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Physics Astrophysics Astrophysics of Galaxies

astro-ph.GA Watch Astrophysics of Galaxies

Tuesday 6 October

Astrophysics of Galaxies

COSMOS-3D: Spectro-photometric Confirmation of Shabgard, a Cool, Metal-poor Brown Dwarf in the COSMOS Field

Bahareh S. Salmasi, Priyanka Jalan, Tomotsugu Goto et al. (+19 more)

Brown dwarfs are unique laboratories for studying cool atmospheres, the low-mass end of star formation, and the transition between stars and giant planets. Identifying and characterizing the coldest brown dwarfs remains challenging, particularly at faint magnitudes in deep extragalactic surveys. We investigate COSMOS2025 ID639188, hereafter Shabgard, a compact source previously selected as a brown-dwarf candidate from COSMOS-Web photometry. We use serendipitous JWST/NIRCam F444W wide-field slitless spectroscopy from COSMOS-3D, combined with COSMOS2025 NIRCam photometry and multi-epoch imaging. We extract a local-background-subtracted, profile-weighted $4.05$-$4.85\mu{\rm m}$ spectrum and compare the data with six atmosphere-model families. The LOWZ atmosphere-grid fit, using PCA interpolation and MCMC sampling, gives $T_{\rm eff}=718.6^{+10.6}_{-10.2}$ K and $[{\rm M/H}]=-1.00^{+0.07}_{-0.08}$, favoring a cool, subsolar-metallicity atmosphere near the late-T/Y transition. A fixed-atmosphere molecular-opacity test strongly favors a contribution from CH$_4$ to the observed $4$-$5 \mu{\rm m}$ spectral shape, although the restricted wavelength coverage prevents a molecule-by-molecule abundance determination. Model-dependent physical estimates span approximately $23$-$52 {\rm M_{\rm Jup}}$ and $140$-$180$ pc. We measure a proper motion of $|\mu|=89.2\pm0.4$ mas yr$^{-1}$, while the LOWZ MCMC flux scaling gives a distance of $176.2^{+18.4}_{-17.8}$pc. Together, these results support the classification of Shabgard as a cool, metal-poor Galactic brown dwarf and demonstrate the ability of JWST deep-field surveys to discover and characterize faint substellar objects.
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All authors Bahareh S. Salmasi, Priyanka Jalan, Tomotsugu Goto, Tetsuya Hashimoto, Feige Wang, Tayyaba Zafar, Koki Kakiichi, Ece Kilerci, Srivardini Ayyappan, Romain A. Meyer, Anilkumar Mailvaganam, Tamal Mukherjee, Xuanyi Lyu, Imam Uzma Aalia, Arihant Raidani, Samuel Carlson, Jose Luis Carrillo Martinez, Andreas L. Faisst, Marko Shuntov, Hollis B. Akins, Zihao Li, the COSMOS-3D Collaboration

Astrophysics of Galaxies

A Graph Attention Network Framework for Simultaneous Prediction of Galaxy and Dark Matter Halo Properties in GAMA DR4

Sandeep Aashish, Abhay V Hegde, Aditya Prakash Gupta et al. (+4 more)

We develop a graph-based deep learning model, GAMANet, to predict galaxy and halo properties using the GAMA DR4 dataset. Multiple catalogues are combined to construct a unified sample of 19,469 galaxies and derive a set of photometric, spectroscopic, morphological, and environmental features. The galaxy sample is represented as a graph, with galaxies as nodes and physically motivated connections based on spatial proximity and group membership as edges. A Graph Attention Network with GATv2 layers is then used to learn information from the galaxy neighbourhood while assigning different weights to neighbouring galaxies. The model simultaneously predicts halo mass, stellar mass, and a specific star formation rate proxy. On the test set, GAMANet achieves $R^2 = 0.958$, $0.985$, and $0.982$ for halo mass, stellar mass, and SSFR proxy, respectively, with an RMSE of $0.237$ dex for halo mass. These results demonstrate that graph-based learning can extract useful information about galaxy and halo properties directly from observational data while incorporating the environmental relationships encoded in the galaxy graph.
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All authors Sandeep Aashish, Abhay V Hegde, Aditya Prakash Gupta, Ashmit Verma, Md Saif, Nishanth Kumaran, Shivang Rayat

Astrophysics of Galaxies

The Role of the Cosmic Web in Galaxy Evolution Across 0.4<z<4

Sina Taamoli, Bahram Mobasher, Hossein Hatamnia et al. (+12 more)

We investigate the dependence of stellar mass and star formation activity on the topology of hosting large-scale structures using mass-complete galaxy samples in four redshift bins spanning from z = 0.4 to z = 4 using the COSMOS2020 catalog. Cosmic web components (clusters, filaments, and field) are identified by applying the Multiscale Morphology Filter to galaxy density maps, and group memberships (central, satellite, isolated) are assigned to each galaxy by the Friends of Friends (FoF) algorithm. Out to z < 1, we find that stellar mass increases toward denser structures, driven primarily by central and satellite galaxies, with differences up to ~0.35 dex between field and clusters. At these redshifts, star formation rate (SFR) is suppressed by ~0.1 dex in clusters compared to field galaxies, particularly for centrals, while the SFR of star-forming galaxies shows a much weaker dependence on the type of host structure, indicating that the quenched population primarily drives the observed environmental dependence. Specific SFR (sSFR) shows the strongest decline for centrals in clusters at z<1, consistent with enhanced quenching in dense environments. At 1<z<2, star formation activity becomes largely independent of environment, suggesting a transitional regime in the role of large-scale structure. At z>2, we observe a mild increase in SFR (~+0.1 dex) and sSFR (~+0.13 dex) from field to cluster galaxies, pointing to a possible reversal of environmental trends during early cosmic epochs. These results highlight the evolving influence of environment on galaxy evolution and set the stage for deeper investigations with upcoming wide-field surveys.
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All authors Sina Taamoli, Bahram Mobasher, Hossein Hatamnia, Nima Chartab, John R. Weaver, Ghassem Gozaliasl, Anton M. Koekemoer, Caitlin M. Casey, Jeyhan S. Kartaltepe, Aryana Haghjoo, Olivier Ilbert, Henry J. McCracken, David B. Sanders, Nicholas Scoville, Sune Toft

Astrophysics of Galaxies

Radiation pressure powers quasar broad absorption line winds but fails to drive galaxy feedback

Braden Seefeldt-Gail, Lluís Mas-Ribas and Norman Murray

Most cosmological simulations invoke quasar-driven winds as the primary mechanism quenching star formation in massive galaxies, assuming wind momentum flux approaches the radiation-pressure maximum, $L/c$. Using 81,388 broad absorption line troughs from the SDSS-IV DR16 quasar catalog and detailed mock trough spectra, we detect line locking, a spectral signature of radiation-pressure-driven outflows, in 60 to 70 percent of troughs, demonstrating that radiative line driving dominates wind acceleration and ruling out alternative mechanisms. Direct measurement of the wind energy budget yields a median kinetic luminosity of only 0.01 percent of bolometric luminosity, 50 times below the level required for effective feedback. Line-driven quasar winds are therefore insufficiently powerful to quench star formation, challenging the primary physical mechanism invoked for black hole feedback in galaxy formation models.
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Astrophysics of Galaxies

