More Like this

1. H i Properties of Field Galaxies at z  ≈ 0.2–0.6: Insights into Declining Cosmic Star Formation

   https://doi.org/10.3847/2041-8213/ae0d8b  

   DePalma, David; Gupta, Neeraj; Chen, Hsiao-Wen; Simcoe, Robert A; Balashev, Sergei; Boettcher, Erin; Cantalupo, Sebastiano; Chen, Mandy C; Combes, Françoise; Faucher-Giguère, Claude-André; et al (October 2025, The Astrophysical Journal Letters)

   Abstract We report statistically significant detection of Hi21 cm emission from intermediate-redshift (z ≈ 0.2–0.6) galaxies. By leveraging multisightline galaxy survey data from the Cosmic Ultraviolet Baryon Survey and deep radio observations from the MeerKAT Absorption Line Survey, we have established a sample of ≈6000 spectroscopically identified galaxies in 11 distinct fields to constrain the neutral gas content at intermediate redshifts. The galaxies sample a broad range in stellar mass, from $\log M_{\mathrm{star}}/M_{\odot }\approx 8$to $\log M_{\mathrm{star}}/M_{\odot }\approx 11$, with a median of ${〈\log M_{\mathrm{star}}/M_{\odot }〉}_{\mathrm{med}}\approx 10$and a wide range in redshift fromz ≈ 0.24 toz ≈ 0.63 with a median of 〈z〉_med = 0.44. While no individual galaxies show detectable Hiemission, the emission line signal is detected in the stacked spectra of all subsamples at greater than 4σsignificance. The observed total Hi21 cm line flux translates to a Himass,M_{H I}≈10¹⁰M_⊙. We find a high Hi-to-stellar-mass ratio ofM_HI/M_star ≈ 6 for low-mass galaxies with $〈\log M_{\mathrm{star}}/M_{\odot }〉\approx 9.3$(>3.7σ). For galaxies with $〈\log M_{\mathrm{star}}/M_{\odot }〉\approx 10.6$, we findM_HI/M_star ≈ 0.3 (>4.7σ). In addition, the redshift evolution of Himass, 〈M_{H I}〉, in both low- and high-mass field galaxies, inferred from the stacked emission-line signal, aligns well with the expectation from the cosmic star formation history. This suggests that the overall decline in the cosmic star formation activity across the general galaxy population may be connected to a decreasing supply of neutral hydrogen. Finally, our analysis has revealed significant 21 cm signals at distances greater than 75 kpc from these intermediate-redshift galaxies, indicating a substantial reservoir of Higas in their extended surroundings. 

   more » « less
2. A Hidden Population of Massive Black Holes in Simulated Dwarf Galaxies

   https://doi.org/10.3847/1538-4357/ac8664  

   Sharma, Ray S; Brooks, Alyson M; Tremmel, Michael; Bellovary, Jillian; Ricarte, Angelo; Quinn, Thomas R (September 2022, The Astrophysical Journal)

   Abstract We explore the characteristics of actively accreting massive black holes (MBHs) within dwarf galaxies in the Romulus25cosmological hydrodynamic simulation. We examine the MBH occupation fraction, X-ray active fractions, and active galactic nucleus (AGN) scaling relations within dwarf galaxies of stellar mass 10⁸M_⊙<M_star< 10¹⁰M_⊙out to redshiftz= 2. In the local universe, the MBH occupation fraction is consistent with observed constraints, dropping below unity atM_star< 3 × 10¹⁰M_⊙,M₂₀₀< 3 × 10¹¹M_⊙. Local dwarf AGN in Romulus25follow observed scaling relations between AGN X-ray luminosity, stellar mass, and star formation rate, though they exhibit slightly higher active fractions and number densities than comparable X-ray observations. Sincez= 2, the MBH occupation fraction has decreased, the population of dwarf AGN has become overall less luminous, and as a result the overall number density of dwarf AGN has diminished. We predict the existence of a large population of MBHs in the local universe with low X-ray luminosities and high contamination from X-ray binaries and the hot interstellar medium that are undetectable by current X-ray surveys. These hidden MBHs make up 76% of all MBHs in local dwarf galaxies and include many MBHs that are undermassive relative to their host galaxy’s stellar mass. Their detection relies on not only greater instrument sensitivity but also better modeling of X-ray contaminants or multiwavelength surveys. Our results indicate that dwarf AGN were substantially more active in the past, despite having low luminosity today, and that future deep X-ray surveys may uncover many hidden MBHs in dwarf galaxies out to at leastz= 2. 

   more » « less
3. Damped Lyα Absorbers in Star-forming Galaxies at z < 0.15 Detected with the Hubble Space Telescope and Implications for Galactic Evolution

   https://doi.org/10.3847/1538-4357/ac5fab  

   Kulkarni, Varsha P.; Bowen, David V.; Straka, Lorrie A.; York, Donald G.; Gupta, Neeraj; Noterdaeme, Pasquier; Srianand, Raghunathan (April 2022, The Astrophysical Journal)

