrsim 30$ hr, as illustrated in Figure 3.

The rapid broadening and decreasing of the $H\alpha$ narrow component testifies to the rapid spectral evolution of SN 2024ggi. By $t \sim 29$ hr, many highly ionized spectral lines emerged in the spectrum, including O IV, N IV, C IV, and even O V. Due to limitations in the signal-to-noise ratio of the spectrum, the He II λ 5411 line can only be marginally detected

at $t < 1$ day, but it becomes visible at $t \sim 29$ hr. The most notable change is that 1 day after explosion, the emission lines of He I λ7065 and C III λ5696 disappeared in the spectra but were replaced by the C IV λλ5801, 5811, C IV λ7110, and N IV λ7122 lines. As shown in Figure 3, He II λ4686 continues to weaken throughout the day, while C IV λλ5801, 5811 progressively gains strength. We notice that the narrow He I λ6678 line disappeared at 29.2 hr, but it reappeared 5 hr later. The He I λ5876 line is not detected, perhaps due to the fact that it coincides with the Na I D absorption of the Milky Way.

At $t \sim 29$ hr, the spectral line flux and profile morphology in this region are unreliable due to a saturation in the wavelength range from $\sim 4610$ to $\sim 4700 \AA$. Therefore, we cannot confirm whether the appearance of N V at this time is genuine, nor can we ascertain the reliability of the intensity contrast between N III and He II. At $t \sim 34$ hr, the narrow lines of He II λ4686 and N III λλ4630, 4641 have comparable intensities and thus formed a double-peak profile. Such a double-peak structure has also been seen in some SNe II. For instance, it appeared in SN 2023ixf at $t \sim 1.2$ days, SN 1998S at $t \sim 2.3$ days, and SN 2018zd at $t \sim 4$ days. SN 2013fs may also exhibit a similar structure at $t \sim 1.4$ days, except that the left peak corresponds to N V λλ4604, 4620 instead of N III in its spectrum (D17; Yaron et al. 2017). The N V doublet is also visible in the spectra of SN 2024ggi taken at $t = 29.2$ hr, 38.3 hr, and 38.7 hr. This indicates that SN 2013fs and SN 2024ggi can reach similar ionization levels, though they may have different shock emissions and CSM environments.

At $t > 38$ hr, the N III doublet becomes weaker than He II λ4686 in SN 2024ggi. However, the N III doublet remains unchanged over the following days, while He II shows a faster decline in intensity. Overall, all narrow emission lines become less distinct after the third day, as they broaden significantly with a velocity exceeding $1000 \text{ km s}^{-1}$. The disappearance of narrow emission lines indicates that the IIn-like phase is ending and SN 2024ggi is gradually entering a new stage of evolution.

The rapid spectral evolution relates in part to a temperature change. We can roughly estimate the temperature of the SN photosphere through fits to the spectral energy distribution (SED) assuming a blackbody emission. In Figure A1, the temperature of SN 2024ggi is inferred to be about 13,500 K during the phase from 13.9 to 16.2 hr, and it increases sharply from $\sim 13,500$ to $\sim 26,000$ K about half a day later (also reported by Chen et al. 2024b). The rapid temperature rise is concomitant with the rapid increase in ionization, and both are attributed to the photoionizing radiation from the shock.

## 2.3. Flash-to-photospheric Phase Transition

Since SN 2024ggi exhibited a rapid evolution, we maintained frequent monitoring until the third day, gradually tapering off the pace after that. As presented in Figure 1, we can only see a few weak “narrow” emission lines, such as $H\alpha$ and the N III λλ4634, 4641 doublet at $\sim 3.6$ days. Meanwhile, P Cygni profiles of $H\beta$, $H\gamma$, and He I λ5876 begin to emerge.

At $3 \text{ days} < t < 15$ days, the spectral lines of SN 2024ggi undergo a significant transition. Initially, these lines exhibited broadening due to electron scattering formed within the unshocked CSM. However, they gradually shift to Doppler broadening, as they initially formed within the fast-moving dense shell and subsequently also arise from the ejecta as the dense shell gradually becomes optically thinner. This evolution
is accompanied by a noticeable blueshift in their emission
peaks. Additionally, these lines begin to appear as P Cygni
profiles, with both absorption and emission components
increasing in strength over time.

Roughly 2 weeks after the explosion, SN 2024ggi developed
spectral features typical of SNe IIP in both optical and near-
infrared (NIR) spectra, i.e., the appearance of broad H I and
He I lines. For example, as seen in Figure A2, the NIR
spectrum of SN 2024ggi, obtained with the 6.5 m Magellan
telescope equipped with Folded-port InfraRed Echelle
(FIRE) at t ~ 14 days, reveals similar spectral features as seen
in SN 2017eaw (Szalai et al. 2019).

