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Alexander de la Vega

Publications and source records attributed to Alexander de la Vega.

At least 19 recordsLinked to original sources

Enhancing WISE Infrared Imaging to Spitzer Resolution Using Deep Learning Super-Resolution

We present a deep-learning framework that performs 4.6x spatial super-resolution from WISE W1 (3.4 micron) toward Spitzer IRAC Ch1 (3.6 micron), and characterize its behavior on the COSMOS field. We train on ~390,000 paired cutouts drawn uniformly within the WISE/Spitzer overlap and report all metrics on a held-out test set of 83,592 cutouts. The framework uses a convolutional neural network trained with a loss function that emphasizes accurate recovery of sources in crowded fields. The model recovers the total flux of the central source in a fixed aperture to a median relative error of 11%, a factor of ~2 better than the interpolation baselines; the gain reaches ~3x on the faintest quartile. The brightness dependence is monotonic: the aperture integrated flux error decreases from 13% on the faintest quartile to 8% on the brightest. At the 3-5 arcsec separations where WISE blends sources that Spitzer separates, the model recovers 35% of the source peaks detectable in the Spitzer truth compared with 9% for interpolation. The characteristic failure mode is oversmoothing of source profiles, which biases integrated flux measurements upward; this pattern is qualitatively similar to that of the interpolation baselines but is quantitatively smaller for the trained model. These results suggest genuine resolution enhancement and source deblending, providing a path toward applying super-resolution across the all-sky area that Spitzer could not cover. An appendix replicates the analysis at W2 -> IRAC Ch2 with consistent results; the trained model and code are publicly available.

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Discovery of Very Minor Merger-Induced Star-forming Clumps at $z=3.33$ with JWST

High-redshift galaxies often contain ''clumps:'' compact, $\sim$kpc-scale regions of intense star formation. These substructures make up significant fractions of the masses and star-formation rates of their host galaxies and are expected to affect the morphologies of their hosts, such as by building up bulges. Studying clumps can therefore inform theoretical models of galaxy evolution. Two main mechanisms have been proposed for their formation: gas fragmentation from disk instabilities and compression of gas during gravitational interactions with other galaxies. In the latter, only mergers with mass ratios greater than 1:10 have been considered. Here, we report James Webb Space Telescope spectroscopy and imaging of JADES 1034862, a galaxy at $z_{\textrm{spec}} = 3.33$, which hosts a clump that formed during an interaction with a companion with a stellar mass that is $\approx28$ times lower than the host. From fits to the spectral energy distributions of the clump and the companion, it is found that both of their star-formation histories rose sharply in the most recent 100-300 Myr, during which the clump formed $\sim10^{7.2} \ M_{\odot}$ in stars and currently makes up $\approx10\%$ of the total star-formation rate of its host. Given the higher frequency of mergers with low mass ratios ($<1:10$) to those with higher ratios, we infer that a substantial fraction of high-redshift clumps could have formed through weak gravitational interactions.

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The Fraction of Clumpy Galaxies in JADES Over $2<z<9$

High-redshift galaxies exhibit compact regions of intense star formation, known as ``clumps,'' which are conspicuous in the rest-frame ultraviolet. Studying them can shed light on how they form and evolve and inform theoretical models of galaxy evolution. We examine the evolution of clumpy galaxies with redshift and stellar mass over ${2<z<9}$ with James Webb Space Telescope (JWST) imaging from the JWST Advanced Extragalactic Survey (JADES). Off-center clumps are detected in the rest-frame near-ultraviolet (NUV) using similar techniques to those in earlier studies based on Hubble Space Telescope (HST) images. This is done for a sample of 9,121 star-forming galaxies with stellar masses $\log\left(M_{\star}/M_{\odot}\right) \geq 8$. The fraction of clumpy galaxies, $f_{\rm{clumpy}}$, increases from ${\sim10\%}$ at ${z\sim7.75}$ to ${ \sim70\%}$ at $z\sim2.75$ at $\log\left(M_{\star}/M_{\odot}\right) \geq 9$. Our $f_{\rm{clumpy}}$ values are generally higher at fixed redshift and increase faster with decreasing redshift than what studies based on HST data found, which we attribute largely to the higher sensitivity of JWST. $f_{\rm{clumpy}}$ correlates with stellar mass. Our $f_{\rm{clumpy}}$ measurements are compared with those from simulations as well as other observations. At low redshifts ($z\lesssim5.75$) and intermediate-to-high stellar masses ($\log\left(M_{\star}/M_{\odot}\right) \geq 9$), our results suggest gas fragmentation due to violent disk instabilities to be the dominant mechanism for forming clumps. At high redshifts and intermediate stellar masses, compression of gas during mergers appears to dominate.