Stirred, not shaken: Dislodging $\omega$ Centauri from the Sausage galaxy through bar resonances

Axel Mannu, Chervin F. P. Laporte, Matthew D. A. Orkney et al. (+2 more)

We test the idea proposed by Laporte & Orkney that $\omega$ Centauri ($\omega$Cen) is the central nuclear star cluster of the Gaia-Sausage-Enceladus galaxy (GSE) which got dislodged by the bar. We run a series of simulations in time-evolving bar potentials with present-day pattern speeds within Portail et al's $\Omega_{b}=39\pm3.5\,\rm{km\,s^{-1}\,kpc^{-1}}$ and rate of slow-down $\dot{\Omega}_{\rm b}= 4.5 \pm 1.4 \,\rm{km \,s^{-1}\,kpc^{-1}\, Gyr^{-1}}$ to study how bar resonances modulate the angular momentum and energies of both GSE-debris and $\omega$Cen's location in $(E,L_{z})$ space. We find that the retrograde $3:2$ resonance is capable of inflicting changes of $\Delta E\approx0.4\times10^{5}\,\rm{km^{2}\, s^{-2}}$, $\Delta L_{z}\approx0.3-0.5\times10^{3}\, \rm{km \, s^{-1}\, kpc^{-1}}$ within timescales of $\Delta t\approx 0.5-1\,\rm{Gyr}$, well within the bar's lifespan $t_{\rm{bar}}\approx10-8\,\rm{Gyr}$. These results are independent of the choice of potential or modelling. We uncover a sample of N-rich stars with metallicities $-2.0<[\rm{Fe/H}]<-1.0$ and aluminium enrichments consistent with $\omega$Cen. In ($E,L_{z}$) space, the map reveals a corridor of trailing N-rich stars clustered about the region connecting $\omega$Cen to the GSE's debris centroid, matching $\omega$Cen's past predicted track. This provides the best evidence yet that $\omega$Cen is the nuclear star cluster of the GSE. Using M54 and $\omega$Cen's chemistry we show that neither first nor second generations follow general galaxy-wide scale chemical enrichment trends. This is empirical evidence that GC chemical trends are a cluster-scale physics problem, not a galaxy-wide one.
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All authors Axel Mannu, Chervin F. P. Laporte, Matthew D. A. Orkney, Yassin Rany Khalil, Benoit Famaey

Monday 5 October

Astrophysics of Galaxies

Barium and europium abundances from the Gaia-ESO Survey using The Cannon

Pradosh Barun Das, Francesca Primas, Gayandhi De Silva et al. (+3 more)

Neutron-capture element abundances encode the time-dependent interplay between slow (s-) and rapid (r-) nucleosynthetic processes and provide critical constraints on Galactic chemical evolution. In particular, barium and europium provide important tracers of s-process and r-process nucleosynthetic enrichment, respectively. Large spectroscopic surveys enable robust statistical studies of these elements; however, determining reliable elemental abundances at intermediate spectral resolution remains challenging. Our aim was to derive homogeneous and statistically well-characterised stellar parameters and chemical abundances of Ba and Eu (hereafter collectively referred to as stellar labels) for the Gaia-ESO Survey HR15N dataset. We applied The Cannon to 36,263 FLAMES/GIRAFFE HR15N spectra (R~19,200), using a model trained on 782 stars with high-quality reference stellar labels drawn from the Gaia-ESO DR5 homogenised catalogue. The model infers Teff, log g, [Fe/H], v sin i, A(Ba), and A(Eu); its reliability was quantified through multi-dimensional quality flags and detection strength of the Ba and Eu features. Our final catalogue contains stellar parameters for 36,263 spectra, with ~21,000 and ~400 reliable Ba and Eu abundances, respectively, satisfying multiple flagging criteria. We obtained median uncertainties of ~38 K in Teff, ~0.08 dex in log g, ~0.04 dex in [Fe/H], ~0.06 dex in A(Ba), and ~0.02 dex in A(Eu). External validation against the Gaia Benchmark Stars, GALAH DR4, and multiple open clusters shows good agreement across independent reference datasets. The resulting catalogue includes newly determined Ba and Eu abundances that complement those available from previous analyses, thereby enabling investigations of neutron-capture enrichment patterns in open clusters. The model framework can readily be adapted to upcoming wide-field spectroscopic surveys such as 4MOST, WEAVE, and WST.
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All authors Pradosh Barun Das, Francesca Primas, Gayandhi De Silva, Guillaume Guiglion, Daniel B. Zucker, Melissa Ness

Astrophysics of Galaxies

The Rogue One: A Dark Matter-Dominated Dwarf in the Trail of Dark Matter-Deficient Galaxies

Daniel Vaz, Arsen Levitskiy, Duncan Forbes et al. (+6 more)

We present the first measurement of the stellar velocity dispersion of LEDA 4014647, a compact dwarf galaxy spatially aligned with the NGC 1052 trail, a linear chain of dark matter-deficient galaxies (DMDGs) proposed to have formed in a single high-speed dwarf$-$dwarf collision $\sim 8$ Gyr ago. Using integral-field spectroscopy obtained with the Keck Cosmic Web Imager (KCWI), we measure a stellar velocity dispersion of $\sigma = 15.8 \pm 2.0$ km s$^{-1}$ within the half-light radius ($R_\mathrm{e}$). We derive a dynamical mass within the 3D half-light radius of $M_\mathrm{dyn}(<r_{1/2}) = (1.5 \pm 0.4) \times 10^8\,M_\odot$, a factor of $\sim 3$ larger than the enclosed stellar mass, implying a DM fraction of $f_\mathrm{DM} = 0.7 \pm 0.1$ at $r_{1/2}$. Full-spectrum stellar population fitting yields an old ($t \approx 13$ Gyr) and metal-poor ($[\mathrm{M/H}] \approx -1.2$ dex) stellar population, consistent with independent photometric estimates and predating the proposed collision epoch. LEDA 4014647 is therefore a DM-dominated system, and cannot have formed from the DM-free, shock-compressed gas that produced the proposed trail DMDGs DF2, DF4, and DF9. Its nature, however, remains an open question: while a chance-projection along the trail axis is a possible explanation, its old stellar population, compact morphology, and DM content are also consistent with the surviving collisionless remnant of one of the progenitor dwarfs of the dwarf$-$dwarf collision. Distinguishing between these two scenarios will ultimately require deeper spectroscopy to tighten the stellar population and DM halo constraints, and targeted $N$-body and hydrodynamical simulations tracking the post-encounter fate of the progenitors.
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All authors Daniel Vaz, Arsen Levitskiy, Duncan Forbes, Aaron J. Romanowsky, Jonah Gannon, Jean P. Brodie, Yimeng Tang, Kenji Bekki, Warrick J. Couch

Astrophysics of Galaxies

Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) III. CH$_3$OH isotopic fractionation in the low-mass protostar BHR71-IRS1

