Galactic conformity and central / satellite quenching, from the satellite profiles of M$^{\ast}$ galaxies at $0.4<z<1.9$ in the UKIDSS UDS

Abstract: We explore the redshift evolution of a curious correlation between the star-formation properties of central galaxies and their satellites (`galactic conformity') at intermediate to high redshift ($0.4<z\<1.9$). Using an extremely deep near-infrared survey, we study the distribution and properties of satellite galaxies with stellar masses, ${\rm log} ({\rm M}_*/{\rm M}_{\odot})\>9.7$, around central galaxies at the characteristic Schechter function mass, ${\rm M} \sim {\rm M}^{\ast}$. We fit the radial profiles of satellite number densities with simple power laws, finding slopes in the range -1.1 to -1.4 for mass-selected satellites, and -1.3 to -1.6 for passive satellites. We confirm the tendency for passive satellites to be preferentially located around passive central galaxies at $3\sigma$ significance and show that it exists to at least $z\sim2$. Meanwhile, the quenched fraction of satellites around star-forming galaxies is consistent with field galaxies of equal stellar masses. We find no convincing evidence for a redshift-dependent evolution of these trends. One simple interpretation of these results is that only passive central galaxies occupy an environment that is capable of independently shutting off star-formation in satellite galaxies. By examining the satellites of higher stellar mass star-forming galaxies (${\rm log} ({\rm M}*/{\rm M}{\odot}) > 11$), we conclude that the origin of galactic conformity is unlikely to be exclusively due to the host dark-matter halo mass. A halo-mass-independent correlation could be established by either formation bias or a more physical connection between central and satellite star-formation histories. For the latter, we argue that a star-formation (or AGN) related outburst event from the central galaxy could establish a hot halo environment which is then capable of quenching both central and satellite galaxies.