   Abstract We report Hubble Space Telescope Cosmic Origins Spectrograph spectroscopy of 10 quasars with foreground star-forming galaxies at 0.02 < z < 0.14 within impact parameters of ∼1–7 kpc. We detect damped/sub-damped Ly α (DLA/sub-DLA) absorption in 100% of cases where no higher-redshift Lyman-limit systems extinguish the flux at the expected wavelength of Ly α absorption, obtaining the largest targeted sample of DLA/sub-DLAs in low-redshift galaxies. We present absorption measurements of neutral hydrogen and metals. Additionally, we present Green Bank Telescope 21 cm emission measurements for five of the galaxies (including two detections). Combining our sample with the literature, we construct a sample of 117 galaxies associated with DLA/sub-DLAs spanning 0 < z < 4.4, and examine trends between gas and stellar properties, and with redshift. The H i column density is anticorrelated with impact parameter and stellar mass. More massive galaxies appear to have gas-rich regions out to larger distances. The specific star formation rate (sSFR) of absorbing galaxies increases with redshift and decreases with M *, consistent with evolution of the star formation main sequence (SFMS). However, ∼20% of absorbing galaxies lie below the SFMS, indicating that some DLA/sub-DLAs trace galaxies with longer-than-typical gas-depletion timescales. Most DLA/sub-DLA galaxies with 21 cm emission have higher H i masses than typical galaxies with comparable M *. High M HI / M * ratios and high sSFRs in DLA/sub-DLA galaxies with M * < 10 9 M ⊙ suggest these galaxies may be gas-rich because of recent gas accretion rather than inefficient star formation. Our study demonstrates the power of absorption and emission studies of DLA/sub-DLA galaxies for extending galactic evolution studies to previously under-explored regimes of low M * and low SFR. 

   more » « less
4. A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE): XX. Star formation in the tidal tail of NGC 4254

   https://doi.org/10.1051/0004-6361/202659007  

   Boselli, A; Lupi, A; Serra, P; Andreani, P; Calura, F; Miville-Deschênes, M-A; Hensler, G; Boquien, M; Fossati, M; Boissier, S; et al (August 2026, Astronomy & Astrophysics)

   ALMA¹²CO(1-0) observations of 42 star-forming regions located outside the disc of the Virgo Cluster galaxy NGC 4254 within an H Igas tail produced during the galaxy’s interaction with another cluster member have revealed the presence of ten giant molecular clouds (GMCs) in four of these regions. All of the GMCs were resolved at the angular resolution of the observations (≃160 pc) and have molecular gas masses ofM(H₂)≃(0.8 − 2.0)×10⁶M_⊙. These ten clouds are characterised by gas column densities [Σ(H₂) ≃ 10 M_⊙pc⁻²] and velocity dispersions [σ_v(CO) ≃ 3–12 km s⁻¹] respectively lower and comparable to those encountered in similar GMCs in the Milky Way. They follow the relation between the gas column density and the star formation activity (Schmidt law) derived using similar data over the stellar disc of NGC 4254 and other local and Virgo cluster galaxies. With analytic calculations and tuned simulations, we show that these clouds are unstable and thus expected to dissolve on relatively short timescales (∼10–30 Myr). We show that they probably formed after the collapse of dense gas clouds in the H Igas tail stripped during the gravitational interaction that the galaxy suffered several hundreds millions of years ago. The clouds are short-lived and isolated given the low density of the surrounding intracluster medium, which cannot confine the gas expelled by stellar feedback. We discuss the implications of these results in the general context of the fate of stripped gas in hostile cluster environments. 

   more » « less
5. Gas properties as a function of environment in the proto-supercluster Hyperion at z  ∼ 2.45

   https://doi.org/10.1051/0004-6361/202553926  

   Gururajan, G; Cucciati, O; Lemaux, B C; Talia, M; Zamorani, G; Pozzi, F; Decarli, R; Forrest, B; Shen, L; De_Lucia, G; et al (June 2025, Astronomy & Astrophysics)

   The cosmic star formation rate density, molecular gas density, and active galactic nucleus (AGN) activity of the Universe peak atz∼2–3, demonstrating the Universe is most active at this epoch. The nature of the galaxies at these redshifts and their properties as a function of their environment are particularly interesting with respect to our understanding of the mechanisms driving their star formation and quenching. Atz∼2.5, the massive (∼4.8×10¹⁵ M_⊙) proto-supercluster Hyperion, consisting of seven groups and peaks and extending over a co-moving volume of 60×60×150 Mpc³, is an excellent laboratory for probing the properties and evolution of galaxies as a function of their environments. We used a large compilation of photometric (optical to radio wavelengths, COSMOS2020, COSMOS-Super-deblended, and A³COSMOS), and spectroscopic (C3VO, HST-Hyperion, VUDS, zCOSMOS, DEIMOS10K, and MAGAZ3NE) data to assign membership and study the relation between the local environment and the molecular gas mass, the star-formation rate (SFR), gas depletion timescales, and quenching mechanisms. We find that the depletion timescales and the molecular gas fractions decrease and SFR increases in denser environments at the ∼2σlevel, suggesting accelerated evolution in the densest regions of this proto-supercluster resulting from gas stripping, over-consumption, and/or cessation of cold flows. Dedicated observations at sub-millimeter (sub-mm) wavelengths will enable further spectroscopic confirmations and better coverage at these wavelengths, thereby offering more conclusive results on the environmental implications on gas reservoirs of galaxies in Hyperion. 

   more » « less