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Exploring the Relationship Between Bars, Star Formation Activity, and Host Galaxy Properties from $\mathbf{z \sim 0}$ to $\mathbf{z \sim 2}$

We present the most comprehensive study to date of the relationship between bars, star formation, and galaxy properties from $z \sim$ 0 to $z \sim$ 2. We use a mass-complete sample of 1,171 galaxies from the JWST CEERS survey with $M_\star > 10^{10} M_\odot$ and repeat the analysis using COSMOS-Web data. Our results are: 1) At high redshift ($z \sim$ $1-2$) barred galaxies tend to have high sSFRs and low Sérsic indices ($n \leq 2$), while at low redshifts barred galaxies emerge with both low sSFR and higher $n$, suggestive of quiescent galaxies with bulges. 2) The fractional contribution of barred quiescent galaxies to the bar fraction rises steeply from $z \sim$ 2 to $z \sim$ 0, while that of barred actively star-forming galaxies falls. 3) The fraction of quiescent galaxies that are barred rises steeply over the last 10 Gyr. 4) Our empirical results show good agreement with the TNG50-1 simulations for bars with $a_{\mathrm{bar}}$ $>$ 1.5 kpc. Our results allow for the possibility that bar-driven secular evolution may lead to quiescence and/or that bars are more likely to persist and grow in gas-poor, quiescent galaxies. The steep rise in the quiescent bar fraction over 10 Gyr may represent an evolutionary sequence whereby gas-rich disks at high redshift first develop short, dynamically young bars and over time, repeated bar-driven gas inflows lead to central starbursts and declining gas fractions that strengthen the bar as the galaxy transitions toward quiescence.

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The Importance of Galaxy-Wide Star Formation in Driving Winds at z~1

In this work, we study winds for a representative sample of 86 star-forming galaxies (SFGs) at z~1 with $M_\star = 10^{9.0}-10^{11.5} M_\odot$, by measuring the Mg II line profiles in deep Keck spectra. A total of 50 (58\%) are found to have winds. Unlike local starburst galaxies, the wind detection rate does not exhibit a threshold in star-formation rate (SFR) density $Σ_\mathrm{SFR}$ at 0.1 Msun/yr/kpc$^2$, but shows a gradual decline around this value. We find correlations between wind velocity $v_\mathrm{wind}$ and SFR, $Σ_\mathrm{SFR}$, and stellar mass, as per previous studies. Intriguingly, the z~1 SFGs appear to follow the same $v_\mathrm{wind}$-SFR relation as local starbursts. A combined fit gives: log $v_\mathrm{wind}$ = 0.16 log SFR + 2.4 (3-sigma significance). This unified relation spans over 4 dex in SFR and agrees with Illustris-TNG. No unified relation is found between $v_\mathrm{wind}$ and stellar mass, sSFR, or $Σ_\mathrm{SFR}$. This suggests winds might be most closely associated with SFR. We examine whether winds in z~1 SFGs are driven by their most compact star-forming regions. To do so, we consider whether the relation between $v_\mathrm{wind}$ and the $Σ_\mathrm{SFR}$ measured from only these regions is stronger than that for the galaxy-wide $Σ_\mathrm{SFR}$. We do not find a stronger correlation, suggesting that winds are most related to $Σ_\mathrm{SFR}$ of the entire galaxy. Collectively, these findings suggest a picture in which galaxy-wide star formation plays an important role in driving winds at z~1. Wind bubbles from all star-forming regions could combine momentum and help lift their entrained gas out of the galaxy.