A. Coutens, F. Cruz-Sáenz de Miera, J. K. Jørgensen et al. (+26 more)

[Abridged] Methanol is a complex organic molecule detected in both ice and gas toward star-forming regions. Due to its high abundance, its rarer isotopologues, including the deuterated ones, are also commonly detected. Our aim is to determine the column density and deuterium fractionation of methanol in the low-mass protostar BHR71-IRS1. We analyzed data from a large spectral survey of BHR71-IRS1 carried out as part of the ALMA Large Program Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS). We used the CASSIS software to identify the lines of the methanol isotopologues and constrain their column densities under the assumption of local thermodynamic equilibrium. We detected CH$_3$OH, $^{13}$CH$_3$OH, CH$_3$$^{18}$OH, CH$_3$OD, CH$_2$DOH, CHD$_2$OH, and CD$_3$OH at the continuum peak position. In addition, CH$_3$$^{17}$OH is tentatively detected, while CD$_3$OD is not detected, although a few faint lines could be present toward a position redshifted from the continuum peak. The column densities of the main isotopologue obtained from $^{13}$CH$_3$OH and CH$_3$$^{18}$OH are consistent, assuming nominal $^{12}$C/$^{13}$C and $^{16}$O/$^{18}$O ratios of 68 and 557, respectively. The CH$_2$DOH/CH$_3$OD column density ratio is equal to 7$\pm$2. The statistically corrected D/H ratio is higher for CH$_2$DOH (1.9$\pm$0.6 %) than for CH$_3$OD (0.8$\pm$0.2 %). The level of deuteration after statistical correction increases with the number of D atoms in the methyl group, reaching 23$\pm$5 % for CD$_3$OH/CH$_3$OH. These trends are similar to other low-mass protostars such as IRAS~16293–2422 and HOPS 373SW, but seem to differ from the eruptive young star V883-Ori. Similar studies with comparable datasets will be carried out for the other COMPASS sources to determine if factors such as environmental conditions and evolutionary states impact the deuteration of methanol.
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All authors A. Coutens, F. Cruz-Sáenz de Miera, J. K. Jørgensen, M. N. Drozdovskaya, J.-E. Lee, A. L. Plunkett, D. Harsono, S. Maret, A. Belloche, J. Ferrer Asensio, M. L. R. van't Hoff, J.-H. Jeong, Á. Kóspál, B. A. McGuire, P. Nazari, S. Spezzano, Z. Telkamp, Y.-L. Yang, H.-S. Yun, S. Zeng, A. Andreu, J. Bergner, C.-H. Kim, N. F. W. Ligterink, Y. Lin, S.-Y. Liu, M. Lützen, P. Marchand, M. Rao

Astrophysics of Galaxies

Life in the Cosmic Neighbourhood: Galactic Habitable Zones in the EAGLE Simulations

Luke A. Barnes, Geraint F.Lewis, Jaime Salcido et al. (+3 more)

When and where is life most likely to form in the universe? We use the EAGLE cosmological hydrodynamical simulations to investigate the prevalence and properties of life-friendly cosmic environments on galactic scales. Our model for the occurrence of observers depends on main-sequence stars, the production and distribution of metals, and the formation of habitable planets. We also take into account five potentially life-extinguishing processes: core-collapse supernovae, Type Ia supernovae, star-star interactions, gamma ray bursts and quasars. We characterise the amount of time that the universe creates on habitable planets per unit mass of baryons and stars in the universe, as a function of galaxy mass and galactocentric distance. As expected, early universe, newly formed, high star formation rate galaxies are the most dangerous. The present-day Milky Way is typical of habitable galaxies: efficient creation of habitable time, and low probability of cosmic catastrophes.
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All authors Luke A. Barnes, Geraint F.Lewis, Jaime Salcido, Pascal Elahi, Miroslav D. Filipovic, Richard G. Bower

Astrophysics of Galaxies

BRAHMa: Bar Recognition And Hatching using Machine learning – a framework for oriented bounding box detection of galactic bars

Rajit Shrivastava, Narendra Nath Patra and Keerthi K

Galactic bars are fundamental components in studies of dark matter, galaxy evolution, and secular dynamics. However, current methods for measuring bar properties suffer from significant limitations. Manual and crowdsourced catalogs are not scalable to next-generation surveys, while recent machine learning (ML) approaches based on pixel-wise segmentation are computationally intensive and require complex post-processing to extract physical parameters. In this paper, we introduce BRAHMa (Bar Recognition And Hatching using Machine Learning), a publicly available tool based on an oriented-bounding-box YOLO11x model, designed for the rapid and objective detection of galactic bars. Our model was initially trained on synthetic data (Shrivastava 2025), validating its efficacy before training with real data. BRAHMa was trained on bar masks from the Galaxy Zoo 3D (GZ-3D) citizen science project, utilizing images from the DESI Legacy Survey. We address the inherent challenge of 'ground truth' in astronomical data by comparing GZ-3D labels with the Hoyle dataset for 586 common galaxies. This comparison yields a baseline mean absolute error of 0.99 kpc between the GZ3D-derived and Hoyle measurements for 586 common galaxies, providing an empirical reference for catalogue-to-catalogue disagreement. Our BRAHMa tool achieves an L1 error of 1.1 kpc on a large set of 3,150 galaxies from the Hoyle dataset, demonstrating performance that approaches this fundamental limit. We further validate the model's robustness by applying it to distinct datasets, proving its generalizability. BRAHMa provides a scalable and reliable method for bar extraction, enabling large-scale morphological studies. The tool is publicly available through the BRAHMa web interface. (https://brahma-bar-detector.netlify.app/)
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Astrophysics of Galaxies

Occam's Razor and a Three-Family Ultralight Fermionic Dark Matter Model

Fabian Hernandez-Gutierrez and Juan Barranco

Inspired by the three generations in the Standard Model, we propose that dark matter particles are simply three non-interacting ultralight fermions. This model is tested at galactic scales with the SPARC catalog of rotational velocity curves. This is done by modeling the dark matter halo as the self-gravitating configuration of three gravitationally coupled fermion species. This configuration is capable of forming more complex rotational curves which gives it flexibility to describe more complex and diverse morphologies of galaxy rotation curves. The core of the halos will be described by degenerate fermions and the transitions from a degenerate core to a non-degenerate halo will be described by either a perfect-fluid-like halo or a NFW density profile. In order to construct the priors of the three-fermion model, first we performed a single-fermion analysis and obtained three characteristic mass regions around 20, 40, and 65-70 eV, for both halo types. It is found that the three-component fermion model improves the goodness of fit for 94.9% of the galaxies considered. To estimate if the additional parameters are favored despite the added complexity by the corresponding improvement in the fit, the information criteria of AICc and BIC were computed and we found that more than $50\%$ of the galaxies analyzed prefer the three-component fermion model over a single fermion model. The three-fermion model is particularly favored for galaxies with larger radial extent and more complex rotation curves. The multicomponent fermionic dark sector is able to describe a large diversity of rotation curves while maintaining characteristic fermion mass scales across the galaxy sample.
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Astrophysics of Galaxies