As observed in SN 2018zd (Zhang et al. 2020) and SN 2023ixf (Zhang et al. 2023), the SN–CSM interaction creates a slow evolution in spectral features after the narrow, electron-scattering broadened emission lines disappear (Figure 2). During shock-wave propagation, the CSM is shocked and compressed into a dense shell, converting a fraction of kinetic energy into thermal energy and heating the CSM as well as boosting the SN luminosity. In the case of SN 2024ggi, photoionization predominantly facilitates this heating process within the first 3 days. Consequently, emission from the postshock gas, such as the cold dense shell (CDS), originates from the release of shock-deposited energy (i.e., deposited at earlier times). The relatively faint spectral lines observed approximately 5 days later are likely attributed to the spectrum primarily forming within the CDS during this period. This weak emission is indicative of a steep density gradient of the CDS (D17).

We note that the evolution speed of the photospheric features in SN 2024ggi is slower than that of the typical SN 1999em but still faster than that in SN 2018zd and SN 2023ixf. As illustrated in Figure 2(b), SN 2024ggi evolves into spectra with prominent P Cygni profiles at around *t* ~ 23 days, and its spectrum at *t* ~ 32 days shows a close resemblance to that of a regular SN IIP. Nevertheless, SN 2024ggi evolves faster in the first month, regardless of early IIn or later photospheric spectra, compared to SN 2018zd and SN 2023ixf, suggesting a more compact CSM.

**3. Insights into the Early Flash Spectroscopy**

3.1. Fluctuation of Ionization States

Within the initial 72 hr after the first light, we obtained a total
of 12 spectra revealing rapid evolution of line strengths, in
particular for lines associated with different ionization levels
(e.g., He I versus He II). During this period, the lower-
ionization species (such as He I, C III, and N III) gradually
weakened and disappeared, while the strength of emission lines
from higher-ionization species (like He II, N IV, and C IV)
increased (see also Section 2.2).

The evolution of these line fluxes or EWs is, however, more
complex. Spectral lines such as He I λ6678, He I λ7065, and
C III λ5696 disappeared at *t* ~ 29.2 hr, while higher-ionization
lines, including C IV λλ5801, 5811 and O V λλ5576, 5598,
emerged. However, 5 hr later, He I λ6678 reappeared, while
O V became more elusive. In the 38.3 hr and 38.4 hr spectra,
roughly 4 hr afterward, He I λ6678 vanished once again and
O V reappeared, while N III significantly diminished and
N V became detectable.

The weakening of N III and the emergence of N V were
already evident in the 29.2 hr spectrum. Considering the

evolution of O V and He I, we believe the relative intensities
of N V and N III observed at 29.2 hr are reliable despite the
detector saturation in this wavelength region. Thus, we
observed a decreasing and increasing ionization across the
29.2, 34.4, and 38.3 hr spectra.

This trend is mirrored in Figure 3, where a similar fluctuation
in C IV λλ5801, 5811 is apparent in the *t* ~ 30 hr spectrum.
These variations suggest that although the ionization evolution
initially arose and fell within the first 72 hr, there were
fluctuations when the ionization reached its maximum at
around 30 hr after the first light. These fluctuations are probably
related to the complicated propagation through a clumpy and
perhaps asymmetric CSM. The alteration in the ionization state
indicates the complex interaction between the shock wave, the
surrounding material, and the high-energy radiation environ-
ment in the early stages of the SN explosion.

3.2. Comparison with Spectra Model

To further understand the evolution of the ionization state of
SN 2024ggi, we compare the observed spectra with the spectral
models presented in D17(as shown in Figure 4). This model set
successfully reproduces the observational manifestations of
SNe II, specifically those featuring short-lived IIn-like spectra,
while taking into account the varying physical conditions of the
progenitor before the explosion. The methodology adopted in
D17 involves creating a comprehensive grid of models for
different CSM characteristics (including radius, mass, density,
density profile, velocity, and composition) based on the same
progenitor and explosion parameters, thereby encompassing a
wide range of the parameter space.

