Nuclear discs as clocks for the assembly history of early-type galaxies: the case of NGC 4458

Abstract

Approximately 20 per cent 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 VIsible MultiObject Spectrograph integral-field spectroscopic observations for the intermediate-mass E0 galaxy NGC 4458 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 disc, or the absence of disruptive encounters since z ∼ 0.6, for a small galaxy such as NGC 4458 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, NGC 4458 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.