ELVES-Field: Isolated Dwarf Galaxy Quenched Fractions Rise Below $M_* \approx 10^7$ $M_\odot$

TL;DR

ELVES-Field investigates whether truly isolated Local Volume dwarfs show significant quenching at low masses. It combines a mass-complete, volume-limited sample within $D<10$ Mpc with robust distance indicators and a projection-based isolation criterion to separate field dwarfs from satellites. The study finds that while dwarfs with $M_* \gtrsim 10^7$ M$_\odot$ are almost all star-forming, about $\sim 30\%$ of dwarfs in the $10^6-10^7$ M$_\odot$ range are quenched, and isolated dwarfs are $\sim 20\%$ smaller than satellites at fixed mass. These results have important implications for low-mass quenching physics and motivate larger-area surveys and space-based programs to obtain resolved-star histories and test proposed quenching mechanisms.

Abstract

We use a new sample of low-mass ($M_* < 10^9$ $M_\odot$) isolated galaxies from the Exploration of Local VolumE Survey - Field (ELVES-Field) to examine the star formation properties and sizes of field dwarf galaxies in the Local Volume (LV; $D<10$ Mpc). This volume-limited sample was selected from nearly 3,000 square degrees of imaging, relying on surface brightness fluctuations to determine distances to the majority of the systems and is complete to $M_* \approx 10^6$ $M_\odot$. Across the surveyed area, we catalog over 2300 candidate LV dwarfs, of which we confirm 95 as genuine LV members and reject over 1600 as background contaminants, with the remaining 600 candidates still requiring a distance measurement. Of the confirmed LV dwarfs, 46 are either new discoveries or confirmed via a distance measurement for the first time here. We explore different environmental criteria to select isolated dwarfs but primarily focus on dwarfs that are $>2\times R_{\mathrm{vir}}$ in projection from any known group with $M_\star > 10^9$ $M_\odot$. We find that, at higher dwarf masses ($M_\star \gtrsim 10^7$ $M_\odot$), essentially all field dwarfs are star-forming as has been found before. In contrast, at $M_\star \lesssim 10^7$ $M_\odot$, $\sim30\%$ of field dwarfs appear to be quenched. Finally, we find that isolated dwarfs are noticeably smaller ($\sim 20\%$) than satellite dwarfs of the same stellar mass, regardless of quenched status.

Figures (4)

• Figure 1: The mass-size relation of isolated field dwarfs from ELVES-Field compared with satellite dwarfs from the ELVES Survey carlsten2022. The top panel also shows the mass-size relation fit from ELVES in carlsten2021a as the dashed black line. As found in ELVES, early- and late-type dwarf satellites exhibit essentially the same mass-size relation, but isolated dwarfs appear to be significantly smaller at fixed stellar mass than satellite dwarfs. The bottom panel shows histograms of the residuals from the fit mass-size relation. The vertical lines show the mean residuals for the three populations. The mean residual for the isolated dwarfs indicates they are $\sim20\%$ smaller than satellites dwarfs at the same stellar mass.
• Figure 2: The optical and NUV colors of confirmed LV dwarfs in ELVES-Field. Points enclosed in black circles represent dwarfs that are 'isolated' according to our fiducial definition from §\ref{['sec:isolation']}. Also shown for reference are satellite dwarfs from the ELVES Survey carlsten2022, split by morphology. Dwarfs not detected in GALEX NUV data are shown as upward arrows at their $2\sigma$ lower limits of $NUV-g$ color. The dashed black lines show the thresholds in color that we use to label dwarfs as 'quenched' or 'star-forming'. When available, we use the threshold in $NUV-g$ with higher priority than that in optical $g-r$.
• Figure 3: The quenched fraction of isolated dwarfs versus dwarf stellar mass. Dwarfs are inferred to be quenched or star-forming based on their $NUV-g$ or $g-r$ color. The curves represent different criteria for a dwarf to be considered 'isolated'. The top panel shows criteria based on dwarfs being outside of $1\times,~ 2\times,$ or $3\times$ the projected virial radii of any massive, $M_\star > 10^9$ M$_\odot$ group. The bottom panel shows the criterion used in geha2012 where dwarfs are required to be $>1.5$ Mpc projected from any $M_\star > 2.5\times 10^{10}$ M$_\odot$ galaxy. The numbers listed in the legends and near each data point show the average number of galaxies contributing to each mass bin after we account for unconfirmed dwarfs without distance measurements. Dashed curves portray the results if this incompleteness correction is not performed, showing that it has a minor effect. Errorbars show the $1\sigma$ Bayesian confidence region. Due to the small number of isolated dwarfs found in the ELVES-Field footprint, the statistics are rather poor. However, we find that high mass dwarfs are essentially all star-forming and find evidence that quenched fractions increase to $\sim30\%$ for $M_\star < 10^7$ M$_\odot$, irregardless of isolation criterion used. The bottom panel shows the results from geha2012 for reference at higher masses.
• Figure 4: The average quenched fraction of dwarfs in the mass range $10^{5.5} < M_\star < 10^{8.5}$ M$_\odot$ as a function of the projected separation of the dwarf to the nearest massive ($M_\star > 1\times 10^{10}$ M$_\odot$) host galaxy. In assigning this separation, we take the minimum projected distance of each dwarf to the massive galaxies in kourkchi2017 that are at a distance consistent within $2\sigma$ to that of the dwarf. We find a non-zero quenched fraction even out to $2-3$ Mpc. To complement the field dwarf sample, within 300 kpc we plot the results of the ELVES Survey.