The physical origin of galactic conformity: from theory to observation

Abstract

ABSTRACT We employ several galaxy formation models, particularly, L-GALAXIES, IllustrisTNG, and EAGLE, as well as observational samples from SDSS and dark energy spectroscopic intstrument (DESI), to investigate galactic conformity, the observed correlation between the star-formation properties of central (primary) galaxies and those of their neighbours. To analyse the models and observations uniformly, we introduce CenSat, a new algorithm to define whether a galaxy is a central or a satellite system. We find that the conformity signal is present, up to at least 5 Mpc from the centres of low- and intermediate-mass centrals in the latest version of L-GALAXIES (Ayromlou et al. 2021b), IllustrisTNG, and EAGLE, as well as in SDSS and DESI observational samples. In comparison, the conformity signal is substantially weaker in an older version of L-GALAXIES (Henriques et al. 2020). One of the main differences between this older model and the other models is that except for satellites within the boundaries of massive cluster haloes, it neglects ram-pressure stripping of the gas reservoirs of galaxies (e.g. in groups and cluster outskirts). Our observational comparisons demonstrate that this difference significantly affects the observed large-scale conformity signal. Furthermore, by examining the contribution of backsplash, fly-by, central, and satellite galaxies to the conformity signal, we show that much, but not all, of it arises from primary galaxies near massive systems. Remaining tensions between the models and observations may be solved by modifying the physical prescriptions for how feedback processes affect the distribution and kinematics of gas and the environment around galaxies out to scales of several Megaparsecs.