Andrew Wetzel edited quiescent_fraction.tex  about 9 years ago

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Figure~\ref{fig:quiescent_fraction} shows, for all satellite galaxies at $\mstar\lesssim10^9\msun$ within $300\kpc$ of the MW or M31, the fraction that are quiescent in 1-dex bins of $\mstar$ \citep[see also][]{Phillips2014, SlaterBell2014}.  We do not correct for any observational completeness versus $\mstar$, because we measure the \emph{relative fraction} in each bin, which is likely unbiased at distances $\lesssim300\kpc$.  %absent significant differential completeness as a function of recent star formation, which is unlikely because star-forming galaxies are generally brighter, but the quiescent fraction is near unity across almost all $\mstar$.  We show fractions for all satellites (blue circles) and separately for those in the MW (violet squares) and M31 (green triangles) halos.  Error bars show 68\% uncertainty for the binomial counts using a beta distribution \citep{Cameron2011}.  Of the 56 satellites, only 4 (7\%) are star-forming/gas-rich: LMC and SMC of the MW, LGS 3 and IC 10 of M31.  Moreover, at $\mstar<8\times10^7\msun$, only 1 (LGS 3) of the 51 satellites is star-forming, and at $\mstar<9\times10^5\msun$ \emph{all} 40 satellites are quiescent.  These near-unity quiescent fractions for satellites of the MW/M31 contrast strongly with the nearly \emph{zero} quiescent fraction observed for isolated (non-satellite) galaxies at $\mstar<10^9\msun$ \citep[][see Introduction]{Geha2012, Phillips2014}.  The only clear exceptions are the quiescent galaxies KKR 25 ($\mstar=1.4\times10^6\msun$) and KKs 3 ($\mstar=2.3\times10^7\msun$) at $\approx2\mpc$ from the MW/M31, although the limited completeness at $\mstar\lesssim10^6\msun$ beyond $300\kpc$ \citep{Tollerud2008, Hargis2014} leaves open the possibility for more such galaxies.% complete within the MW only to 10^5 (again, in the SDSS cone, which we at least hope is a representative sample).   %at 1 Mpc, completeness ony > 10^6, so missing things in the 10^ 5-8 range might indeed be a selection effect (especially given presumably there are more fainter ones).