The universal thickness of mid-infrared bubbles

Michael M. Schulreich and A. Beate C. Patzer

Several hundred ring-like structures seen at $8\mu$m in the Spitzer GLIMPSE surveys share a uniform morphology: the shell thickness $\Delta R$ is a nearly constant fraction of the outer radius $R$, $\Delta R/R\sim0.3$, across a factor of 40 in size and all exciting spectral types. This invariance is unexplained. We ask what sets this thickness and whether its near-constancy is physical or a measurement artifact. We derive analytic constraints on the location, temperature, and heating of the emitting grains, and confront the predicted scaling with radius against the GLIMPSE I and II catalogs (591 objects, 91 with kinematic distances). Grains in radiative equilibrium at the observed radii reach only 20-35 K and fall short of the required $8\mu$m emissivity by more than 20 orders of magnitude; the emission must come from stochastically heated PAHs and very small grains, and therefore from the photon-dominated region, where the ring width is the far-ultraviolet penetration depth. This gives $\Delta R/R=A_V^{\rm PDR}/A_V^{\rm cloud}$, independent of radius; a swept-up shell instead predicts $\Delta R/R\propto(R/R_S)^{3/2}$, with $R_S$ the initial Strömgren radius. We find a pronounced angular-resolution systematic that inflates the ratio for the smallest bubbles; once this is accounted for, $\Delta R/R=0.217$, with a residual dependence on physical radius of $0.03\pm0.05$, consistent with zero. A fixed ambient density is excluded at $20\sigma$, the swept-up shell at 9-28$\sigma$. The implied columns are $N_{\rm cloud}\approx8.7\times10^{21}$ cm$^{-2}$ with only 0.112 dex of scatter. The near-universal appearance of mid-infrared bubbles follows from the ratio of two nearly invariant column densities – the dust-opacity-fixed far-ultraviolet shielding column and the Larson-like column of the parent molecular cloud – and requires no reference to stellar winds.
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Astrophysics of Galaxies · Cross-listed from astro-ph.HE

A spatially resolved view of Fast Radio Burst host metallicities with Integral-field Spectroscopy

Sunil Simha, Hsiao-Wen Chen, Wen-fai Fong et al. (+15 more)

Magnetar sources are the leading theory for Fast Radio Burst (FRB) progenitors. If magnetars are indeed the dominant sources, we may thus expect any metallicity preference for magnetar formation to be reflected in the FRB site in host galaxies. Here, we leverage Integral Field Unit (IFU) spectroscopy to spatially resolve gas-phase metallicity distribution in five FRB host galaxies: FRB 20240210A, FRB 20240312D, FRB 20250227A, FRB 20211127I, and FRB 20250316A, at $z < 0.05$, including the FRB site, with the O3N2 index. We observed the first four sources with the newly commissioned Large Lenslet Array Magellan Spectrograph (LLAMAS) IFU on the Magellan Telescopes and used archival Keck/KCWI data for FRB 20250316A. We find ISM metallicity variations of at least 0.3 dex within individual hosts for the majority of the sample, with median values spanning $12 + log(O/H) \approx$ 8.5–8.7. At the burst sites, four have metallicities of $12 + \log(O/H) \approx 8.5$–8.9, while the fifth has a metallicity below 8.5. These findings reveal substantial metallicity variations both among FRB host galaxies and within the local environments of the bursts. Consequently, our study shows that longslit-based global estimates of host galaxy metallicity do not always represent the true burst-site distributions. We therefore advocate for IFU follow-up of larger host samples to ultimately provide FRB-site-relevant observational constraints on metallicity preferences for FRB occurrence.
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All authors Sunil Simha, Hsiao-Wen Chen, Wen-fai Fong, Sarah Hughes, Yuxin Dong, J. Xavier Prochaska, Nicolas Tejos, Tarraneh Eftekhari, Alexa C. Gordon, Stuart D. Ryder, Mawson W. Sammons, Alice P. Curtin, Victoria M. Kaspi, Adam E. Lanman, Calvin Leung, Kiyoshi W. Masui, Aaron B. Pearlman, Ryan M. Shannon

Astrophysics of Galaxies

Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) VII. First interstellar detection of fully deuterated methanol

A. Belloche, S. Spezzano, A. Coutens et al. (+12 more)

There have been no robust identifications of fully deuterated methanol, CD3OD, in the interstellar medium (ISM) to date. Our goal is continue the search for CD3OD in star-forming regions to determine the deuteration pattern of methanol and set constraints on the deuteration process. We used a sensitive imaging spectral survey of the protostellar system IRAS4A in the NGC1333 molecular cloud performed as part of the large program carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) called Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS). The column densities of methanol and several of its less abundant isotopologs, including deuterated ones, were derived under the assumption of local thermodynamic equilibrium (LTE). Along with the detection of several methanol isotopologs toward the binary component IRAS4A2, we report the first interstellar detection of CD3OD. The deuteration pattern of methanol in IRAS4A2 is similar to the pattern measured in other low-mass protostars, with an increase in the apparent D/H ratio from the singly to multiply deuterated isotopologs. Chemical models of prestellar and protostellar cores focused on deuteration do not reproduce this pattern quantitatively. This discrepancy is even more exacerbated by the detection of CD3OD. Our LTE modeling of the spectra yields low column density ratios of methanol to its 13C and 18O isotopologs while the 13C to 18O and 18O to 17O ratios are consistent with the isotopic ratios expected in the local ISM. We suspect that the column density of methanol is underestimated. One possible explanation is that the emission is clumpy rather than monolithic. The first detection of a fully deuterated COM highlights the power of deuterium fractionation as a diagnostic tool for investigating the formation and inheritance of organic molecules during the process of star formation.
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All authors A. Belloche, S. Spezzano, A. Coutens, M. N. Drozdovskaya, J. K. Jørgensen, J.-E. Lee, A. L. Plunkett, D. Harsono, S. Maret, F. Cruz-Sáenz de Miera, J.-H. Jeong, P. Nazari, Y.-L. Yang, H.-S. Yun, S. Zeng

Astrophysics of Galaxies

Multiple H I 21-cm absorbers towards blazar PKS 1158+007

Abhisek Mohapatra, R. Srianand and R. Dutta

We report an interesting line of sight towards the blazar PKS 1158+007 (J1201+0028) ($z_{\rm em}$ = 1.369), revealing the rare detection of two intervening HI 21-cm absorption systems within a narrow redshift interval ($\Delta z \sim 0.04$) using the MeerKAT radio telescope. The stronger absorber at $z_{\rm abs}$ $=0.5653$ ($\int\tau dv = 1.31\pm0.04$ \kms) is associated with a strong MgII $\lambda2796$ system seen in the Sloan Digital Sky Survey (SDSS) and Dark Energy Spectroscopic Instrument (DESI) spectra, with rest-frame equivalent widths of $W_{2796} = 1.40\pm0.07$~Å\ and $1.21\pm0.11$~Å, respectively. In stark contrast, the second detection at $z_{\rm abs}$ $= 0.5305$ reveals a distinct HI 21-cm absorption feature ($\int\tau dv = 0.31 \pm 0.04$ \kms) with no associated metal absorption detectable in the optical spectra. This system represents a prime example of a "metal-poor” HI 21-cm absorber. Using photometric redshifts from Hyper Suprime-Cam (HSC) PDR3 and DESI Legacy Surveys DR10, we identify a broad galaxy overdensity (at $z \sim0.53\pm0.06$) coincident with these absorbers, suggesting the sightline pierces a complex large-scale structure or filament, which would be interesting to confirm with follow-up observations. These results underscore the power of blind HI 21-cm surveys to uncover the "invisible” cold gas phase of the Universe that optical/UV metal-line selection may systematically miss.
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Astrophysics of Galaxies

Hierarchical Black Hole Mergers at High Redshift: Predictions for LISA and LGWA from SEEDZ