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OCEANS of Absorption: High-resolution NIRSpec Spectroscopy Reveals Diverse Balmer-line Absorption in Little Red Dots

The ``Little Red Dots' (LRDs) that appeared in JWST deep field images have been the subject of significant study since their discovery. In this work, we present high-resolution follow-up spectroscopy from the OCEANS program of 10 LRDs with Ha coverage at 3<z<7 in the CEERS/EGS field. We find Balmer-line absorption in 4 of these LRDs, a detection rate higher than the fractions reported in lower-resolution NIRSpec surveys. All of the absorbers are presented in high-resolution for the first time here and two have Balmer-line absorption detected for the first time. Of the 10 LRDs, 7 are best fit by Ha profiles with exponential wings. We find that absorbers tend to be blue-shifted with a median velocity offset of (-49 km/s) and absorption equivalent width of 5.3 Angstroms. Trends are explored to compare LRD absorption properties along the sequence of LRDs. We confirm an LRD with statistically significant absorption velocity offsets between Ha and Hb. The diversity of absorption properties can be effectively explained by a model with a radial distribution of partial-covering absorbing gas that is often co-located near the broad-line emission regions, along with a radial gradient of close inflow and distant outflow velocities for the absorbing gas. We present other interesting LRDs, including an outflow-dominated LRD and an LRD with relatively blue UV-to-optical colors but clear Balmer-line absorption. This high occurrence of absorbing hydrogen in LRDs, evident by both the Balmer-line absorption features and Balmer break strengths, implies a near-ubiquitous presence of dense, excited n=2 hydrogen.

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Near-infrared emission line diagnostics for AGN from the local Universe to redshift 3

Optical rest-frame spectroscopic diagnostics are usually employed to distinguish between star formation and AGN-powered emission. However, this method is biased against dusty sources, hampering a complete census of the AGN population across cosmic epochs. To mitigate this effect, it is crucial to observe at longer wavelengths in the rest-frame near-infrared (near-IR), which is less affected by dust attenuation and can thus provide a better description of the intrinsic properties of galaxies. AGN diagnostics in this regime have not been fully exploited so far, due to the scarcity of near-IR observations of both AGNs and star-forming galaxies, especially at redshifts higher than 0.5. Using Cloudy photoionization models, we identify new AGN - star formation diagnostics based on the ratio of bright near-infrared emission lines, namely [SIII] 9530 Angstrom, [CI] 9850 Angstrom, [PII] 1.188 $μm$, [FeII] $1.257 μm$, and [FeII] $1.64 μm$ to Paschen lines (either Pa$γ$ or Pa$β$), providing simple, analytical classification criteria. We apply these diagnostics to a sample of 64 star-forming galaxies and AGNs at 0 < z < 1, and 65 sources at 1 < z < 3 recently observed with JWST-NIRSpec in CEERS. We find that the classification inferred from the near-infrared is broadly consistent with the optical one based on the BPT and the [SII]/H$α$ ratio. However, in the near-infrared, we find $\sim 60 \%$ more AGNs than in the optical (13 instead of 8), with 5 sources classified as 'hidden' AGNs, showing a larger AGN contribution at longer wavelengths, possibly due to the presence of optically thick dust. The diagnostics we present provide a promising tool to find and characterize AGNs from z=0 to z=3 with low and medium-resolution near-IR spectrographs in future surveys.

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AGNBoost: A Machine Learning Approach to AGN Identification with JWST/NIRCam+MIRI Colors and Photometry