Although none of the models in D17 can fully reproduce all
of the spectral features, models like r1w5r and r1w6 provide a
good reference for us to track and study the ionization state and
evolution of the CSM. In comparison, we found that the r1w5r
model fits the observations well, especially at t ~ 1.5 days,
when it can reproduce all the observed characteristics of SN
2024ggi. Moreover, at t ~ 1.17 days, high-ionization lines such
as O IV and N IV appeared in the model spectrum, which is
comparable to the spectrum of SN 2024ggi at t ~ 1.35 days,
indicating that the ionization state of SN 2024ggi is consistent
with the model during this period. However, the CSM in the
r1w5r model has a higher ionization state at t < 1 day than that
observed in SN 2024ggi. For example, the r1w5r model does
not show an He I line in the early stage, and in fact, no He I line
appears in any of the D17 models.<fnref n="12" /> In contrast, the He II line in
the r1w5r model is much stronger than the N III line at the
beginning of the explosion.

Interestingly, the blue edge of Hα is quite similar in the
*t* ~ 16 spectrum of SN 2024ggi and the *t* = 15 days r1w5r
model spectrum. The only difference lies in the emission
strength and the extent of the profile to the red. One reason
might be that in the model, the optical depth attenuation is
much more significant, which suggests that the CDS breaks up
and becomes significantly clumpy. This could be resolved if the
CDS was allowed to break up, as would occur in three-

<footnote n="12">¹² The reason is partly because no model was computed during the earliest epochs, when the first radiation from the shock was crossing the CSM (i.e., the radiative precursor). Modeling this phase is quite challenging, and there are inconsistencies in the physics addressed in D17. One such inconsistency relates to the light travel time effect, considered in radiation-hydrodynamics calculations but omitted during the postprocessing phase of radiative transfer calculations for spectra.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Comparison of early-time spectra from SN 2024ggi from 14 hr until 15 days postexplosion with the r1w5r model spectra of D17. Dashed gray lines represent the rest-frame wavelengths of spectral features. In the r1w5r model, the progenitor star radius is $R_{*} = 501 R_{\odot}$, the wind mass-loss rate is $\dot{M} = 5.0 \times 10^{-3} M_{\odot} \text{yr}^{-1}$, and the outer radius of the dense CSM is $R_{\text{CSM}} = 5 \times 10^{14} \text{ cm}$.</figcaption></figure>

dimensional radiation-hydrodynamics with the development of Rayleigh–Taylor instabilities.

Jacobson-Galán et al. (2024b) favored the r1w6 model over the r1w5r model in their comparisons. The mass-loss rates of these two models are quite similar, and their early spectra are also very similar. Both models exhibit a higher ionization in the early stages than observed in SN 2024ggi. The early He II line in the r1w6 model is stronger than that in the r1w5r model, leading us to believe that the r1w5r model performs better in this aspect. Additionally, since the spectra of Jacobson-Galán et al. (2024b) do not extend beyond 10 days after the explosion, they did not observe the distinct P Cygni features that appeared later in SN 2024ggi. It is worth noting that the early-stage spectra of the r1w6 model (up to 14 days) also do not show this characteristic, making it inferior to the r1w5r model.

The r1w5r model is suitable for SN 2024ggi, as it can roughly reproduce the observed main IIn-like spectral features. However, the issue lies in the fact that the early ionization state of the model is too high. The CSM property of SN 2024ggi might be intermediate between r1w5r and r1w6. There may also be additional features ignored in D17, such as asymmetry, the breakup of the CDS, clumping in the CSM, and such. And the radiative transfer modeling could be improved. Therefore, the initial spectral evolution of SN 2024ggi provides a new opportunity for future development of spectroscopic models.

### 3.3. Evolution of Hydrogen Emission

In the early phase, when the SN photosphere is within the unshocked ionized CSM, the spectra are characterized by symmetric emission lines with narrow cores and extended, electron-scattering broadened wings. Figure A3 shows the

double-component fitting of the early $H\alpha$ emission of SN 2024ggi. Note that the broader wings are referred as midwidth components to distinguish them from the even wider emission lines in the later P Cygni profiles Doppler-broadened in the fast-expanding ejecta.

Based on the two-component fitting of Figure A3, we obtain the parameters and evolution of each component. To visually compare this evolution in $H\alpha$, we selected spectra taken by YFOSC+G14 in the first 3 days, as presented in the top panel of Figure 5, to minimize the influence of instrumental effects. All of these spectra are corrected for the redshift and rotation of the host galaxy; see more detail in Appendix A.2.

