Physical origin of the large-scale conformity in the specific star formation rates of galaxies

Abstract

Two explanations have been put forward to explain the observed conformity between the colours and specific star formation rates (SFR/$M_*$) of galaxies on large scales: 1) the formation times of their surrounding dark matter halos are correlated (commonly referred to as "assembly bias"), 2) gas is heated over large scales at early times, leading to coherent modulation of cooling and star formation between well-separated galaxies (commonly referred to as "pre-heating") . To distinguish between the pre-heating and assembly bias scenarios, we search for relics of energetic feedback events in the neighbourhood of central galaxies with different specific star formation rates. We find a significant excess of very high mass ($\log M_* > 11.3$) galaxies out to a distance of 2.5 Mpc around low SFR/$M_*$ central galaxies compared to control samples of higher SFR/$M_*$ central galaxies with the same stellar mass and redshift. We also find that very massive galaxies in the neighbourhood of low SFR/$M_*$ galaxies have much higher probability of hosting radio loud active galactic nuclei. The radio-loud AGN fraction in neighbours with $\log M_* > 11.3$ is four times higher around passive, non star-forming centrals at projected distances of 1 Mpc and two times higher at projected distances of 4 Mpc. Finally, we carry out an investigation of conformity effects in the recently publicly-released Illustris cosmological hydrodynamical simulation, which includes energetic input both from quasars and from radio mode accretion onto black holes. We do not find conformity effects of comparable amplitude on large scales in the simulations and we propose that gas needs to be pushed out of dark matter halos more efficiently at high redshifts.