We present AGNBoost, a machine learning framework utilizing XGBoostLSS to identify AGN and estimate redshifts from JWST NIRCam and MIRI photometry. AGNBoost constructs 66 input features from 7 NIRCam and 4 MIRI bands to predict the fraction of mid-IR $3$--$30\,μ$m emission attributable to an AGN power law ($\text{frac}_{\text{AGN}}$) and photometric redshift. Each model is trained on $10^6$ simulated galaxies from CIGALE. Models are tested on mock CIGALE galaxies, an independent set of empirically-derived templates, and 748 observations from the JWST MIRI EGS Galaxy and AGN (MEGA) survey. On idealized noise-free mock CIGALE galaxies, AGNBoost achieves $15\%$ outlier fractions of $1.63\%$ ($\text{frac}_{\text{AGN}}$) and $0.15\%$ (redshift), with $σ_{\text{RMSE}} = 0.045$ for $\text{frac}_{\text{AGN}}$ and $σ_{\text{NMAD}} = 0.004$ for redshift. When realistic photometric uncertainties are introduced, performance remains robust with median predictions on the 1:1 relation, though outlier fractions increase to $4.38\%$ and $3.35\%$, respectively. On the independent template set, AGNBoost identifies $92.6\%$ of AGN candidates with $\text{frac}_{\text{AGN}} > 0.3$ and $100\%$ with $\text{frac}_{\text{AGN}} > 0.5$, demonstrating generalization beyond the training distribution. On MEGA galaxies with spectroscopic redshifts, AGNBoost achieves $σ_{\text{NMAD}} = 0.056$ and $19.79\%$ outliers. AGNBoost $\text{frac}_{\text{AGN}}$ estimates broadly agree with CIGALE fitting ($σ_{\text{RMSE}} = 0.178$, $11.96\%$ outliers). The flexible framework allows straightforward incorporation of additional photometric bands and re-training for other variables. AGNBoost's computational efficiency makes it well-suited for wide-sky surveys requiring rapid AGN identification and redshift estimation.

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Extreme Emission Line Galaxies in CEERS Are Powered by Star Formation, not AGN

We present a spectroscopic study of photometrically identified extreme emission-line galaxies (EELGs) with observed-frame equivalent widths (EWs) >5000 A of either H alpha or H beta + [OIII] in the CEERS legacy deep field utilizing JWST NIRSpec spectroscopy from the CAPERS, RUBIES, THRILS and CEERS surveys. This master sample allows for performance tests of photometric selections and unveils what types of sources, either AGN or young star formation, were producing excessive ionizing radiation in the early Universe. We identify AGN through broad H alpha emission-lines and report 6 new broad-line AGN at 3.5<z<7 identified by the deep (~8 hr) G395M THRILS survey. We investigate the photometrically selected EELGs in a color-color plot designed for ``Little Red Dot'' selection and demonstrate that it effectively removes AGN with non-extreme lines from the sample. EELGs with and without broad lines show similar optical line ratios. We compare emission-line morphology to EWs and continuum morphologies and find that [OIII] morphology is more compact at higher EW. ~10% of photometrically selected EELGs have broad Balmer lines, jumping to 35% in deep spectroscopy which indicates a significant fraction of photometrically selected EELGs may host AGN. However, many AGN selected as EELGs have incorrectly high photometric EWs. For sources with extreme emission-line EWs that pass our photometric criteria and host an AGN, we find that the narrow H alpha component dominates over the broad, especially in the highest-EW sources. This implies that even when an AGN is present, it does not dominate the extreme emission.

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Mysteries of Capotauro: investigating the puzzling nature of an extreme F356W-dropout

JWST has uncovered a diverse population of extreme near-infrared dropouts, including ultra high-redshift ($z>15$) galaxy candidates, dust-obscured galaxies challenging dust production theories, sources with strong Balmer breaks - possibly compact AGN in dense environments - and cold, sub-stellar Galactic objects. This work presents Capotauro, a F356W-dropout in the CEERS survey with F444W AB magnitude of $\sim27.68$ and a sharp $>3$ mag flux drop between $3.5{-}4.5\,μ$m, undetected below $3.5\,μ$m. We combine JWST/NIRCam, MIRI, and NIRSpec/MSA data with HST/ACS and WFC3 observations to perform a spectro-photometric analysis of Capotauro using multiple SED-fitting codes. Our setup tests $z\geq15$ as well as $z<10$ dusty, Balmer-break or strong-line galaxy solutions, and the possibility of Capotauro being a Milky Way sub-stellar object. Among extragalactic options, our analysis favors interpreting the sharp drop as a Lyman break at $z\sim32$, consistent with the epoch of formation of the first stars and black holes, with only $\sim0.5\%$ of the posterior volume at $z<25$. Lower-redshift solutions struggle to reproduce the extreme break, suggesting that if Capotauro lies at $z<10$, it must show a non-standard combination of strong dust attenuation and/or Balmer breaks, making it a peculiar interloper. Alternatively, its properties match a very cold (Y2-Y3 type) brown dwarf or a free-floating exoplanet with a record-breaking combination of low temperature and large distance ($T_{\mathrm{eff}}<300\,\mathrm{K}$, $d\gtrsim130\,\mathrm{pc}$, up to $\sim2\,\mathrm{kpc}$). While current data cannot determine its nature, Capotauro emerges as a remarkably unique object in all plausible scenarios, and a compelling target for follow-up.

