We investigate the roles of two classes of quenching mechanisms for central and satellite galaxies in the SDSS ($z<0.075$): those involving the halo and those involving the formation of a compact centre. For central galaxies with inner compactness $\Sigma_{ \rm 1kpc} \sim 10^{ 9-9.4}M_{ \odot} { \rm kpc}^{ -2}$, the quenched fraction $f_{ q}$ is strongly correlated with $\Sigma_{ \rm 1kpc}$ with only weak halo mass $M_{ \rm h}$ dependence. However, at higher and lower $\Sigma_{ \rm 1kpc}$, sSFR is a strong function of $M_{ \rm h}$ and mostly independent of $\Sigma_{ \rm 1kpc}$. In other words, $\Sigma_{ \rm 1kpc} \sim 10^{ 9-9.4} M_{ \odot} { \rm kpc}^{ -2}$ divides galaxies into those with high sSFR below and low sSFR above this range. In both the upper and lower regimes, increasing $M_{ \rm h}$ shifts the entire sSFR distribtuion to lower sSFR without a qualitative change in shape. This is true even at fixed $M_{ *}$, but varying $M_{ *}$ at fixed $M_{ \rm h}$ adds no quenching information. Most of the quenched centrals with $M_{ \rm h} > 10^{ 11.8}M_{ \odot}$ are dense ($\Sigma_{ \rm 1kpc} > 10^{ 9}~ M_{ \odot} { \rm kpc}^{ -2}$), suggesting compaction-related quenching maintained by halo-related quenching. However, 21% are diffuse, indicating only halo quenching. For satellite galaxies in the outskirts of halos, quenching is a strong function of compactness and a weak function of host $M_{ \rm h}$. In the inner halo, $M_{ \rm h}$ dominates quenching, with $\sim 90\%$ of the satellites being quenched once $M_{ \rm h} > 10^{ 13}M_{ \odot}$. 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.
Categories: astro-ph.GA