Daxal H. Mehta, Lewis R. Prole, John A. Regan et al. (+13 more)

We investigate the hierarchical assembly of black holes (BHs) in the early Universe using the next-generation SEEDZ cosmological zoom-in simulations. Our suite follows two independent regions (Rarepeak and Normal1) evolved with three BH feedback prescriptions: no feedback (NoFB), weak feedback (WeakFB), and standard thermal isotropic feedback (FidBH). Focusing on BH mergers at $z \geq 10$, we explore merger-driven BH growth and detectability with next-generation gravitational-wave observatories. We find that stronger AGN feedback models dramatically suppress hierarchical BH assembly compared to weaker feedback implementations. In the NoFB simulations, BHs frequently undergo repeated mergers, with some experiencing more than 50 merger events within a few hundred Myr and gaining over $10^8,\mathrm{M_{\odot}}$ through mergers alone. In contrast, BHs in the FidBH simulations rarely undergo more than three mergers. NoFB therefore represents an upper limit to BH merger-driven growth beyond what is expected in the Universe. The suppression of BH mergers in the FidBH case is not driven by differences in galaxy assembly, BH occupation fractions, or BH seed abundances, which remain broadly similar across the simulation suites. Instead, feedback disrupts the dense gas surrounding newly formed BH seeds, suppressing their early accretion-driven growth and preventing them from entering runaway hierarchical merger pathways. Our results demonstrate that early-Universe feedback prescriptions strongly influence hierarchical BH growth through both mergers and accretion, producing distinct high-redshift merger populations. Coupled with seeding choices and other BH subgrid prescriptions, this introduces multiple degeneracies that will need to be broken in advance of LISA and other next-generation GW facilities.
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All authors Daxal H. Mehta, Lewis R. Prole, John A. Regan, Rüdiger Pakmor, Sophie Koudmani, Ricarda S. Beckmann, Michael Tremmel, John H. Wise, Martin A. Bourne, Ralf S. Klessen, Simon C. O. Glover, Martin G. Haehnelt, John Brennan, Pelle Van De Bor, Debora Sijacki, Paul C. Clark

Astrophysics of Galaxies

Spectroscopic Confirmation of the Milky Way Satellites Boötes V and Leo Minor I and the LMC Satellite DELVE 2: Three Small Ultra-Faint Dwarf Galaxies

W. Cerny, M. Geha, J. D. Simon et al. (+22 more)

We present the first multi-object spectroscopy of Boötes V, Leo Minor I, and DELVE 2 – three small (20 pc $\le r_{1/2}\le$ 26 pc), ultra-faint ($-3.2\le M_V\le-2.1$) stellar systems in the outer Galactic halo (71 kpc $\le D_\odot\le$ 102 kpc) with heretofore ambiguous classifications. Leveraging at least two epochs of Keck/DEIMOS or Magellan/IMACS observations targeting each satellite, we identify 13, 7, and 7 spectroscopic members in the three systems, respectively. We tentatively resolve a $\sigma_v=2.1^{+1.5}_{-0.9}$ km s$^{-1}$ velocity dispersion for DELVE 2 but place upper limits of $\sigma_v<2.7$ km s$^{-1}$ and $\sigma_v<7.8$ km s$^{-1}$ for Boötes V and Leo Minor I. We find that all three systems are very metal-poor, with mean iron abundances $-2.85\le\overline{\rm [Fe/H]}\le-2.15$. Both Boötes V and Leo Minor I display unambiguous metallicity spreads ($\sigma_{\rm [Fe/H]}=0.46^{+0.18}_{-0.12}$ dex and $\sigma_{\rm [Fe/H]}=0.57^{+0.23}_{-0.18}$ dex), while DELVE 2 displays a modestly significant spread ($\sigma_{\rm [Fe/H]}=0.52^{+0.29}_{-0.25}$ dex) across three stars. Based primarily on these metallicity measurements, we confidently classify Boötes V and Leo Minor I as dwarf galaxies and determine that DELVE 2 is most likely a dwarf galaxy as well. We explore the orbit of each satellite in a joint Milky Way + LMC potential, confirming for the first time that DELVE 2 is a long-term bound satellite of the LMC. Finally, we review the empirical Milky Way satellite classification landscape and find that nearly every spectroscopically characterized satellite with azimuthally averaged half-light radius $r_{1/2}>15$ pc and surface brightness $\mu_{V,1/2}>26.5$ mag arcsec$^{-2}$ is likely a dwarf galaxy. This result can guide initial probabilistic classifications for satellites discovered by $\textit{Rubin}$, $\textit{Roman}$, and $\textit{Euclid}$.
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All authors W. Cerny, M. Geha, J. D. Simon, T. S. Li, A. B. Pace, R. P. van der Marel, N. Kallivayalil, A. P. Ji, A. Drlica-Wagner, D. Crnojević, G. Limberg, N. E. D. Noël, A. H. Riley, D. J. Sand, G. S. Stringfellow, Y. Choi, P. S. Ferguson, C. E. Martínez-Vázquez, P. Massana, G. E. Medina, B. Mutlu-Pakdil, J. D. Sakowska, C. Y. Tan, A. K. Vivas, J. Bruce

Astrophysics of Galaxies

Molecular hydrogen formation on dust: The impact of gas-dust drift on formation efficiency

Stefan Reissl, Simon C. O. Glover, Ralf S. Klessen et al. (+2 more)

Molecular hydrogen is predominantly formed on dust-grain surfaces in the interstellar medium, where relative gas-dust motion can arise in dynamically active environments. While the dependence of H$_2$ formation on grain temperature and surface properties is well studied, the impact of gas-dust drift has received little attention. We investigate how gas-dust drift modifies H$_2$ formation, focusing on the competition between the drift-enhanced H-atom collision rate and reduced sticking at higher impact energies. We use an event-driven kinetic Monte Carlo model that follows individual H atoms on spherical silicate and carbonaceous grains, including adsorption, surface migration, thermal desorption, and Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) reactions. Drift is described by a shifted Maxwellian velocity distribution, and we compare constant and impact-energy-dependent sticking probabilities. Drift produces increasingly anisotropic distributions of adsorbed H and H$_2$ formation across the grain surface. Assuming constant sticking, increasing drift enhances H$_2$ formation through the higher collision rate, with efficiencies up to $\epsilon=0.3-0.4$. With energy-dependent sticking, strong drift instead suppresses formation on both materials, reducing efficiencies to $\epsilon=0.01-0.03$. Carbonaceous grains remain efficient to higher dust temperatures than silicate grains. ER reactions dominate over most of the investigated parameter space and become increasingly important at strong drift as the reduced surface population suppresses LH reactions. Thus, enhanced collision rates under gas-dust drift do not necessarily increase H$_2$ formation. Models of dynamically active environments should account for both relative gas-dust velocities and their effects on sticking.
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All authors Stefan Reissl, Simon C. O. Glover, Ralf S. Klessen, Liam S. Morrissey, Mordecai-Mark Mac Low

Astrophysics of Galaxies

DIGging into HII regions: the role of ionizing photon leakage and spectral hardening in coupling HII regions and diffuse ionized gas

Dalya Baron, Kathryn Kreckel, Gwen C. Rudie et al. (+15 more)

