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/r(180)). For a given halo mass, the characteristic stellar mass (M-*,M-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-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 Mh remains but the correlation with M-* is absent or becomes positive. For galaxies above M-*,M- ch and with B/Tm fixed, the quenched fraction increases with M-*, but depends only weakly on Mh in both the inner and outer regions. At fixed B/Tm and M-*, the quenched fraction increases with decreasing r/r180 for galaxies with M-* < M-*,M- ch, and depends only weakly on r/r180 for galaxies with M-* > M-*,M-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-*,M- ch and a lower class with M-* < M-*,M- ch. Environmental quenching is important for lower class galaxies, while internal quenching plays the dominating role for the upper class.