SAGAbg III: Environmental Stellar Mass Functions, Self-Quenching, and the Stellar-to-Halo Mass Relation in the Dwarf Galaxy Regime

Abstract

Recent efforts have extended our view of the number and properties of satellite galaxies beyond the Local Group firmly down to $\rm M_\star\sim 10^6 M_\odot$. A similarly complete view of the field dwarf population has lagged behind. Using the background galaxies sample from the Satellites Around Galactic Analogs (SAGA) Survey at $z<0.05$, we take inventory of the dwarf population down to $\rm M_\star \sim 5\times10^6 M_\odot$ using three metrics: the stellar mass function (SMF) as function of environment, the stellar-to-halo mass relation (SHMR) of dwarf galaxies inferred via abundance matching, and the quenched fraction of highly isolated dwarfs. We find that the low-mass SMF shape shows minimal environmental dependence, with the field dwarf SMF described by a low-mass power-law index of $α_1=-1.44\pm0.09$ down to $\rm M_\star \sim 5\times10^6 M_\odot$, and that the quenched fraction of isolated dwarfs drops monotonically to $f_{q} \sim 10^{-3}$ at $\rm M_\star \sim \rm 10^{8.5} M_\odot$. Though slightly steeper than estimates from \HI{} kinematic measures, our inferred SHMR agrees with literature measurements of satellite systems, consistent with minimal environmental dependence of the SHMR in the probed mass range. Finally, although most contemporary cosmological simulations against which we compare accurately predict the \sagalocal{} SHMR, we find that big-box cosmological simulations largely over-predict isolated galaxy quenched fractions via a turnaround in $f_q(\rm M_\star)$ at $\rm 10^8\lesssim M_\star/M_\odot\lesssim 10^9$, underscoring the complexities in disentangling the drivers of galaxy formation and the need for systematic multidimensional observations of the dwarf population across environments.

Figures (12)

• Figure 1: The SAGAbg-SMF sample compared to previous wide-field surveys over the joint distribution in redshift and stellar mass of dwarfs (purple 2D histogram, left), as well as the full stellar mass range (right top) and $r$-band magnitude (right bottom). In each panel, the purple curves correspond to SAGAbg-SMF, grey dashed and hatched curves to GAMA, and dark grey dotted curves to the NASA Sloan Atlas analysis of SDSS spectra. In the main panel at left, we show the mean stellar mass at the survey limiting magnitude as estimated from the mean mass-to-light relation of the SAGAbg-SMF galaxies. In both panels at right, we show this approximate stellar mass completeness at $z=0.05$ (top) and $r$-band magnitude limit (bottom) as colored ticks for each survey.
• Figure 2: The fraction of galaxies that have received a high quality redshift in the parent SAGA photometric catalog as a function of $r$-band magnitude, $r$-band surface brightness, and $(g-r)$ color. The size of each point corresponds to the number of galaxies that contribute to each bin in photometric parameter space. The black contours enclose between 10% and 95% of the observed photometric properties of the SAGAbg-SMF sample, with each contour evenly spaced in enclosed fraction. The black arrow in the corner of each panel shows the median difference in each space of galaxies in our sample between $z=0.005$ and $z=0.05$ due to observational effects alone.
• Figure 3: Completeness-corrected galaxy number densities (colored bars) and best-fit stellar mass function (shaded curves) for the SAGAbg sample as a function of environment. The measurements are compared, where possible, with results from the literature. Completeness un-corrected number densities $n(\rm M_\star)$ are shown as grey lines in each panel; number densities obtained when only the photometric incompleteness term is applied are shown as grey crosses. We fit the SAGAbg SMF with a double Schechter function; the shaded regions show the 68% and 95% confidence intervals of the fit, while the error bars show the 68% and 95% confidence intervals for the binned counts. Each panel shows the best-fit SMF of the full SAGAbg-SMF sample as the dotted purple curve as a visual reference for both slope and overall normalization.
• Figure 4: The SAGAbg-SMF stellar-to-halo mass relation (SHMR, blue) we infer from abundance matching of the SAGAbg-SMFfield sample compared to the SHMR obtained from various abundance matching (AM) and empirical modeling (EM) studies in the literature. Clockwise from top, the panels show a comparison between the SAGAbg-SMF SHMR and relations inferred from environmentally-averaged samples, satellites from beyond the Local Group, and satellites of the Milky Way & M31. Shaded regions show the 68% and 95% confidence intervals of our SHMR posterior. Top: a comparison to empirical and semi-analytic modeling of higher mass populations. Left: our field (blue) and satellite (red) SHMRs compared to studies of the MW & M31 satellite systems. Right: the same as at left, but for satellite environments beyond the MW/M31 systems. Unless noted otherwise, halo mass are given are peak halo masses ($\rm M_{peak}$).
• Figure 5: Our dwarf stellar-to-halo mass relation (SHMR, blue) from abundance matching of the SAGAbg-SMF sample compared to direct observational results obtained via H1 rotation curves and weak lensing. We show the relationship between stellar mass and peak halo mass; the offset between $\rm M_{peak}$ and $\rm M_{halo}($z=0$)$ should be small for the bulk of the sample considered. Shaded regions show the 68% and 95% confidence intervals of our SHMR posterior. Our results are generally consistent with previous findings in the literature up to study-to-study variability in the literature at high stellar masses, and are systematically lower than previous kinematic measurements at low stellar masses. The dashed line shows the extrapolation of our abundance matching results below our limiting stellar mass of $\rm M_\star = 5\times 10^6 M_\odot$.