arXiv subjects

Samir Salim

Publications and source records attributed to Samir Salim.

At least 19 recordsLinked to original sources

arXiv2026

The TNG50-SKIRT Atlas: Spatially resolved synthetic galaxies from the ultraviolet to the submillimetre (DR2)

We present the second data release (DR2) of the TNG50-SKIRT Atlas (TSA), a library of synthetic, spatially resolved galaxy observables. The atlas is constructed by post-processing a stellar-mass-complete ($10^{9.8}~{\text{M}}_\odot < M_\star < 10^{12}~{\text{M}}_\odot$) sample of 1154 $z=0$ galaxies from the TNG50 cosmological hydrodynamical simulation with the Monte Carlo radiative transfer code SKIRT. Compared to the first release, TSA DR2 extends the wavelength coverage from the ultraviolet to the submillimetre, including dust emission, and incorporates updated stellar population models together with an improved treatment of dust-enshrouded star-forming regions. The atlas provides spatially resolved spectral energy distributions, broadband images, and physical property maps for multiple viewing orientations, as well as a catalogue of integrated properties enabling direct comparison with unresolved observations. We validate the data products through extensive quality control, including an assessment of Monte Carlo noise, and demonstrate their internal consistency using diagnostic relations between luminosities and star formation rates. TSA DR2 provides a versatile resource for studies of dust attenuation and emission, star formation tracers, galaxy morphology, and multi-wavelength scaling relations across spatial scales. The atlas and associated data products are publicly released and are intended to support a wide range of observationally oriented studies of galaxy evolution.

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arXiv2026

Summary of Discussion Sessions from "The Dusty Universe 2025: The Fifth Pandust Conference"

"The Dusty Universe: The Fifth Pandust Conference" took place in Tucson (AZ, USA) from November 10 until 14, 2025. The goal of this meeting was to get the dust community together to review where we are, hear exciting new results, and make plans for the future. The meeting encompassed all aspects of dust investigations including observations, theory, modeling, and laboratory studies. The conference consisted of invited review talks, contributed talks and posters. Science topics included interstellar dust (Milky Way & nearby galaxies), circumstellar dust (including Solar System & exoplanets), dust in Galaxies (including high-z), lifecycle of dust, and future needs (laboratory, theory, & observations), with a particular focus on results from JWST and ALMA, and on nanodust (including PAHs). On November 12th, we organized breakout discussion sessions covering a wide range of interesting dust-related topics. The purpose of this document is to capture the main topics/questions that were discussed, the key conclusions of these discussions, the challenges and possible solutions that were brought up, and the open questions that still remain to be answered. We hope that this document records our findings and challenges for the future generation.

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arXiv2026

A Comprehensive Analysis of WISE Mid-Infrared Colors for Obscured AGN Selection

In this paper, we investigate the robustness of WISE mid-IR color selection (W1-W2) for identifying obscured (Type 2) active galactic nuclei (AGNs) at low redshift (z<0.3), using a sample of ~360,000 SDSS galaxies classified via emission lines into Seyfert 2 (Sy2), LINER, and star-forming (BPT-SF) galaxies. We find that the K-correction is essential to remove non-AGN contamination, and once applied the simple W1-W2>0.5 selection emerges as optimal in terms of purity and completeness of AGN selection. However, we confirm that even this lenient cut selects only ~13% of Sy2 galaxies and that achieving W1-W2>0.5 requires AGN contributing >75% of the total infrared luminosity, which is uncommon. Although mid-IR-selected Sy2s tend to be luminous, the high [OIII] luminosity does not guarantee red W1-W2 (nor does any other tested global or NLR-scale parameter), suggesting the critical role of obscuration on smaller scales. <1% of BPT-SF systems (but making ~20% of all mid-IR selected galaxies) exhibit W1-W2>0.5 colors. Such colors cannot be reproduced by models of star-heated dust alone. Red BPT-SFs tend to have higher W4 luminosities than expected from SF, indicating true AGNs. Intriguingly, mid-IR AGNs in massive bulges ($M_{\mathrm{bulge}} \gtrsim 10^{10} M_{\odot}$) predominantly (84%) manifest themselves as BPT-AGNs, whereas those in low-mass bulges ($\lesssim 10^{10} M_{\odot}$) mostly (60%) manifest as BPT-SF. This BPT-AGN vs.\ BPT-SF dichotomy does not extend to total stellar mass. We conclude that although the mid-IR AGN selection is incomplete, its strength lies in identifying optically inconspicuous AGNs with low-mass bulges, regardless of the total mass.