The optical emission-line ratios of star-forming galaxies are broadly understood in the framework of photoionization, yet some systematic trends remain unexplained. Using 50–100 pc resolution PHANGS-MUSE observations of 19 nearby galaxies, we investigate how escaping ionizing radiation couples HII regions to the diffuse ionized gas (DIG), analyzing thousands of HII regions and DIG sightlines. HII regions and the DIG form a continuous sequence across standard line-ratio diagnostic diagrams (e.g., BPT diagrams), where progressively fainter regions show elevated high- and low-ionization line ratios, and the DIG extends the same trends into the composite–LINER regime. Line-ratio variations with global specific star formation rate (sSFR) suggest progressively harder ionizing radiation with increasing star formation activity, contrary to expectations for emission increasingly dominated by older stellar populations. Line-of-sight mixing explains part of the behavior but cannot reproduce all line-ratio trends. We therefore develop a model in which ionizing photons escape HII regions and propagate through the diffuse ISM, undergoing geometric dilution, attenuation, and spectral hardening. The model predicts that progressively fainter HII regions are increasingly ionized by leaked radiation from neighboring luminous regions, implying radiative coupling between HII regions. Joint modeling of HII regions and the DIG reproduces the HII region luminosity distribution and DIG surface brightness for escape fractions of $\sim$0.4$-$0.6 and ionizing-photon mean free paths of $\sim$0.5$-$1 kpc, and qualitatively reproduces the observed line-ratio sequences and their dependence on sSFR. Our results suggest that ionizing photon leakage may provide an important non-local feedback channel linking compact star-forming sites to large-scale galaxy disks.
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All authors Dalya Baron, Kathryn Kreckel, Gwen C. Rudie, Karin M. Sandstrom, Alberto D. Bolatto, Guillermo A. Blanc, Oleg V. Egorov, Eva Schinnerer, Ashley T. Barnes, Enrico Congiu, Daniel A. Dale, Simon C. O. Glover, Jaeyeon Kim, Ralf S. Klessen, José Eduardo Méndez-Delgado, Justus Neumann, Ismael Pessa, Thomas G. Williams

Astrophysics of Galaxies · Cross-listed from astro-ph.HE

Uncovering the Physical Origins of Black Holes in the Milky Way for Astronomical Surveys

Ilia Kiato, Pierson Lipschultz, Vicky Kalogera et al. (+5 more)

The observational landscape of stellar-mass black holes (BHs) in the Milky Way is expanding beyond X-ray binaries, with Gaia DR4 and Roman expected to open new windows onto BH populations. We use the binary population synthesis framework POSYDON to predict the present-day Galactic BH population and connect it to its evolutionary origins. We test the robustness of these predictions to uncertainties in binary evolution and BH formation, including core-collapse mechanisms, natal kicks, common-envelope efficiency, metallicity, and initial binary distributions. Across our models, we find that most Galactic BHs are isolated, with stellar mergers providing the largest contribution and binary disruptions the second largest. Among surviving binaries, detached BH-BH systems dominate. We identify a distinct, narrow feature at higher BH masses ($\simeq13\,\mathrm{M}_{\odot}$ in the fiducial model) produced by wind-driven evolution of massive Wolf-Rayet progenitors. This feature is particularly prominent among detached BH-BH binaries and persists across variations in binary-evolution and BH-formation physics, but depends strongly on metallicity. At lower masses, these BH-BH systems predominantly originate from progenitors that undergo stable mass transfer before BH formation. Finally, for more direct observational applications, we embed the synthetic populations in a Galactic model, evolve their kinematics, perform first-order microlensing calculations for BH-BH lenses, and examine BH systems with luminous companions in the context of the Gaia BHs. In the latter, we uncover a robust stable-mass-transfer population largely absent in rapid population-synthesis predictions. These results provide an evolutionary framework for interpreting the emerging Galactic BH population across complementary observational channels.
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All authors Ilia Kiato, Pierson Lipschultz, Vicky Kalogera, Jeff J. Andrews, Max M. Briel, Abhishek Chattaraj, Seth Gossage, Philipp M. Srivastava

Astrophysics of Galaxies · Cross-listed from astro-ph.CO

And Then There WEre Baryons (ATWEB): The Baryon Budget of the Universe in Halos and the IGM over 13 Billion Years of Cosmic Time

Mohammadreza Ayromlou, G. Mark Voit, Thomas H. Reiprich et al. (+2 more)

Observations of the local Universe show that, except in the most massive galaxy clusters, the baryon fraction within halos falls below the cosmic mean. This deficit can be characterized using the previously established closure radius, the halocentric distance within which the enclosed baryon fraction reaches the cosmic value. Using the TNG, EAGLE and SIMBA simulations, we examine how, when, and on what scales the baryonic and dark matter distributions diverge. At z=0 less than 30% of cosmic baryons reside within galaxy-hosting halos, compared with ~45% of the dark matter, leaving over 70% of all baryons in the IGM. Building on the closure radius, we introduce three further characteristic scales: the compensation radius, closure envelope, and compensation envelope. The compensation radius marks where the enclosed total mass recovers its gravity-only value, and today it typically lies inside the closure radius in all three models. The closure and compensation envelopes are the largest median radii across halo mass. Galaxy groups provide the largest contribution per dex of halo mass to IGM enrichment with halo-associated baryons and redistribute baryons over the largest distances. They set the closure envelope, which reaches ~2-6.5 Mpc at z=0, depending on the model. The closure radius usually increases with cosmic time at fixed halo mass. Tracking individual halos, we find open baryon cycles with substantial redistribution beyond halo boundaries across all halo masses, while even the progenitors of today's most massive clusters fall below the cosmic baryon fraction. The compensation envelope grows toward low redshift, reaching ~1.5-5.5 Mpc at z=0. Across cosmic time, it coincides with the scale on which the matter power spectrum recovers its gravity-only form, establishing a direct link between the spatial extent of baryon redistribution and the impact of feedback on large-scale structure.
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All authors Mohammadreza Ayromlou, G. Mark Voit, Thomas H. Reiprich, Cristiano Porciani, Dominique Eckert

Astrophysics of Galaxies

Cracking open the "dragon egg": ALMA 1.3mm observations of a compact, young high-mass core candidate at 30mas resolution

A. T. Barnes, J. D. Henshaw, Á. Sánchez-Monge et al. (+14 more)

Massive prestellar cores are a central assumption of core-accretion models of high-mass star formation, yet convincing observational examples remain rare. The "dragon egg" (C2c1a) core in the infrared-dark cloud G028.37+00.07 has been proposed as a promising compact high-mass core candidate. We present new ALMA Band 6 observations at $0.035''$-$0.078''$ ($\sim175$-400 au at 5 kpc), providing one of the most stringent high-resolution tests of such a candidate. The region resolves into multiple fragments, but the brightest component, C2c1a(a), remains a single compact continuum peak down to $\sim175$ au, with a deconvolved radius of $\sim87$ au and a nearest neighbouring fragment at a projected separation of $\sim2400$ au. The continuum peak reaches a brightness temperature of $\sim40$ K, exceeding the $\sim10$ K gas temperatures inferred from lower-resolution NH$_3$ observations and indicating a strong radial temperature gradient. Continuum-based mass estimates range from $\sim2$ to $\sim30$ $M_\odot$ (implying extreme densities of $n_\mathrm{H2} > 10^9$cm$^{-3}$), depending on temperature, while lower-resolution data indicate a much larger mass reservoir of $\sim110$ $M_\odot$ on $\sim10^4$ au scales. The spectrum shows no strong hot-core tracers and only weak, spatially extended H$_2$CO and CO emission; no high-velocity outflow can be unambiguously associated with C2c1a(a). We conclude that C2c1a(a) is an exceptionally compact core embedded within a massive cold reservoir, with no additional compact fragments detected within the present resolution and sensitivity limits, potentially tracing the earliest formation of a central protostellar "seed" within a still-dominant reservoir. [abridged]
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All authors A. T. Barnes, J. D. Henshaw, Á. Sánchez-Monge, P. Caselli, I. Jiménez-Serra, J. E. Pineda, K. Immer, M. De Simone, R. J. Parker, F. Fontani, E. Redaelli, S. N. Longmore, L. Böhm, Q. Zhang, C. Y. Law, G. Cosentino, J. C. Tan