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Testing Photometric Techniques for Measuring the Rest-Frame UV Spectral Slope Against JWST PRISM Spectroscopy

We present a sample of 53 galaxy spectra at z_spec ~ 5-12 from the JWST CEERS and RUBIES surveys, combining NIRSpec PRISM spectroscopy with NIRCam photometry. We aim to use these data to establish best practices for measuring the UV spectral slope ($β$) in the era of JWST. We adopt power-law fits to the rest-frame UV continuum from the spectroscopic data as our fiducial, or `true', $β$ values, and compare them to photometric estimates derived through four methods: (1) photometric power-law fitting, (2) power-law fitting to an SED model fitted to the photometry, (3) single-color fitting near the Lyman break, and (4) single-color fitting at fixed rest-frame wavelengths. We find that photometric power-law fitting most closely recovers the spectroscopic slopes, with minimal bias and scatter. SED fitting performs moderately well, and can be preferable in cases of low signal-to-noise where photometric power-law fitting may become unreliable. Single-color estimates, while commonly used in past studies, show the most significant deviations and are not recommended when more than a single color is available. Our results highlight the limitations and strengths of each approach and provide practical guidance for measuring $β$ from photometry when spectra are unavailable or are of insufficient quality.

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Large-Scale Structure in COSMOS-Web: Tracing Galaxy Evolution in the Cosmic Web up to $z \sim 7$ with the Largest JWST Survey

We present a reconstruction of the large-scale structure using the James Webb Space Telescope's (JWST) COSMOS-Web program to trace environmentally driven galaxy evolution up to $z\sim7$. We applied a weighted kernel density estimation method to 160,000 galaxies with robust photometric redshifts. We find that stellar mass has a positive correlation with density at all redshifts, stronger for quiescent galaxies (QGs) at $z\lesssim2.5$, while at higher redshifts ($2.5\lesssim z\lesssim5.5$) this trend is confined to extreme overdense environments, consistent with early mass assembly in proto-clusters. The star-formation rate (SFR) shows a negative trend with density for QGs at $z\lesssim1.2$, reversing at $z\gtrsim1.8$, while star-forming galaxies (SFGs) show a mild positive correlation up to $z\sim5.5$. The specific SFR remains nearly flat for SFGs and declines with density for QGs at $z\lesssim1.2$. Moreover, mass and environmental quenching efficiencies show that mass-driven processes dominate at $z\gtrsim2.5$, the two processes act with comparable strength between $0.8\lesssim z\lesssim2.5$, and environmental quenching becomes stronger for low-mass galaxies ($M_\star\lesssim10^{10} M_\odot$) at $z\lesssim0.8$. These findings reveal that large-scale structure drives galaxy evolution by enhancing early mass assembly in dense regions and increasingly suppressing star formation in low-mass systems at later times, establishing the environmental role of the cosmic web across cosmic history. COSMOS-Web, the largest JWST survey, provides accurate and deep photometric redshifts, reaching 80% mass completeness at $\log(M_\star/M_\odot)\sim8.7$ at $z\sim7$, enabling the first view of how environments shaped galaxy evolution from the epoch of reionization to the present day.

