Marcin Sawicki

We present the evolution of the resolved mass-weighted age, stellar metallicity, and specific star formation rate (sSFR) of 872 Milky Way analog (MWA) progenitors up to redshift z = 5 from the Canadian Unbiased Cluster Survey. The metallicity and mass-weighted ages were obtained via spatially resolved spectral energy distribution fitting with the nonparametric code Dense Basis . We split the sample into mergers versus nonmergers using the merger parameter from the Gini– M ₂₀ plane obtained through Gini– M ₂₀ analysis of the morphology of the stellar mass maps with Statmorph . Across our redshift range, nonmergers have negative or flat average age gradients from −0.022 to 0.005 dex kpc ⁻¹ , and positive or flat sSFR gradients from −0.089 to 0.092 dex kpc ⁻¹ , consistent with inside-out assembly. The average log ( Z / Z ⊙ ) gradients for nonmergers range from −0.029 to 0.044 dex kpc ⁻¹ ; however, positive gradients only appear between 2 < z < 3. At every redshift epoch, mergers typically have flatter age gradients, more negative sSFR gradients, and similar metallicity gradients compared to nonmergers. We divide the property maps of ongoing mergers into separate regions based on their component galaxies, and find little to no difference between the components’ average ages or metallicities, but the less massive of the merging system is on average 0.1−0.4 dex higher in sSFR. Our results point to major mergers contributing some momentary disruption to the general trend of inside-out mass assembly, but this does not upend the overall picture of MWA disks growing inside-out over cosmic time.

Little red dots (LRDs) are a heterogeneous class of objects, and several scenarios have been proposed for their physical nature and evolution. While these theories have been tested on individual LRDs using limited spectral features, a systematic Bayesian analysis of the LRD population incorporating the different models across a broad wavelength range is still lacking. We conducted a consistent ultraviolet (UV)-to-optical continuum fitting analysis of 99 LRDs at 2 < z < 6 using JWST/NIRSpec PRISM spectroscopy. Employing a modified version of the code Bagpipes , including blackbody (BB) emission affected by Balmer absorption, stellar and nebular emission attenuated by dust, and an active galactic nucleus (AGN) component, we assessed the performance of the black hole star (BH ^∗ ) model in describing the LRD population. We adopted broad priors and therefore did not impose any specific physical scenario. Our results show that only ∼4% of LRDs with statistically robust solutions (81 objects in total) are best fit by a BH ^∗ in the optical and a host galaxy in the UV. About 9% of the LRDs show BB-dominated optical continua, but lack a stellar component or exhibit AGN UV leakage. Most LRDs are dominated by stellar and/or AGN emission in the optical, with a minor BB contribution. When we adopted a prior that disfavored a strong AGN continuum to enforce BH ^∗ -like solutions, the percentage of BH ^∗ systems increased to ∼35%, highlighting the strong degeneracy between a BH ^∗ solution and alternative scenarios. Even when BH ^∗ -like solutions were enforced, many LRDs still required a stellar-dominated optical continuum. This might indicate limitations in the BH ^∗ model, as further supported by independent evidence, such as the detection of high-ionization emission lines in some LRDs.

