Modelling galactic conformity with the colour-halo age relation in the Illustris simulation

Citation:

Bray AD, Pillepich A, Sales LV, Zhu E, Genel S, Rodriguez-Gomez V, Torrey P, Nelson D, Vogelsberger M, Springel V, et al. Modelling galactic conformity with the colour-halo age relation in the Illustris simulation. Monthly Notices of the Royal Astronomical Society. 2016;455 :185-198.

Date Published:

January 1, 2016

Abstract:

Comparisons between observational surveys and galaxy formation modelsfind that dark matter haloes' mass can largely explain their galaxies'stellar mass. However, it remains uncertain whether additionalenvironmental variables, known as assembly bias, are necessary toexplain other galaxy properties. We use the Illustris simulation toinvestigate the role of assembly bias in producing galactic conformityby considering 18 000 galaxies with Mstellar > 2 ×109 M. We find a significant signal ofgalactic conformity: out to distances of about 10 Mpc, the mean redfraction of galaxies around redder galaxies is higher than around bluergalaxies at fixed stellar mass. Dark matter haloes exhibit an analogousconformity signal, in which the fraction of haloes formed at earliertimes (old haloes) is higher around old haloes than around younger onesat fixed halo mass. A plausible interpretation of galactic conformity isthe combination of the halo conformity signal with the galaxycolour-halo age relation: at fixed stellar mass, particularly towardsthe low-mass end, Illustris' galaxy colours correlate with halo age,with the reddest galaxies (often satellites) preferentially found in theoldest haloes. We explain the galactic conformity effect with a simplesemi-empirical model, assigning stellar mass via halo mass (abundancematching) and galaxy colour via halo age (age matching). Regardingcomparison to observations, we conclude that the adoptedselection/isolation criteria, projection effects, and stackingtechniques can have a significant impact on the measured amplitude ofthe conformity signal.

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