Abstract
Comparisons between observational surveys and galaxy formation models find that the mass of dark matter haloes can largely explain galaxies' stellar mass. However, it remains uncertain whether additional environmental variables, generally referred to as assembly bias, are necessary to explain other galaxy properties. We use the Illustris Simulation to investigate the role of assembly bias in producing galactic conformity by considering 18,000 galaxies with $M_{stellar}$ > $2 \times 10^9$ $M_{\odot}$. We find a significant signal of galactic conformity: out to distances of about 10 Mpc, the mean red fraction of galaxies around redder galaxies is higher than around bluer galaxies at fixed stellar mass. Dark matter haloes exhibit an analogous conformity signal, in which the fraction of haloes formed at earlier times (old haloes) is higher around old haloes than around younger ones at fixed halo mass. A plausible interpretation of galactic conformity can be given as a combination of the halo conformity signal with the galaxy color-halo age relation: at fixed stellar mass, particularly toward the low-mass end, Illustris' galaxy colors correlate with halo age, with the reddest galaxies (often satellites) being preferentially found in the oldest haloes. In fact, we can explain the galactic conformity effect with a simple semi-empirical model, by assigning stellar mass based on halo mass (abundance matching) and by assigning galaxy color based on halo age (age matching). We investigate other interpretations for the galactic conformity, particularly its dependence on the isolation criterion and on the central-satellite information. Regarding comparison to observations, we conclude that the adopted selection/isolation criteria, projection effects, and stacking techniques can have a significant impact on the measured amplitude of the conformity signal.
Highlights
Previous investigations of the demographics and distribution of dark matter haloes in a cold dark matter universe have found that the clustering properties of these haloes have a dependence on formation time, in addition to the more significant dependence on halo mass (Gao et al 2005; Wechsler et al 2006; Croton et al 2007; Li et al 2008)
In this paper we have presented a series of phenomenological measurements of the galactic and halo clustering signals and of the relation between galaxy colors and dark-matter halo ages from the redshift z = 0 snapshot of the Illustris simulation
This is a full-volume cosmological hydrodynamical simulation from which we have selected a sample of about 18,000 galaxies with stellar mass > 2 × 109M in a 75 h−1 Mpc box
Summary
Previous investigations of the demographics and distribution of dark matter haloes in a cold dark matter universe have found that the clustering properties of these haloes have a dependence on formation time, in addition to the more significant dependence on halo mass (Gao et al 2005; Wechsler et al 2006; Croton et al 2007; Li. Recent observations at low redshift have found a signal of galactic conformity in which the sSFR and gas fractions of neighboring galaxies correlate with the respective properties of the central galaxy, both within and beyond the virial radius (e.g., Weinmann et al 2006; Kauffmann et al 2013; Lacerna et al 2014; Hartley et al 2015; Knobel et al 2015).
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