Massive relic galaxies prefer dense environments

Abstract

We study the preferred environments of $z \sim 0$ massive relic galaxies ($M_\star \gtrsim 10^{10}~\mathrm{M_\odot}$ galaxies with little or no growth from star formation or mergers since $z \sim 2$). Significantly, we carry out our analysis on both a large cosmological simulation and an observed galaxy catalogue. Working on the Millennium I-WMAP7 simulation we show that the fraction of today massive objects which have grown less than 10 per cent in mass since $z \sim 2$ is ~0.04 per cent for the whole massive galaxy population with $M_\star > 10^{10}~\mathrm{M_\odot}$. This fraction rises to ~0.18 per cent in galaxy clusters, confirming that clusters help massive galaxies remain unaltered. Simulations also show that massive relic galaxies tend to be closer to cluster centres than other massive galaxies. Using the New York University Value-Added Galaxy Catalogue, and defining relics as $M_\star \gtrsim 10^{10}~\mathrm{M_\odot}$ early-type galaxies with colours compatible with single-stellar population ages older than 10 Gyr, and which occupy the bottom 5-percentile in the stellar mass-size distribution, we find $1.11 \pm 0.05$ per cent of relics among massive galaxies. This fraction rises to $2.4 \pm 0.4$ per cent in high-density environments. Our findings point in the same direction as the works by Poggianti et al. and Stringer et al. Our results may reflect the fact that the cores of the clusters are created very early on, hence the centres host the first cluster members. Near the centres, high-velocity dispersions and harassment help cluster core members avoid the growth of an accreted stellar envelope via mergers, while a hot intracluster medium prevents cold gas from reaching the galaxies, inhibiting star formation.