Characteristic mass in galaxy quenching: environmental versus internal
  effects
  A clear transition feature of galaxy quenching is identified in the
multi-parameter space of stellar mass ($M_*$), bulge to total mass ratio
($B/T_{\rm m}$), halo mass ($M_{\rm h}$) and halo-centric distance
($r/r_{180}$). For given halo mass, the characteristic stellar mass ($M_{*, \rm
ch}$) for the transition is about one-fifth of that of the corresponding
central galaxy, and almost independent of $B/T_{\rm m}$. Once $B/T_{\rm m}$ is
fixed, the quenched fraction of galaxies with $M_{*} < M_{*, \rm ch}$ increases
with $M_{\rm h}$, but decreases with $M_*$ in the inner part of halos
($r/r_{180} < 0.5$). In the outer part ($r/r_{180} > 0.5$), the trend with
$M_{\rm h}$ remains but the correlation with $M_*$ is absent or becomes
positive. For galaxies above $M_{\rm *, ch}$ and with $B/T_{\rm m}$ fixed, the
quenched fraction increases with $M_{\rm *}$, but depends only weakly on
$M_{\rm h}$ in both the inner and outer regions. At fixed $B/T_{\rm m}$ and
$M_*$, the quenched fraction increases with decreasing $r/r_{180}$ for galaxies
with $M_{*} < M_{*, \rm ch}$, and depends only weakly on $r/r_{180}$ for
galaxies with $M_{*} > M_{*, \rm ch}$. Our finding provides a
physically-motivated way to classify galaxies in halos into two classes based
on their quenching properties: an `upper class' with $M_{*} > M_{\rm *,ch}$ and
a `lower class' with $M_{*} < M_{\rm *,ch}$. Environmental quenching is
important for `lower class' galaxies, while internal quenching plays the
dominating role for the `upper class'.
