Nuclear discs as clocks for the assembly history of early-type galaxies:
  the case of NGC4458
  Approximately 20% of early-type galaxies host small nuclear stellar discs
that are tens to a few hundred parsecs in size. Such discs are expected to be
easily disrupted during major galactic encounters, hence their age serve to
constrain their assembly history. We use VIMOS integral-field spectroscopic
observations for the intermediate-mass E0 galaxy NGC4458 and age-date its
nuclear disc via high-resolution fitting of various model spectra. We find that
the nuclear disc is at least 6 Gyr old. A clue to gain narrow limits to the
stellar age is our knowledge of the nuclear disc contribution to the central
surface brightness.
  The presence of an old nuclear disk, or the absence of disruptive encounters
since z~0.6, for a small galaxy such as NGC4458 which belongs to the Virgo
cluster, may be consistent with a hierarchical picture for galaxy formation
where the smallest galaxies assembles earlier and the crowded galactic
environments reduce the incidence of galaxy mergers. On the other hand, NGC4458
displays little or no bulk rotation except for a central kpc-scale
kinematically-decoupled core. Slow rotation and decoupled core are usually
explained in terms of mergers. The presence and age of the nuclear disc
constraint these mergers to have happened at high redshift.