Context. UV slopes ( β ) are a powerful diagnostic for galaxies at the epoch of reionisation, tracing star formation, interstellar medium ionisation, and the escape fraction f _esc of ionising photons. Studies at low and intermediate z suggested a gradual reddening of the UV slope with time and steeper slopes for fainter galaxies. The James Webb Space Telescope ( JWST ) enabled measurements of the UV slope for statistically significant samples of galaxies deep into the epoch of reionisation up to redshift z ≳ 10 and for increasingly faint galaxies. Recent studies of β with JWST at z > 7 revealed a flattening of the trend of the UV slope with respect to redshift and UV magnitude. Aims. We measure β at z > 7.5 using the strong lensing around massive galaxy clusters to observe high-redshift and faint galaxies. The low-brightness regime is of particular interest for reionisation, as most of the recent models of this process posit that numerous faint galaxies are the prime drivers of reionisation Methods. We used NIRCam and NIRSpec observations from CANUCS, Technicolor, JUMPS, Silver Bullet, UNCOVER, and MEGA-SCIENCE in seven strong-lensing fields in order to find galaxies down to M _UV ∼ −16 and 7.5 < z ≲ 12.5. We measured their β with a forward-modelling procedure. We estimated the f _esc of a subsample with emission line data using a relation with UV slope, galaxy size, and H β equivalent width calibrated from a low-redshift sample. Results. We found 378 visually confirmed galaxies, including 43 galaxies with a spectrum. We obtained an average UV slope < β > = −2.3 ± 0.4, an average redshift < z > = 8.5 ± 1.0, and an average AB magnitude M _UV = −18 ± 1. We found no significant evolution of β in our redshift range, suggesting a flattening of the β – z trend above z ∼ 7.5. We found a weak trend between β and M _UV . Conclusions. The flat trend of β at z > 7.5 indicates a similarity in average galaxy properties between 300 and 600 Myr after the Big Bang. The weak trend between β and M _UV suggests an analogous composition for the interstellar medium of low- and high-mass galaxies, likely due to a lack of time to build up a dust reservoir.

From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts z > 10 and as high as z = 14. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at z > 15 may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright z ∼ 14 galaxies should be visible at redshifts as high as z ∼ 20–30, showing in the data as F277W and F356W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with Y dwarf templates with temperatures 272–351 K and and 445–525 K (using ATMO2020 and Sonora Elf Owl models) and distances of ∼150–650 pc. The first is one of the lowest-temperature brown dwarfs known, and the lowest-temperature brown dwarf detected spectroscopically outside the solar neighborhood. With additional NIRCam imaging taken ∼1 yr later, we also detect their proper motions of (49 ± 8) mas yr ⁻¹ and (24 ± 3) mas yr ⁻¹ , further indicating that at least some F277W and F356W dropouts are substellar cold Milky Way objects such as brown dwarfs.

External interactions drive galaxy stellar mass growth and morphological evolution. As stellar halos—assembled largely via hierarchical accretion—preserve signatures of these processes, their growth probes how environment regulates galaxy evolution. We investigate how cluster environments influence quiescent galaxy (QG) stellar halo assembly over 0.1 ≤ z ≤ 1.0 in a sample of 2168 cluster and 94,479 field QGs of log M ⋆ ≥ 9.66 . Extended emission is traced via rest-frame g -band surface brightness ( μ _g ) profiles extracted from deep Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) grizy imaging. We study stellar halo assembly trends by linking median μ _g profile evolution to the underlying mass growth in galaxy subpopulations. Over 0.1 ≤ z ≤ 1.0, cluster QGs build up stellar halos faster than field QGs, with a ∼23% and ∼40% larger increase in integrated stellar halo luminosity ( L _halo ) in the low-mass ( 9.66 ≤ log M ⋆ < 10.5 ) and high-mass ( log M ⋆ ≥ 10.5 ) samples, respectively. High-mass cluster QGs host more luminous stellar halos than the field (mean cluster-to-field L _halo ratio of ∼1.2), while low-mass cluster QGs host less luminous stellar halos than their field counterparts (mean ratio of ∼0.87). Among cluster QGs of log M ⋆ ≥ 10 , L _halo increases with host cluster mass, but decreases for cluster QGs of log M ⋆ < 10 . These results suggest higher-mass cluster QGs ( log M ⋆ ≥ 10 ) experience enhanced stellar halo growth over 0.1 ≤ z ≤ 1.0 fueled by increased merger-driven accretion, likely from minor mergers in cluster outskirts or in preinfall group and filament environments. Lower-mass cluster QGs ( 9.66 ≤ log M ⋆ < 10 ) instead have suppressed stellar halo growth in clusters and likely lose outer stellar material to environmental stripping or accretion by high-mass galaxies during mergers.

