The SAGA Survey. VI. The Size-Mass Relation for Low-Mass Galaxies Across Environments

TL;DR

This study probes whether Milky Way–like environments imprint structural differences on low-mass galaxies by comparing SAGA satellites to environmentally averaged SAGAbg and SDSS/NSA isolated galaxies using a uniform pysersic-based pipeline. The authors quantify the size–mass relation across samples, finding satellites are typically larger by $\sim$0.05–0.12 dex at fixed mass, with a parallel trend for both star-forming and quenched populations. Quenched satellites show a shallower size–mass relation and, at low masses, rounder morphologies, suggesting environmental quenching is accompanied by mild structural evolution. The results indicate environmental processes in MW-like halos can measurably reshape the stellar distributions of low-mass satellites, informing models of galaxy growth, quenching, and halo–galaxy co-evolution with practical implications for interpreting satellite populations in the local universe.

Abstract

We investigate how Milky Way-like environments influence the sizes and structural properties of low-mass galaxies by comparing satellites of Milky Way analogs from the Satellites Around Galactic Analogs (SAGA) Survey with two control samples: an environmentally agnostic population from the SAGA background (SAGAbg) sample and isolated galaxies from the SDSS NASA-Sloan Atlas. All sizes and structural parameters are measured uniformly using pysersic to ensure consistency across samples. We find the half-light sizes of SAGA satellites are systematically larger than those of isolated galaxies, with the magnitude of the offset ranging from 0.05 to 0.12 dex (10-24%) depending on the comparison sample and completeness cuts. This corresponds to physical size differences between 85-200 pc at 10^7.5 solar masses and 220-960 pc at 10^10 solar masses. This offset persists among star-forming galaxies, suggesting that environment can influence the structure of low-mass galaxies even before it impacts quenching. The intrinsic scatter in the size-mass relation is lower for SAGA satellites than isolated galaxies, and the Sérsic index distributions of satellites and isolated galaxies are similar. In comparison to star-forming satellites, quenched SAGA satellites have a slightly shallower size-mass relation and rounder morphologies at low-mass, suggesting that quenching is accompanied by structural transformation and that the processes responsible differ between low- and high-mass satellites. Our results show that environmental processes can imprint measurable structural differences on satellites in Milky Way-mass halos.

Figures (14)

• Figure 1: Distribution of $r$-band apparent magnitude ($r_{o}$, top) and stellar mass ($\log_{10}(M_\star/\mathrm{M}_\odot)$, bottom) as a function of spectroscopic redshift ($z_\mathrm{spec}$) for SAGA Gold satellites (purple, circles), SAGAbg environmentally averaged galaxies (orange, triangles), and SDSS/NSA isolated galaxies (green, stars). Luminosity distance is shown for reference as the upper $x$-axis. The black dashed and dot-dashed lines in both panels represent magnitude limits of $r_o = 20.7$ and $r_o = 17.77$, indicating approximate spectroscopic limits for the SAGA and SDSS surveys, respectively. The vertical dotted lines highlight the redshift criteria for the SAGAbg and SDSS/NSA isolated samples. We employ a redshift cutoff of $z<0.025$ for the SAGAbg sample (orange dotted line) and $z<0.01$ for SDSS/NSA isolated galaxies (green dotted line). These redshift ranges are selected such that the samples are stellar mass-matched to our SAGA satellite sample (above $\log_{10}(M_\star/\mathrm{M}_\odot) = 7.5$).
• Figure 2: Stellar mass distributions for the observed and completeness-corrected samples of SAGA satellites (first panel, purple), SAGAbg galaxies (second panel, orange), and SDSS/NSA isolated galaxies (third panel, green). The hatched histograms represent the observed (uncorrected) distributions, while the solid filled histograms show the completeness-corrected distributions. Dotted lines indicate Gaussian kernel density estimates (KDEs) for the observed distributions, and solid lines indicate KDEs for the completeness-corrected distributions. Given the large completeness correction required for the SDSS/NSA sample, we verify in Section \ref{['sec:incompleteness-tests']} that incompleteness cannot fully account for our results.
• Figure 3: Example galaxy in Legacy Survey imaging. The axes in each panel correspond to physical distances in kiloparsecs (kpc) relative to the center of the galaxy. First panel: Optical color image ($g-$, $r-$, and $z-$bands) with Sérsic model ellipses corresponding to $R_{\mathrm{eff}}$, $R_{80}$, and $3\times R_{\mathrm{eff}}$. Second panel: Masked $r$-band image. Third panel: Best-fit pysersic model to $r$-band image. Fourth panel: Residual between pysersic model image and $r$-band image.
• Figure 4: The size--mass relation for SAGA satellite galaxies. Star-forming (blue circles) and quenched (maroon squares) populations are both shown and fit independently. Open symbols correspond to unconfirmed candidates with high probability of being satellites ($p_\mathrm{sat} > 0.5$), with open blue circles and maroon squares indicating predicted star-forming and quenched candidates, respectively. Vertical dashed lines indicate the SAGA Silver ($\log_{10}(M_\star/\mathrm{M}_\odot) = 6.75$) and SAGA Gold ($\log_{10}(M_\star/\mathrm{M}_\odot) = 7.5$) mass thresholds. The black line shows our completeness corrected size--mass relation for all SAGA satellites above $\log_{10}(M_\star/\mathrm{M}_\odot) = 7.5$, and the relations for star-forming and quenched satellites are shown by the blue dashed and maroon dot-dashed lines, respectively.
• Figure 5: Half-light size ($R_{r,\mathrm{eff}}$, top), ellipticity ($e$, middle) and Sérsic index ($n$, bottom) distributions of SAGA satellites. Left panels show the binned medians as a function of stellar mass, with shaded regions indicating the 16th--84th percentile ranges. Right panels show the normalized distributions for satellites in the mass range $\log_{10}(M_\star/\mathrm{M}_\odot) > 7.5$. Gaussian KDE density functions are included to illustrate the overall shape of each distribution. In all panels, distributions show the completeness corrected satellite population, split by star-forming (SAGA SF, blue) and quenched satellites (SAGA Q, maroon).