Given the low spectral resolution, an instrumental correction is necessary to accurately determine the FWHM of the $H\alpha$ line via $\text{FWHM}_{\text{cor}} = (\text{FWHM}_{\text{obs}}^2 - \text{FWHM}_{\text{inst}}^2)^{1/2}$, where instrumental FWHM is estimated through the skyline O I $\lambda$6300.3. This method has been proved effective in the study of SN 2023ixf, where the value we obtained through midresolution spectra (Zhang et al. 2023) matches well with that from the high-resolution spectra (Smith et al. 2023), as seen in the middle panel of Figure 5. In this panel, the corrected FWHM of the narrow component broadens from about $50 \text{ km s}^{-1}$ to approximately $200 \text{ km s}^{-1}$ within hours on the first day. Initially, the $\text{FWHM}_{\text{obs}}$ is close to the instrumental $\text{FWHM}_{\text{inst}}$, making corrections potentially inaccurate. We propose $50 \text{ km s}^{-1}$ as an upper limit of FWHM before radiation broadening, and the actual FWHM is likely narrower at $t < 14$ hr.

Despite a measurement uncertainty of around 50% in the third spectrum, there is a discernible broadening of spectral lines, indicating the onset of electron-scattering effects as well as the potential radiative acceleration of the unshocked CSM
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Top: Hα emission spectra of SN 2024ggi captured at *t* < 3 days obtained by LJT+YFOSC(G14). The black dashed line denotes the zero-velocity mark. The instrumental FWHM (460 km s⁻¹) of these spectra is indicated by the gray dotted-dashed lines. Middle: the evolution of the corrected FWHM of the narrow Hα component of SN 2024ggi compared with that of SN 2023ixf derived by the midresolution spectra of Zhang et al. (2023) and a high-resolution spectrum (at *t* ≈ 62.4 hr) of Smith et al. (2023). Bottom: velocity evolution derived from the Hα emission of SN 2024ggi at *t* < 3 days. The dashed line represents a linear fit to the velocity data. The derived velocities have been corrected for skylines O I λ6300.3, as well as adjusted for the host galaxy’s rotation at *v* = −60 ± 10 km s⁻¹, which was determined through analysis of the Na I D absorption in midresolution spectra.</figcaption></figure>

(D17). At this point, using the FWHM of the narrow lines to constrain the original CSM velocity becomes unfeasible.

Pessi et al. (2024) measured an FWHM ≈ 55 km s⁻¹ of Hα at *t* ~ 23.6 hr (the epoch is calculated by the first light adopted in this Letter) with high-resolution spectroscopy. They found

that this line broadened to 61 km s⁻¹ 7 hr later. Their measurements indicate that although the low-resolution results are not precise enough, they can provide certain constraints at the earliest hours.

At *t* ~ 38.7 hr, a midresolution spectrum revealed that the FWHM of Hα had increased significantly, rendering instrumental broadening negligible. Remarkably, the narrow spectral lines of SN 2024ggi expanded to over 700 km s⁻¹ within 48 hr. In contrast, the Hα FWHM of SN 2023ixf during a similar timescale after the first light was only 55 km s⁻¹ (*t* ~ 43 hr; Zhang et al. 2023); see the middle panel of Figure 5. Furthermore, the broadening of the Hα narrow-line component in SN 2023ixf occurred much slower, increasing only from 55 to 140 km s⁻¹ within the first 3 days and reaching close to 900 km s⁻¹ when the narrow line was almost disappearing on the fifth day. Similarly to SN 2023ixf, the FWHM of the narrow Hα of SN 2020pni increased from 250 to nearly 1000 km s⁻¹ in the first 5 days (Terreran et al. 2022). The more rapid and drastic broadening observed in SN 2024ggi suggests a less extended CSM than that of SN 2023ixf and SN 2020pni.

Although we cannot use the FWHM to limit the CSM velocity of SN 2024ggi, a noticeable blueshift was observed in the Hα emission. This shift is evident in the top panel of Figure 5, with precise measurements detailed in the middle panel. After some velocity adjustments and a refined wavelength calibration using the skylines O I λ6300.3, an initial blueshift velocity of −12 ± 20 km s⁻¹ was measured at *t* ~ 13.9 hr. The resolution of the second spectrum at *t* ~ 14.7 hr is slightly higher, and the measured velocity is −20 ± 10 km s⁻¹. Considering the measurement errors in galactic rotation, the lower limit of the stellar wind velocity observed at the outer layer of CSM around SN 2024ggi is 20 ± 15 km s⁻¹. A fast expansion of the narrow Hα component was also seen in the SNe II with a short-lived IIn-like feature, e.g., SN 2020pni.

