arXiv subjects

H. B. Akins

Publications and source records attributed to H. B. Akins.

6 recordsLinked to original sources

arXiv2026

COSMOS-Web: From early star-formation enhancement to late suppression in galaxy groups

Galaxy groups trace dense environments where interactions, gas removal, and reduced accretion may drive quenching. Common diagnostics trace star formation over short timescales ($\lesssim100$ Myr); time-resolved star formation histories (SFHs) separate recent changes from longer-term evolution at fixed mass and redshift. Using COSMOS-Web data, we test how group environment correlates with star-formation activity and its evolution with cosmic time and group-centric distance, contrasting high-richness groups with excluded field galaxies. We combine COSMOS2025/COSMOS-Web stellar masses and non-parametric SFHs with group detections and memberships from AMICO (Adaptive Matched Identifier of Clustered Objects), comparing stacked SFHs of matched group and field galaxies and measuring how the SFH offset evolves with group-centric distance for the richest groups. Massive galaxies ($\log(M_\star/M_\odot)>10.5$) show the largest offsets but are already quenched in both environments. For lower masses ($8.1<\log(M_\star/M_\odot)<10.5$), suppression is clearest at $z<1.5$ (deficit up to 0.8 dex), while at $z>1.5$ SFHs show weak suppression or occasional enhancement. The radial signal evolves similarly: quenching is broad at low $z$, the inner-outer contrast sharpens at $z\gtrsim1$, and any ordering at $z\gtrsim2$ is tentative given growing uncertainties in the AMICO centroids. These results suggest an evolving picture: groups are more mixed at early epochs, with both suppressed and occasionally elevated SFHs, partly reflecting high-$z$ uncertainties, while suppression dominates from $z\lesssim1.5$, most clearly for low-to-intermediate-mass galaxies. Inner-region galaxies likely spend more time in the group potential, undergo more passages through dense intra-group gas, and accrete less pristine cold gas, making quenching progressively clearer with cosmic time.

astro-ph.GA↗

arXiv2025

Active galactic nuclei-heated dust revealed in "little red dots"

Little red dots (LRDs) are a puzzling population of extragalactic sources whose origin is highly debated. In this {work}, we performed a comprehensive stacking analysis of NIRCam, MIRI, and ALMA images of a large and homogeneously selected sample of LRDs from multiple JWST Legacy fields. We report clear evidence of hot-dust emission in the median stacked spectral energy distribution (SED) that features a rising near-infrared continuum up to rest-frame $λ_{\rm rest}$$\sim$ 3$μ$m, which is best explained by a standard dusty active galactic nucleus (AGN) structure. Although LRDs are likely to be a heterogeneous population, our findings suggest that most ($\gtrsim$50 %) LRDs show AGN-heated dust emission, regardless of whether the optical and ultraviolet (UV) continua are stellar or AGN-dominated. In either case, the best-fit dusty-AGN SED, combined with the lack of X-ray detection in the deep Chandra stacks, suggests that Compton-thick ($N_{\rm H}$$>$3$\times$10$^{24}$ cm$^{-2}$) gas obscuration is common, and likely confined within the dust sublimation radius ($R$$_{\rm sub}$$\sim$0.1 pc). Therefore, we argue that AGN-heated dust does not directly obscure either the optical-UV continuum or the broad-line region emission, in order to explain the observed blue UV slopes and prominent Balmer features. While a gas-dust displacement is in line with several models, the formation scenario (in-situ or ex-situ) of this pre-enriched hot dust remains unclear.

astro-ph.GA↗

arXiv2025

Testing for Intrinsic Type Ia Supernova Luminosity Evolution at z>2 with JWST

The James Webb Space Telescope} (JWST) is opening new frontiers of transient discovery and follow-up at high-redshift. Here we present the discovery of a spectroscopically confirmed Type Ia supernova (SN Ia; SN $2023$aeax) at $z=2.15$ with JWST, including a NIRCam multi-band light curve. SN $2023$aeax lands at the edge of traditional low-$z$ cosmology cuts because of its blue color (peak rest-frame $B-V\sim-0.3$) but with a normal decline rate ($Δm_{15}(B)\sim1.25$), and applying a fiducial standardization with the BayeSN model we find the SN $2023$aeax luminosity distance is in $\sim0.1σ$ agreement with $Λ$CDM. SN $2023$aeax is only the second spectroscopically confirmed SN Ia in the dark matter-dominated Universe at $z>2$ (the other is SN $2023$adsy), giving it rare leverage to constrain any potential evolution in SN Ia standardized luminosities. Similar to SN $2023$adsy ($B-V\sim0.8)$, SN $2023$aeax has a fairly extreme (but opposite) color, which may be due to the small sample size or a secondary factor, such as host galaxy properties. Nevertheless, the SN $2023$aeax spectrum is well-represented by normal low-$z$ SN Ia spectra and we find no definitive evolution in SN Ia standardization with redshift. Still, the first two spectroscopically confirmed $z>2$ SNe Ia have peculiar colors and combine for a $\sim1σ$ distance slope relative to $Λ$CDM, though in agreement with recent SN Ia cosmological measurements.