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arXiv2026

GalSyn I: A Forward-Modeling Code for Synthetic Galaxy Observations from Hydrodynamical Simulations and First Data Release from IllustrisTNG

We present GalSyn (Galaxy Synthesizer), a modular and flexible Python package for generating synthetic observations from hydrodynamical galaxy simulations. GalSyn generates synthetic spectrophotometric data cubes for individual galaxies from simulation cutouts, employing a particle-by-particle spectral modeling approach that enables the rapid production of large synthetic datasets required for statistical population studies, offering a computationally efficient alternative to full radiative transfer codes. Users have full control over the spectral modeling choices, including the stellar population synthesis engine, stellar isochrones, spectral libraries, and initial mass functions. Dust attenuation is modeled at spatially resolved scales using a line-of-sight column-density method, with a comprehensive suite of fixed and adaptive attenuation laws. A decoupled kinematics model independently Doppler-shifts the stellar and nebular components, enabling realistic synthetic integral field unit data cubes. It also provides features to add observational realism, including PSF convolution and noise simulation. Beyond generating imaging and spectroscopic data cubes, GalSyn reconstructs spatially resolved physical property maps and star formation histories (SFHs) of a galaxy. Alongside this paper, we present the first data release of synthetic imaging observations and resolved SFHs generated from the IllustrisTNG simulations. This release includes four mock extragalactic survey fields together with data cubes of the individual galaxies within them, data cubes of 290 local massive galaxies and their progenitors tracked across $0<z<5$, and 259 major-merger systems. Each galaxy data cube contains imaging across 47 filters spanning HST, JWST, Euclid, Rubin/LSST, and the Roman Space Telescope. GalSyn is publicly available at https://github.com/aabdurrouf/GalSyn.

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arXiv2026

Ashes of FIRE: Modeling Dust Grain Size Evolution in the Local Group with FIRE

We introduce a new, discretized grain size evolution model, incorporated into the GIZMO code and coupled with FIRE-3 stellar feedback and ISM physics, to investigate variations in dust abundance, chemical composition, and grain sizes observed in the Local Group. This model tracks the size evolution of specific dust species, and includes stellar production of dust, dust growth through gas-phase metal accretion, dust destruction by sputtering, SNe shocks, and astration, grain-grain collisional shattering and coagulation, and turbulent dust diffusion. Using idealized galaxy simulations, we test the dependence of MW dust properties on variations in each dust process and find that our model uniquely predicts a bimodal grain size distribution. This bimodality is due to our simulation's ability to resolve each dust process and where they occur in the ISM, unlike other works. We find that Local Group dust abundances are determined by dust growth and destruction, with little dependence on coagulation or shattering, explaining why models that do not include these processes can match abundance observations. We also find that variations in Local Group extinction curve slopes are determined by coagulation, with inefficient coagulation leading to steeper slopes. However, inefficient coagulation also results in stronger extinction curve bumps, which are not observed. We also do not predict a population of very small (${<}1$ nm) carbonaceous grains, required for MIR emission features, due to their rapid growth by accretion. These results highlight the possible necessity of ``top-down'' PAH formation from preexisting grains as a means to inhibit carbonaceous dust growth.