We averaged the measurements daily to reduce uncertainties, yielding mean velocities of 42 ± 29 km s⁻¹, 99 ± 23 km s⁻¹, and 133 ± 17 km s⁻¹ in the first 3 days, respectively. These measures reveal the acceleration of the unshocked CSM by the radiation from the shock. The acceleration implies that the earlier observations can more genuinely reflect the movement of CSM before the explosion, representing the progenitor’s stellar wind speed in its final phase. Thus, the inferred CSM velocity of SN 2024ggi does not exceed 40 km s⁻¹, consistent with the high-dispersion observation results (e.g., 37 km s⁻¹; Shrestha et al. 2024). Given the lower limit estimated before, the stellar wind velocity derived by our initial observation suggests that the progenitor wind velocity of SN 2024ggi is between 20 and 40 km s⁻¹.

## 4. Discussion

The initial spectral transformation and ionization processes observed in this SN provide a unique and valuable perspective into the final moments of a massive star. The rapid transition from IIn-like spectra to the photospheric phase in SN 2024ggi suggests that the fast-expanding ejecta quickly engulfed the CSM, and the CSM density decreased sharply at large distances.

Based on the preceding analysis, we have sketched the CSM of SN 2024ggi. The blueshifted Hα emission indicates a wind velocity of the outer CSM at 20 km s⁻¹ < *v* < 40 km s⁻¹. The comparison with the D17 model suggests a mass-loss rate at the
inner CSM on the order of $5 \times 10^{-3}-10^{-2} M_{\odot} \text{yr}^{-1}$ to produce the observed IIn-like spectral features.

The duration and strength of the narrow emission lines depend on the radius and density of the CSM, providing valuable clues for the mass-loss rate of the progenitor. The narrow emission lines with electron-scattering broadened wings of SN 2024ggi vanished approximately 6 days after the explosion. Based on subsequent measurements of the hydrogen P Cygni absorption component, the maximum ejecta velocity of SN 2024ggi is $8000 \text{ km s}^{-1}$. Adopting this value, we can confidently infer that the distribution range of the CSM does not exceed $4 \times 10^{14} \text{ cm}$. This aligns with the photosphere radius inferred from the bolometric luminosity on the sixth day, as seen in Figure A1. Considering the upper limit of the stellar wind speed of $40 \text{ km s}^{-1}$, it would take at least 3 yr for the stellar wind to reach that distance, but much longer if we adopt a slowly accelerating wind.

Comparison of SN 2024ggi with other SNe II exhibiting short-lived IIn-like spectra highlights the diversity in CSM properties. SN 2024ggi stands out for the fast transition from the IIn phase to the phase when spectral lines appear broad and dominated by Doppler broadening, which is consistent with a compact surrounding CSM. For example, SN 2024ggi has a more compact CSM than that of SN 2023ixf (e.g., with a CSM distribution region of $7 \times 10^{14}$; Zhang et al. 2023; Zimmerman et al. 2024) and SN 2018zd (e.g., with a CSM distribution region of $10^{15}$; Zhang et al. 2020).

The analysis of the $H\alpha$ emission line reveals significant broadening within 63 hr, and the FWHM increases from ~50 to ~1500 km s$^{-1}$, which is due to the radiative acceleration of the CSM. This is why the narrow component quickly disappears.

The early spectral (and photometric; S. Yan et al. 2024, in preparation) evolution of SN 2024ggi indicates that we caught the SN during the SBO. In particular, the four spectra taken between 13.9 and 16.2 hr showed spectra with lines of a low ionization level. Taking into account the influence of spectral resolution, it can be assumed that the spectral lines and blackbody temperature of these four spectra remained almost unchanged. Combined with the significant increase in ionization level observed in the 29.2 hr spectrum, it can be inferred that the SBO occurred during this period. The photosphere radius at this time was approximately $10^{14}$ cm. Subsequently, fluctuations in high-ionization lines were observed at 29.2 hr, 34.4 hr, and 38.2 hr, which may be related to the end of the SBO. The corresponding photosphere radius at this time was $1.5 \times 10^{14}$ cm (Figure A1). In other words, the SBO of SN 2024ggi, as observed from the IIn-like spectrum, occurred within the region between $1 \times 10^{14}$ and $1.5 \times 10^{14}$ cm.

In summary, the study of SN 2024ggi will contribute to our understanding of the late stages of stellar evolution, the pivotal role of CSM in shaping the SN observations, the intriguing diversity of SNe II, and the process of shock-wave propagation in CSM.

