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_m), halo mass (M_h) and halo-centric distance (r/r₁₈₀). For a given halo mass, the characteristic stellar mass ( ${M}_{* ,\mathrm{ch}}$ ) for the transition is about one-fifth of that of the corresponding central galaxy, and almost independent of B/T_m. Once B/T_m is fixed, the quenched fraction of galaxies with ${M}_{* }\lt {M}_{* ,\mathrm{ch}}$ increases with M_h, but decreases with M_* in the inner part of halos (r/r₁₈₀ < 0.5). In the outer part (r/r₁₈₀ > 0.5), the trend with M_h remains but the correlation with M_* is absent or becomes positive. For galaxies above ${M}_{* ,\mathrm{ch}}$ and with B/T_m fixed, the quenched fraction increases with ${M}_{* }$ , but depends only weakly on M_h in both the inner and outer regions. At fixed B/T_m and M_*, the quenched fraction increases with decreasing r/r₁₈₀ for galaxies with ${M}_{* }\lt {M}_{* ,\mathrm{ch}}$ , and depends only weakly on r/r₁₈₀ for galaxies with ${M}_{* }\gt {M}_{* ,\mathrm{ch}}$ . Our finding provides a physically motivated way to classify galaxies in halos into two classes based on their quenching properties: an "upper class" with ${M}_{* }\gt {M}_{* ,\mathrm{ch}}$ and a "lower class" with ${M}_{* }\lt {M}_{* ,\mathrm{ch}}$ . Environmental quenching is important for "lower class" galaxies, while internal quenching plays the dominating role for the "upper class."