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arXiv2025

Highly Efficient Identification of Extreme Emission Line Galaxies in the Local Universe: >8000 New Green Pea Candidates at 0.12 < z < 0.36

The currently known compact extreme emission-line galaxies (the "Green Peas", GPs) in SDSS are rare and were mostly found among serendipitous spectroscopic targets, thus leaving open the possibility that a substantial population of GPs is missed. A significantly larger number of identified GPs in the Local Universe might provide a better characterization of their high-redshift analogs and Lyman continuum escape. In this paper, we confront the challenges of robustly identifying GPs without spectroscopic information, a needed approach considering the incompleteness of spectroscopic surveys for compact sources. The principal difficulty stems from a significant contamination of photometric candidates by stars and quasars of similar color. To solve this, we introduce an SED matching method, which separates candidate GPs from contaminants on the basis of SDSS and WISE photometry of spectroscopically confirmed stars, quasars and galaxies. The method has an effectiveness of 85%, and a contamination rate of ~10%. With it we identify ~9600 GP candidates expected to lie in the 0.12 < z < 0.36 range - a tenfold increase over what would be selected using SDSS DR18 spectra. Some of the new GPs are as bright as r~19, and 1200 are predicted to have [OIII]5007 equivalent widths in excess of 500 A. The new population contains many "Extended Peas", which are absent among known GPs and possibly represent merging systems. We provide catalogs containing 8313 newly identified GP candidates, as well as 917 GPs confirmed using SDSS spectroscopy and 521 GPs with spectroscopic redshifts from LAMOST and other sources.

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arXiv2024

Galaxy evolution in the post-merger regime. III -- The triggering of active galactic nuclei peaks immediately after coalescence

Galaxy mergers have been shown to trigger AGN in the nearby universe, but the timescale over which this process happens remains unconstrained. The Multi-Model Merger Identifier (MUMMI) machine vision pipeline has been demonstrated to provide reliable predictions of time post-merger (T_PM) for galaxies selected from the Ultraviolet Near Infrared and Optical Northern Survey (UNIONS) up to T_PM=1.76 Gyr after coalescence. By combining the post-mergers identified in UNIONS with pre-coalescence galaxy pairs, we can study the triggering of AGN throughout the merger sequence. AGN are identified using a range of complementary metrics: mid-IR colours, narrow emission lines and broad emission lines, which can be combined to provide insight into the demographics of dust and luminosity of the AGN population. Our main results are: 1) Regardless of the metric used, we find that the peak AGN excess (compared with a matched control sample) occurs immediately after coalescence, at 0 < T_PM < 0.16 Gyr. 2) The excess of AGN is observed until long after coalescence; both the mid-IR selected AGN and broad line AGN are more common than in the control sample even in the longest time bin of our sample (0.96 < T_PM < 1.76 Gyr). 3) The AGN excess is larger for more luminous and bolometrically dominant AGN, and we find that AGN in post-mergers are generally more luminous than secularly triggered events. 4) A deficit of broad line AGN in the pre-merger phase, that evolves into an excess in post-mergers is consistent with evolution of the covering fraction of nuclear obscuring material. Before coalescence, tidally triggered inflows increase the covering fraction of nuclear dust; in the post-merger regime feedback from the AGN clears (at least some of) this material. 5) The statistical peak in the triggering of starbursts occurs contemporaneously with AGN, within 0.16 Gyr of coalescence.

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arXiv2024

A Dusty Dawn: Galactic Dust Buildup at $z\gtrsim5$

Over the last decade, the Atacama Large Millimeter Array has revealed massive, dusty star-forming galaxies at $z\gtrsim5$, and the James Webb Space Telescope is primed to uncover even more information about them. These observations need dust evolution theory to provide context and are excellent benchmarks to test this theory. Here, we investigate the evolution of galactic dust budget at cosmic dawn using a suite of cosmological zoom-in simulations of moderately massive, high-redshift ($M_{\rm star}\gtrsim10^9 M_{\odot}$; $z\gtrsim5$) galaxies from the FIRE project, the highest resolution ($m_{\rm b} \approx 7100\, M_{\odot}$) of such simulations to date. Our simulations incorporate a dust evolution model that accounts for the dominant sources of dust production, growth, and destruction and follows the evolution of specific dust species, allowing it to replicate a wide range of present-day observations. We find, similar to other theoretical works, that dust growth via gas-dust accretion is the dominant producer of dust mass for these massive, $z\gtrsim 5$ galaxies. However, our fiducial model produces $M_{\rm dust}$ that fall ${\gtrsim}1$ dex below observations at any given $M_{\rm star}$ (typical uncertainties are ${\sim}1$ dex), which we attribute to reduced accretion efficiencies caused by a combination of low galactic metallicities and extremely bursty star formation. Modest enhancements (i.e., within observational/theoretical uncertainties) to accretion and SNe II dust creation raise $M_{\rm dust}$ by ${\lesssim}1$ dex, but this still falls below observations which assume $T_{\rm dust}\sim25$ K. One possibility is that inferred dust masses for $z\gtrsim4$ galaxies are overestimated, and recent observational/analytical works that find $T_{\rm dust}\sim50$ K along with metallicity constraints tentatively support this.