Astrophysics of Galaxies · Cross-listed from astro-ph.EP

First Direct Identification of a Multi-Star Microlens System Hosting a Planet

Sean K. Terry, David P. Bennett, Jean-Philippe Beaulieu et al. (+11 more)

Nearly 25 years have passed since gravitational microlensing delivered its first exoplanet discoveries. The relative proper motion between lens and source systems allows them to be resolved as they separate on the sky over many years. This enables scientists to study light coming directly from the lenses and place tight constraints on the physical properties of these planetary systems. In this paper we present an analysis of high angular resolution images of the microlensing target OGLE-2006-BLG-284 using Keck adaptive optics and HST data taken more than 15 years after the event. We clearly detect the lens system and measure its motion across the Keck and HST epochs. We measure a lens-source relative proper motion of $5.80 \pm 0.15$ mas yr$^{-1}$. A combination of the light curve parameters and the host star(s) brightnesses in $V,\, I$, and $K$ passbands gives primary and secondary host star masses of $M_{\rm 1} = 0.43 \pm 0.05\,M_{\odot}$ and $M_{\rm 2} = 0.12 \pm 0.02\,M_{\odot}$, and a planetary companion mass of $m_p = 165^{+40}_{-33}\,M_{\oplus}$. Although we are unable to determine if the planet orbits one or both stars in the system, our analysis motivates next-generation high-precision RV measurements that can distinguish these two possibilities. This is the first direct detection of flux coming from a two-star microlensing system which hosts a planet.
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All authors Sean K. Terry, David P. Bennett, Jean-Philippe Beaulieu, T. Dex Bhadra, Aparna Bhattacharya, Joshua W. Blackman, Ian A. Bond, Andrew A. Cole, Przemek Mróz, Kansuke Nunota, Clément Ranc, Natalia E. Rektsini, Daisuke Suzuki, Aikaterini Vandorou

Astrophysics of Galaxies

Probing departures from Schwarzschild geometry with the S301 star and kilometre-baseline interferometry

Rodrigo P. Silva, Alexandre C. M. Correia and Paulo J. V. Garcia

The S301 star reaches a pericentre one tenth that of S2, opening a new regime for probing black-hole spin and departures from Schwarzschild. We develop a post-Newtonian secular Hamiltonian framework for bound geodesics in stationary, axisymmetric spacetimes and compute the maximum projected astrometric deviation over one orbit. At the expected GRAVITY+ precision, S301 can probe several nonrotating geometries and sizeable spin-induced deviations, while kilometre-baseline interferometry can probe much smaller departures, establishing S301 as a powerful test of the spacetime geometry of Sgr A*.
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Astrophysics of Galaxies · Cross-listed from astro-ph.IM

Armillary: A Label-Free Coordinate System for Stellar Spectra

Yuan-Sen Ting and Serat Saad

A stellar spectrum is determined by a few parameters, so a set of spectra lies on a low-dimensional manifold with those parameters as its coordinates. We introduce Armillary, which generalises the Sequencer of Baron and Ménard from an ordering to multidimensional coordinates, constructed from the spectra alone. Two stars are compared by the Wasserstein distance between their signed absorption profiles. Shortest paths through a nearest-neighbour graph measure distance along the manifold and give the coordinates, and the same graph transfers the labels of a few stars to all the others. The coordinates are more nearly linear in $T_{\rm eff}$ and $\log g$ than principal components, t-SNE, UMAP or an autoencoder. The label transfer, without deep learning, is more precise than the first three and comparable to the autoencoder. On 202,932 APOGEE DR17 stars, an illustrative draw of 32 training stars recovers the ASPCAP $T_{\rm eff}$, $\log g$ and [M/H] of held-out dwarfs and giants, with $1\sigma$ errors of 100 K, 0.18 and 0.083 dex. Resolving the two $\alpha$ sequences of the disc in [$\alpha$/M] for giants takes 128 training stars, and with 2048 the four errors improve to 33 K, 0.06, 0.026 and 0.022 dex. On 104,799 DESI stars, 32 training labels from APOGEE recover the stellar parameters to 129 K, 0.25 and 0.203 dex, even for spectra degraded to a signal-to-noise of about 20. At a signal-to-noise of about 100, 2048 training labels resolve the two $\alpha$ sequences of the disc in DESI giants, with errors against APOGEE of 51 K, 0.07, 0.047 and 0.026 dex. The code is public at https://github.com/tingyuansen/armillary.
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Astrophysics of Galaxies

EMIR confirmation of the high-z Ly{\alpha} blob SHARDS20018464

S.Zarattini, F. Garzón and C. Muñoz-Tuñón

A spectroscopic follow-up of the Ly$\alpha$ blob SHARDS20018464 was performed using the infrared spectrograph EMIR (Espectrógrafo Multiobjeto Infra-Rojo) at the 10.4 m GTC telescope, equipped with its newly upgraded H2RG focal plane array detector. We observed the blob in the Ks band, which corresponds, at the photometric redshift of $z=3.49$ presented in the literature, to the rest-frame visible part of the spectrum. Thanks to the sensitivity of EMIR, we were able to identify the [OIII] emission doublet at the position of the blob and the companion galaxy. The flux of the detected lines is of the order of $10^{-17}$ erg s$^{-1}$ cm$^{-2}$, obtained with 4-hour exposure. The mean redshift derived by analysing the two lines individually is $z=3.59\pm0.01$, slightly higher than the photometric one, although in agreement within the uncertainties. We also estimated the flux of the two lines and their ratio, which was found to be $R = 2.92$, very close to the theoretical prediction of 2.98. This is a strong analytical confirmation that the observed emissions correspond to the [OIII] doublet. Within uncertainties, the redshift of the blob confirms the photometric one reported in the literature, as well as the physical properties inferred for it. Its remarkably small size (5-8 kpc vs. $\sim 100$ kpc for typical samples) and its position aside the companion galaxy also confirm the previously-proposed idea that this object could be the first of a new class of compact Ly$\alpha$ emitters. Moreover, the data support the scenario of photo-ionisation due to a cluster of young and massive stars.
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Astrophysics of Galaxies

Unveiling the Origins of the Molecular Gas Reservoirs in Recently Quenched Massive Galaxies