## Acknowledgments

This work is supported by the National Key R&D Program of China with No. 2021YFA1600404, the National Natural Science Foundation of China (12173082), the science research grants from the China Manned Space Project with No. CMS-CSST-2021-A12, the Yunnan Fundamental Research Projects (grants 202201AT070069 and 202401BC070007), the Top-notch Young

Talents Program of Yunnan Province, the Light of West China Program provided by the Chinese Academy of Sciences, the International Centre of Supernovae, and Yunnan Key Laboratory (No. 202302AN360001). X.W. is supported by the National Natural Science Foundation of China (NSFC grants 12288102 and 1203300) and the Tencent Xplorer Prize. Y.-Z.C. is supported by the National Natural Science Foundation of China (NSFC; grant No. 12303054) and the Yunnan Fundamental Research Projects (grant No. 202401AU070063). Z.G. is supported by the ANID FONDECYT Postdoctoral program No. 3220029. This work was funded by ANID, Millennium Science Initiative, AIM23-0001. This work has made use of the University of Hertfordshire's high-performance computing facility (http://uhhpc.herts.ac.uk). L.W. is sponsored (in part) by the Chinese Academy of Sciences (CAS), through a grant to the CAS South America Center for Astronomy (CASSACA) in Santiago, Chile.

We acknowledge the support of the staff of the LJT, VLT, and TNG. Funding for the LJT has been provided by the CAS and the People's Government of Yunnan Province. The LJT is jointly operated and administrated by YNAO and the Center for Astronomical Mega-Science, CAS.

**Facilities:** YAO:2.4m, TNG, VLT:Antu, Magellan:Baade.

**Software:** PyRAF (Science Software Branch at STScI 2012), NumPy (Harris et al. 2020), Matplotlib (Hunter 2007), Astropy (Astropy Collaboration et al. 2013, 2018, 2022).

## Appendix

### A.1. Data Reduction

Figure 1 displays the spectra of SN 2024ggi obtained by LJT, TNG, and VLT UT1, with further specifics outlined in Table A1. All of these spectra are produced in the standard way in IRAF, including precise wavelength and flux calibration, and have been corrected for telluric absorption and redshift. The wavelength was double-checked by the skylines (e.g., $OI \lambda5577.3$, $OI \lambda6300.3$, and $OI \lambda6363.8$). The flux of the spectra was double-checked by the SED of *ugriz*-band photometry, as seen in Figure A1. During the spectroscopic monitoring of LJT, we got high-cadence photometry quasi-simultaneously. The flux of VLT and TNG are checked with the SED interpreted by the photometry of LJT and Burst Observer and Optical Transient Exploring System (S. Yan et al. 2024, in preparation). Figure A1 presents the bolometric light curve of SN 2024ggi based on the *ugriz*-band photometry and blackbody fitting. The temperature derived from the blackbody fitting and the photospheric radius estimated via luminosity and temperature are also plotted. We got one near-infrared spectrum from the Magellan telescope with FIRE (Folded-port InfraRed Echellette) on April 24, 2024 (t ~ 14 d), as seen in Figure A2, which was reduced with the FIREHOST V2.0 pipeline (Gagné et al. 2015).

Figure A3 displays two-components fitting of $H\alpha$ emission in SN 2024ggi presented in spectra obtained in the first two days.

### A.2. Na I D Absorption and Extinctions

We observed three sets of absorption lines in the midresolu-tion spectrum of SN 2024ggi. Based on their wavelength relationships, we can infer that these three sets of lines are Na I D absorption lines from different redshifts, indicating dust extinction in the line-of-sight direction of SN 2024ggi, as seen in Figure A4. The first set at z ~ 0.000036 is the absorption
<figure><img src="image_9.png" /><figcaption>**Figure A1.** Bolometric light curve, blackbody temperature, and photospheric radius calculations for SN 2024ggi based on photometric data of LIT at the same time of spectral observation. Top panel: bolometric light curves determined through *ugriz*-band photometry and fitted to a blackbody curve. Middle panel: temperature determined from the blackbody fit of the photometry data. Bottom panel: photospheric radius determined using luminosity and temperature measurements.</figcaption></figure>

from the Milky Way. Based on the redshift, $z \sim 0.00039$, the second set of absorption lines may originate from a molecular cloud within the Milky Way at a recession velocity of approximately $120 \text{ km s}^{-1}$. Given the redshift of NGC 3621 ($z = 0.002435 \pm 0.000007$; from NASA/IPAC Extragalactic Database), the third set of Na I D lines at $z \sim 0.002235$ should be from the host galaxy with a rotation velocity at $-60 \pm 10 \text{ km s}^{-1}$, where the velocity error is derived by the estimation of the skyline $\Omega I \lambda 5577.3$.