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arXiv2024

Assessment of SDSS-Derived Galaxy Morphologies Using HST Imaging

The Sloan Digital Sky Survey (SDSS) was foundational to the study of galaxy evolution, having revealed the bimodality of galaxies and the relationship between their structure and star-forming activity. However, ground-based optical surveys like SDSS are limited in resolution and depth which may lead to biases or poor quality in the derived morphological properties, potentially impacting our understanding of how and why galaxies cease their star formation (quench). We use archival HST imaging of ~2,000 SDSS objects to assess the reliability of SDSS-derived morphologies, taking advantage of both SDSS statistical samples and of HST's superior resolution and sensitivity. Single Sersic fitting and bulge-disk decomposition is performed on HST images for direct comparison with SDSS results. Of the three catalogs of SDSS-derived morphologies considered, none are significantly more accurate than the others. For disk-dominated galaxies (n<2.5), global Sersic indices (n) from Meert et al. 2015 (M15) are preferred. For bulge-dominated galaxies (n>2.5), Simard et al. 2011 (S11) and M15 overestimate n by ~20%, and NYU-derived global n are preferred. Global R_eff from S11 are preferred, but overestimate R_eff for the largest galaxies by 0.1 dex. SDSS-derived single-component parameters are generally significantly more robust than SDSS-derived two-component parameters. The bulge Sersic index (n_bulge) cannot be reliably constrained from SDSS imaging. The bulge-to-total (B/T) ratio can be reliably inferred from SDSS for galaxies with SDSS B/T<0.6 provided that n_bulge=4 is enforced. The difference in global n between HST and SDSS depends strongly on B/T; an empirical correction based only on it accounts for most of the systematics in global n.

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arXiv2024

FORECASTOR -- II. Simulating Galaxy Surveys with the Cosmological Advanced Survey Telescope for Optical and UV Research

The Cosmological Advanced Survey Telescope for Optical and UV Research (CASTOR) is a planned flagship space telescope, covering the blue-optical and UV part of the spectrum. Here we introduce the CASTOR image simulator, a Python GalSim package-based script capable of generating mock CASTOR images from an input catalogue. We generate example images from the CASTOR Wide, Deep, and Ultra-Deep surveys using simulated light-cones from the Santa Cruz Semi-Analytic Model. We make predictions for the performance of these surveys by comparing galaxies that are extracted from each image using Source Extractor to the input catalogue. We find that the Wide, Deep, and Ultra-Deep surveys will be 75% complete for point sources down to ~27, 29 and 30 mag, respectively, in the UV, u, and g filters, with the UV-split and u-split filters reaching a shallower depth. With a large area of ~2200 deg$^2$, the Wide survey will detect hundreds of millions of galaxies out to z~4, mostly with $M_\ast \gtrsim 10^9 M_\odot$. The Ultra-Deep survey will probe to z~5, detecting galaxies with $M_\ast \gtrsim 10^7 M_\odot$. These galaxy samples will enable precision measurements of the distribution of star formation in the cosmic web, connecting the growth of stellar mass to the assembly of dark matter halos over two thirds of the history of the Universe, and other core goals of CASTOR's legacy surveys. These image simulations and the tools developed to generate them will be a vital planning tool to estimate CASTOR's performance and iterate the telescope and survey designs prior to launch.

