Andrew Wetzel edited observations.tex  about 9 years ago

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\subsection{Observations}  Words. To understand the star-formation, cold-gas, and morphological properties of the dwarf galaxies in the Local Group, we use the compilation from \citet{McConnachie2012}, which includes stellar masses, cold (atomic) gas masses, morphologies, and distances for all galaxies known at that time that are within $3 \mpc$ of the Sun.  We select ``dwarf'' galaxies as $\mstar \lesssim 10^9 \msun$, thus including the LMC ($\mstar = 1.5 \times 10^9 \msun$) but not M33 ($\mstar = 5 \times 10^{10} \msun$).  \citet{McConnachie2012} includes distances from both the MW and M31, and we defined ``satellite'' galaxies as those within $300 \kpc$ of either host, motivated by the relatively sharp transition in star formation, gas, and morphological properties within this distance.  This distance also coincides well with the virial radius, $\rvir$, for our simulated MW/M31 halos (seeb below).  In addition to the data in \citet{McConnachie2012}, we also include the more recent measurements/upper-limits of cold atomic gas mass from \citet{Spekkens2014}.  We do not include the recently-discovered ultra-faint satellites from the Dark Energy Survey (REFS).  The observed satellites show a tight correlation between their morphology, star formation, and cold (atomic) gas mass, such that all (?) of the dwarf spheroidals have little-to-no detectable gas ($\mgas / \mstar < 0.1$) or ongoing star formation, and all (?) of the dwarf irregulars have a significant fraction of their baryonic mass in cold (atomic) gas and ongoing star formation.  Thus, we define ``quiescent'' satellites as those that have $\mgas / \mstar < 0.1$ or are classified as having spheroidal morphology if they have no atomic gas constraints.  Thus, by this definition, the only star-forming, gas-rich satellites are: LMC ($\mstar = 1.5 \times 10^9 \msun$, $\mgas / \mstar \approx 0.3$) and SMC ($\mstar = 4.6 \times 10^8 \msun$, $\mgas / \mstar \sim 1$) around the MW, and LGS 3 ($\mstar = 9.6 \times 10 ^ 5 \msun$, $\mgas / \mstar \approx 0.4$) and IC 10 ($\mstar = 9 \times 10 ^ 7$, $\mgas / \mstar \approx 0.6$) around M31.  While Sculptor has $\mgas / \mstar \approx 0.096$, we consider it quiescent based both on its morphology and lack of significant recent star formation \citep{Weisz2014}.  (Dan - can you say something about comparing these cuts with either SFR metrics or your SFHs?)  In examining the fraction of satellite dwarfs that are quiescent, we them into 1-dex-wide bins of $\mstar$.  We do not attempt any correction for completeness of this samples as a function of mass, but we do take into account the maximum observed distances when we compare with the simulations (see below).