E. Savitch, D. Narayanan, D. J. Setton et al. (+13 more)

Observations of quenched galaxies across cosmic time have revealed systems containing substantial molecular gas reservoirs, challenging the traditional picture of gas-poor, red and dead galaxies. Using the Simba cosmological simulation, we investigate three possible hypotheses for the origin of molecular gas-rich quenched systems: (i) quiescence is temporary and the galaxy will rejuvenate, (ii) molecular gas masses are overestimated due to an underestimated CO-to-H$_2$ conversion factor, or (iii) star formation is ongoing but heavily obscured by dust. Across $M_*>3\times10^9M_\odot$ galaxies from $z=0-2$, we find that all three mechanisms are plausible. Many seemingly quiescent, molecular gas-rich galaxies host dust-obscured star formation, with visual optical depths increasing on average from $\tau_V\sim0.5-2.3$ from $z=0-2$. Assuming a Milky Way-like CO-to-H$_2$ conversion factor can overestimate molecular gas masses by a factor of $\sim\!5$ in non-star-forming galaxies, compared to $\sim\!1.45$ in star-forming galaxies. Temporary quiescence can also occur in gas-rich transitioning galaxies on timescales ranging from $100\,\text{Myr}-2\,\text{Gyr}$, although this is most prominent at high redshift and is relatively rare overall. Further work is needed to disentangle the relative contributions of these mechanisms across cosmic time.
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All authors E. Savitch, D. Narayanan, D. J. Setton, K. E. Whitaker, R. Bezanson, E. Cenci, R. Davé, V. R. D'Onofrio, R. Feldmann, J. E. Greene, M. Kriek, M. Parente, J. S. Spilker, K. A. Suess, D. Valentín-Martínez, D. Zimmerman

Astrophysics of Galaxies

Forming Molecular Hydrogen in a Galaxy Halo Through Star Formation Wind Interactions

Brad Koplitz, Sanchayeeta Borthakur, Timothy Heckman et al. (+5 more)

Molecular hydrogen (H$_2$) is the key ingredient for star formation in galaxies. However, how H$_2$ is distributed in and around galaxies is an open question in astronomy. We report the detection of H$_2$ (covering excitation levels $J = 0$ to 3) in a unique environment at a distance of 139 kpc from a starbursting luminous infrared galaxy. This is the furthest from a galaxy that H$_2$ has been directly detected to date. This H$_2$ system is unlike any seen before, with a high molecular-to-atomic hydrogen ratio (2%) and an accompanying low neutral hydrogen column density (log$_{10}[N($H I$)/\mathrm{cm}^{-2}] = 18.52^{+0.13}_{-3.58}$). Our measurements reveal that the H$_2$ bearing cloud is traveling at $\sim$100 km s$^{-1}$ relative to the host galaxy, has a super-solar metallicity, and excites at a single, cold temperature ($245^{+25}_{-21}$ K) indicating a lack of the ”typical” core-envelope structure seen in other H$_2$ clouds. Furthermore, our analysis points to the H$_2$ likely being formed through interactions between galactic winds and the gaseous halo surrounding the galaxy. The properties of this cloud suggest that the H$_2$ is short-lived, yet it presents crucial information on the microphysics of the wind-CGM interactions.
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All authors Brad Koplitz, Sanchayeeta Borthakur, Timothy Heckman, Frances H. Cashman, Evan Scannapieco, Jason Tumlinson, Hsiao-Wen Chen, Molly Peeples

Astrophysics of Galaxies

Disentangling the assembly of the central and diffuse stellar components in galaxy clusters through dynamical friction and tidal stripping

Frazer R. Pearce, Steven P. Bamford, Garreth Martin et al. (+7 more)

In galaxy clusters, it is common to conceptually distinguish the central, brightest cluster galaxy (BCG) from the more diffuse intracluster light (ICL). So far, however, this distinction has lacked a clear physical basis, confusing both theoretical and observational studies. We address this by presenting a dynamically motivated definition of the two stellar components, evaluated using cluster zoom simulations from the Three Hundred project. We start with a motivational hindcasting approach, robustly tracing stars from the centres of 107 present-day clusters back through the merger hierarchy. We find that the majority of the final BCG stellar mass is typically contributed by the central galaxies of only 3 to 5 progenitor haloes. Most accreted galaxies contribute no stars to the BCG, instead surviving as satellites or depositing stellar material throughout the cluster. This naturally leads to a definition of the BCG stellar population as originating from the central galaxies of the most massive infalling systems. These are delivered to the cluster centre via efficient dynamical friction and subsequently undergo violent relaxation. The ICL arises from the complementary population of stars tidally stripped from the outskirts of all infalling systems. We therefore develop a forecasting method, identifying stars in the BCG and ICL via the dynamical fates of their originating stellar systems. Applying this to 261 clusters, we find the resulting components remain spatially interleaved, but occupy distinct regions of phase space. Our physically defined BCG matches conventional aperture-based selections with high completeness, while providing a direct interpretation in terms of hierarchical assembly.
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All authors Frazer R. Pearce, Steven P. Bamford, Garreth Martin, Yannick M. Bahé, Harley J. Brown, Madeline Butler, Ana Contreras-Santos, Weiguang Cui, Nina Hatch, Alexander Knebe

Astrophysics of Galaxies

PAH emission from star-forming and quiescent galaxies at $z=0.5-0.8$ probed by JWST/MIRI

Chayan Mondal, Chian-Chou Chen, Kanak Saha et al. (+4 more)

We investigate emission from polycyclic aromatic hydrocarbon (PAH) dust molecules in 224 MIRI-selected star-forming and quiescent galaxies at $z=0.5-0.8$ in the GOODS-South field using multi-wavelength data from the AstroSat/UVIT, JWST, HST, Herschel, and MUSE. We define an observed diagnostic parameter, f$_{\rm PAH}$, as the ratio of JWST/MIRI F1280W to the average of F770W and F2550W band fluxes, and demonstrate that it effectively traces the PAH mass fraction (q$_{\rm PAH}$) by capturing 7.7$\mu$m PAH emission at our targeted redshift. We observe a bimodal distribution of similar nature in both f$_{\rm PAH}$ and q$_{\rm PAH}$, suggesting that PAH enrichment in galaxies goes through a rapid transition phase from a PAH-poor to a PAH-rich system, at about 70% solar metallicity. For the star-forming galaxies (SFGs; i.e., $\sim$78% of the total sample), we find that f$_{\rm PAH}$ increases with stellar mass and rest-frame FUV luminosity, identifying massive, UV-bright galaxies to have higher PAH mass fraction. However, the majority of the massive quiescent galaxies (QGs) show lower PAH mass fraction relative to that of SFGs of similar mass, suggesting PAH and smaller dust grains are more susceptible to destruction during quenching processes. Among SFGs, we observe an anti-correlation between f$_{\rm PAH}$ and the O32 emission line ratio, suggesting PAH photo-dissociation in strongly ionized ISM. This anti-correlation, along with a correlation between f$_{\rm PAH}$ and gas-phase metallicity 12+log(O/H), indicates that low-mass, metal-poor SFGs, which are increasingly common at even higher redshifts, host more extreme conditions less favorable for PAH production and survival.
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All authors Chayan Mondal, Chian-Chou Chen, Kanak Saha, Thomas S.-Y. Lai, Irene Shivaei, Hiddo S. B. Algera, Bovornpratch Vijarnwannaluk