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arXiv2024

X-ray AGNs with SRG/eROSITA: Multi-wavelength observations reveal merger triggering and post-coalescence circumnuclear blowout

Major mergers between galaxies are predicted to fuel their central supermassive black holes (SMBHs), particularly after coalescence. However, determining the prevalence of active galactic nuclei (AGNs) in mergers remains a challenge, because AGN diagnostics are sensitive to details of the central structure (e.g., nuclear gas clouds, geometry and orientation of a dusty torus) that are partly decoupled from SMBH accretion. X-rays, expected to be ubiquitous among accreting systems, are detectable through non-Compton-thick screens of obscuring material, and thus offer the potential for a more complete assessment of AGNs in mergers. But, extant statistical X-ray studies of AGNs in mergers have been limited by either sparse, heterogeneous, or shallow on-sky coverage. We use new X-ray observations from the first SRG/eROSITA all-sky data release to characterize the incidence, luminosity, and observability of AGNs in mergers. Combining machine learning and visual classification, we identify 923 post-mergers in Dark Energy Camera Legacy Survey (DECaLS) imaging and select 4,565 interacting galaxy pairs (with separations <120 kpc and mass ratios within 1:10) from the Sloan Digital Sky Survey. We find that galaxies with X-ray AGNs are 2.0+/-0.24 times as likely to be identified as post-mergers compared to non-AGN controls, and that post-mergers are 1.8+/-0.1 times as likely to host an X-ray AGN as non-interacting controls. A multi-wavelength census of X-ray, optical, and mid-IR-selected AGNs suggests a picture wherein the underlying AGN fraction increases during pair-phase interactions, that galaxy pairs within ~20 kpc become heavily obscured, and that the obscuration often clears post-coalescence.

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arXiv2024

Strategies for obtaining robust SED fitting parameters for galaxies at z~1 and z~2 in the absence of IR data

Robust estimation of star formation rates (SFRs) at higher redshifts (z>1) using UV-optical-NIR photometry is contingent on the ability of spectral energy distribution (SED) fitting to simultaneously constrain the dust attenuation, stellar metallicity, and star formation history (SFH). IR-derived dust luminosities can help break the degeneracy between these parameters, but IR data is often not available. Here, we explore strategies for SED fitting at z>1 in the absence of IR data using a sample of log M*>10.2 star-forming galaxies from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) for which 24mu data are available. We adopt the total IR luminosity (L_TIR) obtained from 24mu as the 'ground truth' that allows us to assess how well it can be recovered (as L_dust) from UV-optical-NIR SED fitting. We test a variety of dust attenuation models, stellar population synthesis models, metallicity assumptions, and SFHs separately to identify which assumptions maximize the agreement (correlation and linearity) between L_TIR and L_dust. We find that a flexible dust attenuation law performs best. For stellar populations, we find that BC03 models are favored over those of BPASS. Fixing the stellar metallicity at solar value is preferred to other fixed values or leaving it as a free parameter. For SFHs, we find that minimizing the variability in the recent (<100 Myr) SFH improves the agreement with L_TIR. Finally, we provide a catalog of galaxy parameters (including M* and SFR) for CANDELS galaxies with log M*>8 and 0.7<z<1.3 obtained using the models we found to be the most robust.

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arXiv2023

The Near-Infrared Extinction Law at High and Low Galactic Latitudes

The Milky Way extinction curve in the near-infrared (NIR) follows a power law form, but the value of the slope, $\beta_\text{NIR}$, is debated. Systematic variations in the slope of the Milky Way UV extinction curve are known to be correlated with variations in the optical slope (through $R_V$), but whether such a dependence extends to the NIR is unclear. Finally, because of low dust column densities, the NIR extinction law is essentially unconstrained at high Galactic latitudes where most extragalactic work takes place. In this paper, we construct extinction curves from 56,649 stars with SDSS and 2MASS photometry, based on stellar parameters from SDSS spectra. We use dust maps to identify dust-free stars, from which we calibrate the relation between stellar parameters and intrinsic colors. Furthermore, to probe the low-dust regime at high latitudes, we use aggregate curves based on many stars. We find no significant variation of $\beta_\text{NIR}$ across low-to-moderate dust columns ($0.02 4$. Finally, we find $R_H=0.345\pm0.007$ and comment on its bearing on Cepheid calibrations and the determination of $H_0$.