The EW of interstellar Na I D1 and D2 lines can be used to estimate the dust extinctions, e.g., the empirical relations in Poznanski et al. (2012). We derived the $E(B-V)$ of SN 2024ggi depending on the three sets of Na I D absorption, as listed in Table A2. The estimations from the same set of D1 and D2 are

averaged. Based on this, we derived that the Galactic extinction is $E(B-V)_{MW} = 0.048 \pm 0.009$ mag, which is close to the result of Schlafly & Finkbeiner (2011; i.e., $E(B-V)_{MW} = 0.071$ mag). The extinctions of the host galaxy and the intermediate cloud are $E(B-V)_{Host} = 0.063 \pm 0.005$ mag and $E(B-V)_{IMC} = 0.050 \pm 0.009$ mag, respectively. Adopting the Galactic extinction of Schlafly & Finkbeiner (2011) and our host and intermediate cloud estimates, the total extinction of SN 2024ggi is $E(B-V) = 0.18 \pm 0.01$ mag, which is adopted in the related calculation of this paper. This measurement is consistent with the extinction results obtained by Pessi et al. (2024), i.e., $E(B-V) = 0.16 \pm 0.02$ mag, and Shrestha et al. (2024), i.e., $E(B-V) = 0.15 \pm 0.02$ mag, through high-dispersion spectroscopic observations of the Na I D lines.
<figure><img src="image_10.png" /><figcaption>**Figure A2.** Optical-NIR spectrum of SN 2024ggi compared with SN 2017eaw (Szalai et al. 2019). The epoch of optical and NIR observation is marked after the name of each SN. The blue dotted line represents a blueshift corresponding to a velocity of −8000 km s⁻¹.</figcaption></figure>

<table><caption>**Table A1**
Journal of Spectroscopic Observations of SN 2024ggi</caption><thead><tr><th>Date (UT)</th><th>MJD</th><th>Epoch<sup>a</sup></th><th>Range (Å)</th><th>Spec. Res.</th><th>Telescope+Inst.</th></tr></thead><tbody><tr><td>Apr 11</td><td>60411.608</td><td>13.9 hr</td><td>3600–8800</td><td>410</td><td>LJT+YFOSC(G3)</td></tr><tr><td>Apr 11</td><td>60411.641</td><td>14.7 hr</td><td>3900–7600</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 11</td><td>60411.677</td><td>15.5 hr</td><td>3900–7600</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 11</td><td>60411.706</td><td>16.2 hr</td><td>3900–7600</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 11</td><td>60412.248</td><td>29.2 hr</td><td>3300–9200</td><td>370</td><td>VLT UT1+FORS2(300V)</td></tr><tr><td>Apr 11</td><td>60412.463</td><td>34.4 hr</td><td>3400–8100</td><td>410</td><td>TNG+LRB</td></tr><tr><td>Apr 12</td><td>60412.623</td><td>38.2 hr</td><td>3900–8870</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 12</td><td>60412.641</td><td>38.7 hr</td><td>3500–9500</td><td>3500</td><td>LJT+YFOSC(E9G10)</td></tr><tr><td>Apr 12</td><td>60413.141</td><td>50.7 hr</td><td>3300–9200</td><td>370</td><td>VLT UT1+FORS2(300V)</td></tr><tr><td>Apr 12</td><td>60413.151</td><td>50.9 hr</td><td>3600–5100</td><td>1200</td><td>VLT UT1+FORS2(1200b)</td></tr><tr><td>Apr 13</td><td>60413.620</td><td>62.2 hr</td><td>3500–9500</td><td>3500</td><td>LJT+YFOSC(E9G10)</td></tr><tr><td>Apr 13</td><td>60413.652</td><td>62.9 hr</td><td>3900–8870</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 14</td><td>60414.631</td><td>3.60 days</td><td>3400–9150</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 14</td><td>60414.638</td><td>3.61 days</td><td>3850–9160</td><td>410</td><td>LJT+YFOSC(G3)</td></tr><tr><td>Apr 15</td><td>60415.634</td><td>4.60 days</td><td>3400–9150</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 15</td><td>60416.078</td><td>5.05 days</td><td>3400–9300</td><td>370</td><td>VLT UT1+FORS2(300V)</td></tr><tr><td>Apr 16</td><td>60416.676</td><td>5.65 days</td><td>3850–9160</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 16</td><td>60416.989</td><td>5.96 days</td><td>3300–9200</td><td>370</td><td>VLT UT1+FORS2(300V)</td></tr><tr><td>Apr 16</td><td>60417.002</td><td>5.97 days</td><td>3600–5100</td><td>1200</td><td>VLT UT1+FORS2(1200b)</td></tr><tr><td>Apr 18</td><td>60418.627</td><td>7.60 days</td><td>3900–7600</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>Apr 22</td><td>60422.598</td><td>11.57 days</td><td>3900–8870</td><td>410</td><td>LJT+YFOSC(G3)</td></tr><tr><td>Apr 24</td><td>60424.980</td><td>13.95 days</td><td>8400–24000</td><td>6000</td><td>Magellan+FIRE</td></tr><tr><td>Apr 25</td><td>60425.626</td><td>14.60 days</td><td>3500–9500</td><td>3500</td><td>LJT+YFOSC(E9G10)</td></tr><tr><td>Apr 26</td><td>60426.554</td><td>15.52 days</td><td>3600–8800</td><td>410</td><td>LJT+YFOSC(G3)</td></tr><tr><td>Apr 29</td><td>60429.589</td><td>18.56 days</td><td>3900–7600</td><td>650</td><td>LJT+YFOSC(G14)</td></tr><tr><td>May 3</td><td>60433.557</td><td>22.53 days</td><td>3600–8800</td><td>410</td><td>LJT+YFOSC(G3)</td></tr><tr><td>May 8</td><td>60438.566</td><td>27.54 days</td><td>3600–8800</td><td>410</td><td>LJT+YFOSC(G3)</td></tr><tr><td>May 12</td><td>60442.556</td><td>31.53 days</td><td>3600–8800</td><td>410</td><td>LJT+YFOSC(G3)</td></tr></tbody></table>

