Relic galaxy analogues in Illustris TNG50: the formation pathways of
  surviving red nuggets in a cosmological simulation
  Relic galaxies are massive compact quiescent galaxies that formed at
high-redshift and remained almost unchanged since then. In this work, we search
for analogues to relic galaxies in the TNG50 cosmological simulations to
understand relic formation and test the ability of TNG50 to reproduce such rare
objects. Using stellar mass, age, radius, quiescence and stellar assembly
criteria, we find 5 subhalos in TNG50 that could be potential relic analogues.
We compare their properties with other constraints imposed by a sample of 13
observed relic galaxies. We find one analogue in TNG50 that simultaneously
satisfies most of the available observational constraints, such as metallicity
and morphology. It also shows similarities to the confirmed relic NGC 1277,
regarding environment and dark matter fraction. By taking into account a degree
of relicness, a second relic analogue may also be considered. However, the
central parts of the brightness and density profiles of the analogues are less
steep than that of real relic galaxies, possibly due to limited numerical
resolution. We identify two formation pathways of relic analogues in TNG50
depending on their environment: they either have their remaining gas stripped
during the infall into a cluster at $z \lesssim 1.2$ or consume it before $z >
1.5$. They are then deprived of significant star formation, leaving their
stellar populations almost unaltered during the last 9 Gyr. We also find that
the analogue progenitors at $z \sim 4$ inhabit more massive halos than
progenitors of quiescent galaxies with similar stellar mass at $z \sim 0$.