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arXiv2023

$\Sigma_{\mathrm{SFR}}$-M* Diagram: A Valuable Galaxy Evolution Diagnostic to Complement (s)SFR-M* Diagrams

The specific star formation rate (sSFR) is commonly used to describe the level of galaxy star formation (SF) and to select quenched galaxies. However, being a relative measure of the young-to-old population, an ambiguity in its interpretation may arise because a small sSFR can be either because of a substantial previous mass build up, or because SF is low. We show, using large samples spanning 0 < z < 2, that the normalization of SFR by the physical extent over which SF is taking place (i.e., SFR surface density, $\Sigma_{\mathrm{SFR}}$) overcomes this ambiguity. $\Sigma_{\mathrm{SFR}}$ has a strong physical basis, being tied to the molecular gas density and the effectiveness of stellar feedback, so we propose $\Sigma_{\mathrm{SFR}}$-M* as an important galaxy evolution diagram to complement (s)SFR-M* diagrams. Using the $\Sigma_{\mathrm{SFR}}$-M* diagram we confirm the Schiminovich et al. (2007) result that the level of SF along the main sequence today is only weakly mass dependent - high-mass galaxies, despite their redder colors, are as active as blue, low-mass ones. At higher redshift, the slope of the "$\Sigma_{\mathrm{SFR}}$ main sequence" steepens, signaling the epoch of bulge build-up in massive galaxies. We also find that $\Sigma_{\mathrm{SFR}}$ based on the optical isophotal radius more cleanly selects both the starbursting and the spheroid-dominated (early-type) galaxies than sSFR. One implication of our analysis is that the assessment of the inside-out vs. outside-in quenching scenarios should consider both sSFR and $\Sigma_{\mathrm{SFR}}$ radial profiles, because ample SF may be present in bulges with low sSFR (red color).

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arXiv2023

VLT-MUSE spectroscopy of AGNs misclassified by BPT diagnostic or with weak emission lines

Despite powerful X-ray emission, some AGNs are known to either lack optical emission lines (so-called 'optically dull' AGNs) or have lines that fall on the star-forming branch of the BPT diagram ('misclassified' AGNs). Aperture effects have been proposed to explain such atypical spectra, especially when based on SDSS (3'') fibers. We use observations from VLT-MUSE with Adaptive Optics to explore the spatially resolved optical emission line properties of 4 optically dull and 1 misclassified X-ray AGN candidates. VLT-MUSE IFU spectra allow us to investigate the extent to which the aperture size affects the emission line measurements. The optically dull AGNs become detectable in deeper VLT-MUSE spectroscopic apertures having the same size (3'') as SDSS fibers, suggesting no AGN is truly lineless. However, in no case does the line become more detectable as the aperture decreases, as would be expected if dilution by strong continuum was responsible for making the lines appear weak. We also show that the misclassified X-ray AGN retains the same position on the BPT diagram in smaller apertures (down to 0.''5), demonstrating that its misclassification is not the result of the dilution by HII regions. Thus, we conclude that continuum swamping or star formation dilution, i.e., aperture effects, are not responsible for atypical lines. Rather, the AGN lines are intrinsically weak.