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The Abundance and Properties of Barred Galaxies out to $z \sim$ 4 Using $\textit{JWST}$ CEERS Data

We present the first estimate of the observed fraction and properties of bars out to $ z\sim4$ using $\textit{JWST}$ CEERS NIRCam images. We analyze 1770 galaxies with $M_\star > 10^{10} M_\odot$ at $0.5 \leq z \leq 4$ and identify barred galaxies from 839 moderately inclined disk galaxies via ellipse fits and visual classification of both F200W and F444W images. Our results apply mainly to bars with projected semi-major axis $a_{\rm bar}$ $> 1.5 $ kpc ($\sim$ 2 $\times$ PSF in F200W images) that can be robustly traced by ellipse fits. For such bars, the observed bar fraction at $z\sim$ 2-4 is low ($\lesssim 10\%$), and they appear to be emerging at least as early as $z\sim 4$. Our observed bar fraction is consistent with the bar fraction predicted by TNG50 simulations for large bars with $a_{\rm bar}$ $> 1.5$ kpc at $z \sim$ 0.5-4 and with the bar fraction from Auriga simulations out to $z \sim$ 1.5. However, TNG50 simulations predict a large population of smaller bars that our data cannot robustly detect. If such bars exist, the true bar fraction at $z \sim$ 2-4 may be significantly higher than our results. At $z \ge 1.5$, many barred galaxies show nearby neighbors, suggesting bars may be tidally triggered. From $z \sim$ 4-0.5, the observed bar fraction, average projected bar length, and projected bar strength rise. Our results highlight the early emergence and evolution of barred galaxies and the rising importance of bar-driven secular evolution from $z \sim$4 to today.

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Improved SED-Fitting Assumptions Result in Inside-Out Quenching at $z\sim0.5$ and Quenching at All Radii Simultaneously at $z\sim1$

Many studies conclude that galaxies quench from the inside-out by examining profiles of specific star-formation rate (sSFR). These are usually measured by fitting spectral energy distributions (SEDs) assuming a fixed dust law and uniform priors on all parameters. Here, we examine the effects of more physically motivated priors: a flexible dust law, an exponential prior on the dust attenuation $A_V$, and Gaussian priors that favor extended star-formation histories. This results in model colors that better trace observations. We then perform radial SED fits to multiband flux profiles measured from Hubble Space Telescope images for 1,440 galaxies at $0.4<z<1.5$ of stellar masses $10^{10}-10^{11.5}\ M_{\odot}$ using both the traditional and the more physically motivated assumptions. The latter results in star formation rate and $A_V$ profiles that agree with measurements from spectroscopy and $A_V$ profiles that behave correctly as a function of inclination. Since green valley galaxies at $z\sim1.3$ are expected to evolve into quiescent galaxies at $z\sim0.9$, we compare their sSFR profiles using the more physically motivated assumptions. Their slopes are similar at all masses ($0.06 - 0.08~\textrm{dex}~\textrm{kpc}^{-1}$), and the normalizations for the quiescent galaxies are lower. Therefore, the sSFR profiles decline with time as quenching occurs at all radii simultaneously. We compare profiles of green valley galaxies at $z\sim0.9$ and quiescent galaxies at $z\sim0.5$. The former are shallower at all masses by $\sim0.1~\textrm{dex}~\textrm{kpc}^{-1}$. The sSFR profiles steepen with time as galaxies quench from the inside-out. In summary, at $z\sim0.9-1.3$, galaxies quench at all radii simultaneously, and at $z\sim0.5-0.9$, they quench from the inside-out.

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Strong spectral features from asymptotic giant branch stars in distant quiescent galaxies

Dating the ages and weighting the stellar populations in galaxies are essential steps when studying galaxy formation through cosmic times. Evolutionary population synthesis models with different input physics are used for this purpose. Moreover, the contribution from the thermally pulsing asymptotic giant branch (TP-AGB) stellar phase, which peaks for intermediate-age 0.6-2 Gyr, has been debated for decades. Here we report the detection of strong cool-star signatures in the rest-frame near-infrared spectra of three young (~1Gyr), massive (~10^10Msun) quiescent galaxies at large look-back time, z=1-2, using JWST/NIRSpec. The coexistence of oxygen- and carbon-type absorption features, spectral edges and features from rare species, such as vanadium and possibly zirconium, reveal a strong contribution from TP-AGB stars. Population synthesis models with a significant TP-AGB contribution reproduce the observations better than those with a weak TP-AGB, which are commonly used. These findings call for revisions of published stellar population fitting results, as they point to populations with lower masses and younger ages and have further implications for cosmic dust production and chemical enrichment. New generations of improved models are needed, informed by these and future observations.