astro-ph.CO↗

arXiv2025

COSMOS-Web: The emergence of the Hubble Sequence

Leveraging the wide area coverage of the COSMOS-Web survey, we quantify the abundance of different morphological types from $z\sim 7$ with unprecedented statistics and establish robust constraints on the epoch of emergence of the Hubble sequence. We measure the global (spheroids, disk-dominated, bulge-dominated, peculiar) and resolved (stellar bars) morphologies for about 400,000 galaxies down to F150W=27 using deep learning, representing a two-orders-of-magnitude increase over previous studies. We then provide reference Stellar Mass Functions (SMFs) of different morphologies between $z\sim 0.2$ and $z\sim 7$ and best-fit parameters to inform models of galaxy formation. All catalogs and data are made publicly available. (a)At redshift z > 4.5, the massive galaxy population ($\log M_*/M_\odot>10$) is dominated by disturbed morphologies (~70%) -- even in the optical rest frame -- and very compact objects (~30%) with effective radii smaller than ~500pc. This confirms that a significant fraction of the star formation at cosmic dawn occurs in very dense regions, although the stellar mass for these systems could be overestimated.(b)Galaxies with Hubble-type morphologies -- including bulge and disk-dominated galaxies -- arose rapidly around $z\sim 4$ and dominate the morphological diversity of massive galaxies as early as $z\sim 3$. (c)Using stellar bars as a proxy, we speculate that stellar disks in massive galaxies might have been common (>50%) among the star-forming population since cosmic noon ($z\sim2$-2.5) and formed as early as $z\sim 7$ (d)Massive quenched galaxies are predominantly bulge-dominated from z~4 onward, suggesting that morphological transformations briefly precede or are simultaneous to quenching mechanisms at the high-mass end. (e) Low-mass ($\log M_*/M_\odot<10$) quenched galaxies are typically disk-dominated, pointing to different quenching routes in the two ends of the stellar mass spectrum from cosmic dawn.

astro-ph.GA↗

arXiv2025

COSMOS-Web: The Role of Galaxy Interactions and Disk Instabilities in Producing Starbursts at z<4

We study of the role of galaxy-galaxy interactions and disk instabilities in producing starburst activity in galaxies out to z = 4. For this, we use a sample of 387 galaxies with robust total star formation rate measurements from Herschel, gas masses from ALMA, stellar masses and redshifts from multi-band photometry, and JWST/NIRCam rest-frame optical imaging. Using mass-controlled samples, we find an increased fraction of interacting galaxies in the starburst regime at all redshifts out to z = 4. This increase correlates with star formation efficiency (SFE), but not with gas fraction. However, the correlation is weak (and only significant out to z = 2), which could be explained by the short duration of SFE increase during interaction. In addition, we find that isolated disk galaxies make up a significant fraction of the starburst population. The fraction of such galaxies with star-forming clumps ("clumpy disks") is significantly increased compared to the main-sequence disk population. Furthermore, this fraction directly correlates with SFE. This is direct observational evidence for a long-term increase of SFE maintained due to disk instabilities, contributing to the majority of starburst galaxies in our sample and hence to substantial mass growth in these systems. This result could also be of importance for explaining the growth of the most massive galaxies at z > 6.

astro-ph.GA↗

arXiv2024

COSMOS-Web: stellar mass assembly in relation to dark matter halos across $0.2<z<12$ of cosmic history

We study the stellar mass function (SMF) and the co-evolution with dark matter halos via abundance matching in the largest redshift range to date $0.2 5$, we find increased abundances of massive (log$\, M_{\star}/M_{\odot}>10.5$) implying integrated star formation efficiencies (SFE) $ε_{\star}\equiv M_{\star}\, f_{\rm b}^{-1} M_{\rm halo}^{-1} \gtrsim 0.5$. We find a flattening of the SMF at the high-mass end that is better described by a double power law at $z>5.5$. At $z \lesssim 5.5$ it transitions to a Schechter law which coincides with the emergence of the first massive quiescent galaxies in the Universe. We trace the cosmic stellar mass density (SMD) and infer the star formation rate density (SFRD), which at $z>7.5$ agrees remarkably with recent \JWST{} UV luminosity function-derived estimates. However, at $z \lesssim 3.5$, we find significant tension ($\sim 0.3$ dex) with the cosmic star formation (SF) history from instantaneous SF measures, the causes of which remain poorly understood. We infer the stellar-to-halo mass relation (SHMR) and the SFE from abundance matching out to $z=12$, finding a non-monotonic evolution. The SFE has the characteristic strong dependence with mass in the range of $0.02 - 0.2$, and mildly decreases at the low mass end out to $z\sim3.5$. At $z\sim3.5$ the SFE increases sharply from $\sim 0.1$ to approach high SFE of $0.8-1$ by $z\sim 10$ for log$(M_{\rm h}/M_{\odot})\approx11.5$, albeit with large uncertainties. Finally, we use the SHMR to track the SFE and stellar mass growth throughout the halo history and find that they do not grow at the same rate -- from the earliest times up until $z\sim3.5$ the halo growth rate outpaces galaxy assembly, but at $z>3.5$ halo growth stagnates and accumulated gas reservoirs keep the SF going and galaxies outpace halos.

astro-ph.GA↗