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arXiv2023

Measuring the dust attenuation law of galaxies using photometric data

Fitting model spectral energy distributions (SED) to galaxy photometric data is a widely used method to recover galaxy parameters from galaxy surveys. However, the parameter space used to describe galaxies is wide and interdependent, and distinctions between real and spurious correlations that are found between these parameters can be difficult to discern. In this work, we use the SED fitting code BAGPIPES to investigate degeneracies between galaxy parameters and the effect of the choice of different sets of photometric bands. In particular, we focus on optical to infrared wavelength coverage, and on two parameters describing the galaxies' dust attenuation law: $A_V$ and $\delta$, which characterize dust column density and the slope of a flexible dust attenuation law, respectively. We demonstrate that 1) a degeneracy between the residual (the difference between truth and recovered value) $A_V$ and star formation rate exists, but this is lifted when WISE bands are included; 2) BAGPIPES is able to accurately recover the input $A_V$ and $\delta$ distributions and relations (differences in slope of less than 1.7$\sigma$ for a flat relation, less than 1.2$\sigma$ for an observationally-motivated relation from Salim et al. 2018) and is not introducing spurious correlations between these parameters. Our findings suggest that the information needed to constrain $A_V$ and $\delta$ well enough individually exists in the data, especially when IR is added. This indicates that recent works finding a correlation between $A_V$ and $\delta$ are not being misled by fitting degeneracies from their SED fitting code.

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arXiv2023

Improved GALEX UV Photometry for 700,000 SDSS Galaxies

The Galaxy Evolution Explorer (GALEX) satellite performed the first and only large-area UV survey, which in tandem with the Sloan Digital Sky Survey (SDSS) has facilitated modeling of the spectral energy distributions (SEDs) of low-redshift galaxies and the determination of various galaxy properties, in particular the star formation rate. However, the relatively crude angular resolution of GALEX (5") made its images susceptible to blending of sources, resulting in potentially biased far-UV (FUV) and near-UV (NUV) pipeline photometry. To remedy this issue and take advantage of model-fit photometry, we use the EMphot software to obtain forced GALEX photometry for ~700,000 SDSS galaxies at z<0.3. Positional priors of target galaxies and potentially contaminating neighbors were taken from SDSS. New photometry is based on the best-fitting of three model profiles: optical-like, exponential and flat. New photometry mitigates blending present in the original pipeline catalogs, which affected 16% of galaxies at a level of >0.2 mag and 2% at a level of >1 mag. Pipeline NUV magnitudes are severely affected (>1 mag) when the neighbor is brighter than the target galaxy and within 10", or when the neighbor is fainter and within ~3" of the target. New photometry fixes edge-of-detector bias, which affected pipeline photometry by up to 0.1 mag in NUV. We present catalogs with new photometry for GALEX observations of different depths, corresponding to the all-sky imaging survey (AIS), medium imaging survey (MIS) and deep imaging survey (DIS). Catalogs feature combined magnitudes for multiple detections of the same galaxy in a survey.

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arXiv2023

Star-formation rate and stellar mass calibrations based on infrared photometry and their dependence on stellar population age and extinction

The stellar mass ($M_\star$) and the star-formation rate (SFR) are among the most important features that characterize galaxies. Measuring these fundamental properties accurately is critical for understanding the present state of galaxies, and their history. This work explores the dependence of the IR emission of galaxies on their extinction, and the age of their stellar populations (SPs). It aims at providing accurate IR SFR and $M_\star$ calibrations that account for SP age and extinction while quantifying their scatter. We use the CIGALE spectral energy distribution (SED) fitting code to create models of galaxies with a wide range of star-formation histories, dust content, and interstellar medium properties. We fit the relations between $M_\star$ and SFR with IR and optical photometry of the model-galaxy SEDs with the MCMC method, and perform a machine-learning random forest analysis on the same data set in order to validate the latter. This work provides calibrations for the SFR using a combination of the WISE bands 1 and 3, or the JWST F200W and F2100W bands. It also provides mass-to-light ratio calibrations based on the WISE band-1, or the JWST band F200W, along with the optical $u-r$ or $g-r$ colors. These calibrations account for the biases attributed to the SP age, while they are given in the form of extinction-dependent and extinction-independent relations. They show robust estimations while minimizing the scatter and biases throughout a wide range of SFRs and stellar masses. The SFR calibration offers better results, especially in dust-free or passive galaxies where the contributions of old SPs or biases from the lack of dust are significant. Similarly, the $M_\star$ calibration yields significantly better results for dusty/high-SFR galaxies where dust emission can otherwise bias the estimations.

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