Nature versus nurture: relic nature and environment of the most massive
  passive galaxies at $z < 0.5$
  Relic galaxies are thought to be the progenitors of high-redshift red nuggets
that for some reason missed the channels of size growth and evolved passively
and undisturbed since the first star formation burst (at $z>2$). These local
ultracompact old galaxies are unique laboratories for studying the star
formation processes at high redshift and thus the early stage of galaxy
formation scenarios. Counterintuitively, theoretical and observational studies
indicate that relics are more common in denser environments, where merging
events predominate. To verify this scenario, we compared the number counts of a
sample of ultracompact massive galaxies (UCMGs) selected within the third data
release of the Kilo Degree Survey, that is, systems with sizes $R_{\rm e} < 1.5
\, \rm kpc$ and stellar masses $M_{\rm \star} > 8 \times 10^{10}\, \rm
M_{\odot}$, with the number counts of galaxies with the same masses but normal
sizes in field and cluster environments. Based on their optical and
near-infrared colors, these UCMGs are likely to be mainly old, and hence
representative of the relic population. We find that both UCMGs and normal-size
galaxies are more abundant in clusters and their relative fraction depends only
mildly on the global environment, with denser environments penalizing the
survival of relics. Hence, UCMGs (and likely relics overall) are not special
because of the environment effect on their nurture, but rather they are just a
product of the stochasticity of the merging processes regardless of the global
environment in which they live.
