Two conditions for galaxy quenching: compact centres and massive haloes

We investigate the roles of two classes of quenching mechanisms for central and satellite galaxies in the Sloan Digital Sky Survey (z < 0.075): those involving the halo and those involving the formation of a compact centre. For central galaxies with inner compactness Sigma(1) (kpc) similar to 10(9-9.4) M-circle dot kpc(-2), the quenched fraction f(q) is strongly correlated with Sigma(1) (kpc) with only weak halo mass M-h dependence. However, at higher and lower Ei kre, specific star formation rate (sSFR) is a strong function of Mh and mostly independent of Sigma(1) (kpc). In other words, Sigma(1) (kpc) similar to 10(9) (9.4) M-circle dot kpc(-2) divides galaxies into those with high sSFR below and low sSFR above this range. In both the upper and lower regimes, increasing Mh shifts the entire sSFR distribution to lower sSFR without a qualitative change in shape. This is true even at fixed Mt, but varying M* at fixed Mh adds no quenching information. Most of the quenched centrals with M-h > 10(11.8) M-circle dot are dense (Sigma(1) (kpc), > 10(9) M-circle dot kpc(-2)), suggesting compaction-related quenching maintained by halo-related quenching. However, 21 per cent are diffuse, indicating only halo quenching. For satellite galaxies in the outskirts of haloes, quenching is a strong function of compactness and a weak function of host M-h. In the inner halo, M-h dominates quenching, with similar to 90 per cent of the satellites being quenched once M-h > 10(13) M-circle dot. This regional effect is greatest for the least massive satellites. As demonstrated via semi-analytic modelling with simple prescriptions for quenching, the observed correlations can be explained if quenching due to central compactness is rapid while quenching due to halo mass is slow.