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JWST/CEERS sheds light on dusty star-forming galaxies: forming bulges, lopsidedness and outside-in quenching at cosmic noon

We investigate the morphology and resolved physical properties of a sample of 22 IR-selected DSFG at cosmic noon using the JWST/NIRCam images obtained in the EGS field for the CEERS survey. The resolution of the NIRCam images allowed to spatially resolve these galaxies up to 4.4um and identify their bulge even when extinguished by dust. The goal of this study is to obtain a better understanding of the formation and evolution of FIR-bright galaxies by spatially resolving their properties using JWST in order to look through the dust and bridge the gap between the compact FIR sources and the larger optical SFG. Based on RGB images from the NIRCam filters, we divided each galaxy into several uniformly colored regions, fitted their respective SEDs, and measured physical properties. After classifying each region as SF or quiescent, we assigned galaxies to three classes, depending on whether active SF is located in the core, in the disk or in both. We find (i) that galaxies at a higher z tend to have a fragmented disk with a low core mass fraction. They are at an early stage of bulge formation. When moving toward a lower z, the core mass fraction increases, and the bulge growth is associated with a stabilization of the disk: the NIRCam data clearly point toward bulge formation in preexisting disks. (ii) Lopsidedness is a common feature of DSFGs. It could have a major impact on their evolution; (iii) 23% of galaxies have a SF core embedded in a quiescent disk. They seem to be undergoing outside-in quenching, often facilitated by their strong lopsidedness inducing instabilities. (iv) We show that half of our galaxies with SF concentrated in their core are good SMG counterpart candidates, demonstrating that compact SMGs are usually surrounded by a larger, less obscured disk. (v) Finally, we found surprising evidence for clump-like substructures being quiescent or residing in quiescent regions.

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Characterizing the Average Interstellar Medium Conditions of Galaxies at $z\sim$ 5.6-9 with UV and Optical Nebular Lines

Ultraviolet (UV; rest-frame $\sim1200-2000$ A) spectra provide a wealth of diagnostics to characterize fundamental galaxy properties, such as their chemical enrichment, the nature of their stellar populations, and their amount of Lyman-continuum (LyC) radiation. In this work, we leverage publicly released JWST data to construct the rest-frame UV-to-optical composite spectrum of a sample of 63 galaxies at $5.6<z<9$, spanning the wavelength range from 1500 to 5200 A. Based on the composite spectrum, we derive an average dust attenuation $E(B-V)_\mathrm{gas}=0.16^{+0.10}_{-0.11}$ from \hb/\hg, electron density $n_e = 570^{+510}_{-290}$ cm$^{-3}$ from the [O II] doublet ratio, electron temperature $T_e = 17000^{+1500}_{-1500}$ K from the [O III] $\lambda4363$/ [O III] $\lambda5007$ ratio, and an ionization parameter $\log(U)=-2.18^{+0.03}_{-0.03}$ from the [O III]/[O II] ratio. Using a direct $T_e$ method, we calculate an oxygen abundance $12+\log\mathrm{(O/H)}=7.67\pm0.08$ and the carbon-to-oxygen (C/O) abundance ratio $\log\mathrm{(C/O)}=-0.87^{+0.13}_{-0.10}$. This C/O ratio is smaller than compared to $z=0$ and $z=2$ - 4 star-forming galaxies, albeit with moderate significance. This indicates the reionization-era galaxies might be undergoing a rapid build-up of stellar mass with high specific star-formation rates. A UV diagnostic based on the ratios of C III] $λ\lambda1907,1909$/He II $\lambda1640$ versus O III] $\lambda1666$/He II $\lambda1640$ suggests that the star formation is the dominant source of ionization, similar to the local extreme dwarf galaxies and $z\sim2$ - 4 He II-detected galaxies. The [O III]/[O II] and C IV/C III] ratios of the composite spectrum are marginally larger than the criteria used to select galaxies as LyC leakers, suggesting that some of the galaxies in our sample are strong contributors to the reionizing radiation.

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