**Note.**

<footnote><sup>a</sup> The epoch is relative to the first-light date, MJD = 60411.03.</footnote>
<figure><img src="image_11.png" /><figcaption>**Figure A3.** Gaussian and Lorentzian fits to the narrow and medium components of the Hα line in the spectra of SN 2024ggi at six selected phases. The spectral resolution of each spectrum is marked.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure A4.** Three sets of Na I D absorption in the middle-resolution spectra of SN 2024ggi. The dashed, dashed-dotted, and dotted lines represent the Na I D at different redshifts as marked in the legend.</figcaption></figure>
<table><caption>**Table A2**
Parameters of Na I D Lines and the Extinctions of SN 2024ggi</caption><thead><tr><th>Line<sup>a</sup></th><th>Obs. λ<br>(Å)</th><th>Δλ<br>(Å)</th><th>EW<br>(Å)</th><th>ΔEW<br>(Å)</th><th>E(B-V)<br>(mag)</th><th>ΔE(B-V)<br>(mag)</th></tr></thead><tbody><tr><td>D2<sub>MW</sub></td><td>5889.91</td><td>0.24</td><td>0.30</td><td>0.03</td><td>0.053</td><td>0.006</td></tr><tr><td>D2<sub>IMC</sub></td><td>5892.30</td><td>0.21</td><td>0.33</td><td>0.01</td><td>0.064</td><td>0.000</td></tr><tr><td>D1<sub>MW</sub></td><td>5895.98</td><td>0.62</td><td>0.16</td><td>0.02</td><td>0.042</td><td>0.003</td></tr><tr><td>D1<sub>IMC</sub></td><td>5898.26</td><td>0.16</td><td>0.22</td><td>0.03</td><td>0.062</td><td>0.006</td></tr><tr><td>D2<sub>Host</sub></td><td>5903.08</td><td>0.28</td><td>0.25</td><td>0.01</td><td>0.043</td><td>0.001</td></tr><tr><td>D1<sub>Host</sub></td><td>5909.10</td><td>0.52</td><td>0.21</td><td>0.01</td><td>0.056</td><td>0.002</td></tr></tbody></table>

**Note.**

<sup>a</sup> MW represents the absorption from the Milky Way, IMC represents absorption from intermediate clouds, and Host represents absorption from the host galaxy.

**ORCID iDs**

Jujia Zhang https://orcid.org/0000-0002-8296-2590

Luc Dessart https://orcid.org/0000-0003-0599-8407

Xiaofeng Wang https://orcid.org/0000-0002-7334-2357

Yi Yang https://orcid.org/0000-0002-6535-8500

Zhen Guo https://orcid.org/0000-0003-0292-4832

Lingzhi Wang https://orcid.org/0000-0002-1094-3817

Zeyi Zhao https://orcid.org/0009-0005-2963-7245

Shengyu Yan https://orcid.org/0009-0004-4256-1209

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