One of the most surprising results of early James Webb Space Telescope (JWST) observations is the discovery of an abundance of red, compact, broad-line objects dubbed “little red dots” (LRDs) at z > 4. Their spatial density (∼10 ⁻⁴ –10 ⁻⁵ cMpc ⁻³ ) is 100 times more abundant than UV-selected quasars at that redshift if one extrapolates the quasar luminosity function (QLF) down to the LRD regime. However, whether LRDs dominate black hole accretion at quasar-like luminosities ( L _bol ≳ 10 ⁴⁵⁻⁴⁶ erg s ⁻¹ ) remains unanswered, as probing the bright end of the LRD luminosity function requires a much larger area than that which is able to be surveyed by JWST. In this work, we present our search for the brightest LRDs ( K < 23.7) at 4.5 < z < 4.9 using wide-area multiwavelength imaging surveys from the near-UV to the infrared bands. With over 15 deg ² of sky coverage, we only identify one single LRD candidate at z _phot ≈ 4.6, which translates into a spatial density of n ( M ₅₁₀₀ < −23.5) ≈ 10 ⁻⁸ cMpc ⁻³ —this is nearly 10 times less abundant than the UV-selected quasars at similar optical luminosity. When combined with the LRD sample identified by JWST at the same redshift range, we find a sharp cutoff in the optical luminosity function at λL ₅₁₀₀ ≈ 2.2 × 10 ⁴⁴ erg s ⁻¹ , while the quasar luminosity function (QLF) turnover occurs at ≳20 times higher luminosity. We therefore confirm the exclusively low-luminosity nature of LRDs, ruling out that LRDs are the counterparts of quasars. Furthermore, we speculate that, if the shape of the luminosity function holds up, it points to LRDs being powered by low-mass black holes with a narrow range of Eddington-level accretion rates.

The One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey is conducting the widest-field deep narrowband (NB) imaging of the equatorial and southern skies. ODIN uses three custom-built NB filters that sample Ly α -emitting galaxies (LAEs) within thin cosmic slices centered at z = 2.4, 3.1, and 4.5. In this work, we utilize extensive DESI spectroscopy of ODIN-selected galaxies in the COSMOS and XMM-LSS fields to validate our LAE selection. Exposures of 2–4 hr with DESI yielded redshift confirmation of 3075 ODIN LAE candidates with NB magnitudes brighter than 26 mag. Restricting to objects that yield high-confidence redshifts, the confirmation rates are (93%, 96%, 92%) at z = (2.4, 3.1, 4.5). The primary contaminants consist of active galactic nuclei at the expected Ly α- redshift range and lower redshifts (C iv , C iii ]), with the remainder being star-forming galaxies ([O ii ] and [O iii ]). We find minimal contamination from [O ii ] emitters in our sample (≲1%), implying that our rest-frame equivalent width (REW) > 20 Å NB excess photometry requirement is sufficient to remove them.

The discovery of a population of massive, ancient quiescent galaxies within the first 2 Gyr of the Universe’s history has led to significant tensions with models of galaxy formation. However, these analyses are often based on slit spectroscopy, which typically captures only the centermost region of these galaxies and, crucially, assumes these cores are representative of the entire galaxy. To illustrate the varying stellar populations present throughout these galaxies, we present an analysis of color gradients in four z > 3, log ( M ⋆ / M ⊙ ) > 11 quiescent galaxies which previous works have argued are in tension with models. Using medium-band photometry from MINERVA JWST observations, we measure resolved photometry in a series of elliptical annuli out to 0 . ″ 7 (∼4 R _e ). We find negative color gradients in three galaxies, and for the most extreme color gradient (Δ( U − V )/Δ R = −0.126 ± 0.030 mag kpc ⁻¹ ), we find the stellar mass is 0.1 dex lower when compared to photometry measured within NIRSpec slits. In the limiting case where these color gradients are entirely driven by age, we find lessened tensions with extreme value statistics models out to z ∼ 9.5, though different stellar population modeling choices also contribute significantly. Ultimately, these findings highlight the need for integral field unit spectroscopy. Spatially resolved spectra can provide the evidence needed to break the age–dust–metallicity degeneracy, and reliably separate the effects of the observed color gradients from the effects of different physical modeling assumptions on the formation histories of these galaxies.