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 ($M_{\ast }$), bulge to total mass ratio ($B\mathrm{/}{T}_{\mathrm{m}}$), halo mass ($M_{\mathrm{h}}$) and halo-centric distance ($r\mathrm{/}{r}_{180}$). 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 $B\mathrm{/}{T}_{\mathrm{m}}$. Once $B\mathrm{/}{T}_{\mathrm{m}}$ is fixed, the quenched fraction of galaxies with $M_{\ast }<M_{\ast ,\mathrm{c}\mathrm{h}}$ increases with $M_{\mathrm{h}}$, but decreases with $M_{\ast }$ in the inner part of halos ($r\mathrm{/}{r}_{180}<0.5$). In the outer part ($r\mathrm{/}{r}_{180}>0.5$), the trend with $M_{\mathrm{h}}$ remains but the correlation with $M_{\ast }$ is absent or becomes positive. For galaxies above $M_{\ast ,\mathrm{c}\mathrm{h}}$ and with $B\mathrm{/}{T}_{\mathrm{m}}$ fixed, the quenched fraction increases with $M_{\ast }$, but depends only weakly on $M_{\mathrm{h}}$ in both the inner and outer regions. At fixed $B\mathrm{/}{T}_{\mathrm{m}}$ and $M_{\ast }$, the quenched fraction increases with decreasing $r\mathrm{/}{r}_{180}$ for galaxies with $M_{\ast }<M_{\ast ,\mathrm{c}\mathrm{h}}$, and depends only weakly on $r\mathrm{/}{r}_{180}$ for galaxies with $M_{\ast }>M_{\ast ,\mathrm{c}\mathrm{h}}$. 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 $M_{\ast }<M_{\ast ,\mathrm{c}\mathrm{h}}$. Environmental quenching is important for lower class' galaxies, while internal quenching plays the dominating role for the upper class’.