Early-type Galaxy Evolution: The Last 8 Billion Years
Abstract
I review our current understanding of the star formation histories of early-type galaxies, in the context of recent observational studies of their rest-frame ultraviolet (UV) properties. By combining GALEX and SDSS photometry at low reshift, and exploiting (deep) optical surveys (MUSYC/COMBO-17/GEMS/COSMOS) at intermediate redshift, we are able to put unprecedented constraints on the formation and evolution of these galaxies over the last 8 billion years. In agreement with previous (optical) studies, the results indicate that the bulk of the stellar mass in early-types forms at high redshift (z > 1), possibly over short timescales (< 1 Gyr). Nevertheless, early-types of all luminosities form stars over the lifetime of the Universe, with most luminous (-23 < M(V) -21) systems forming up to 10-15% of their stellar mass after z = 1 (with a scatter to higher values), while their less massive counterparts form up to 30-60% of their mass in the same redshift range. The intensity of recent star formation and the UV colour distribution is quantitatively consistent with what might be expected from minor mergers (mass ratios < 1:3) in an LCDM cosmology. This is supported by visual inspection of HST images of early-types around z 0.5 which show a remarkable correspondence between the presence of morphological disturbances and UV excess. We use our results to speculate on the potentially significant role of minor merging on the evolution of the massive galaxy population at late epochs and the possible characteristics that future surveys will have to possess to study the minor merger process.
This research was supported by a Fellowship from the Royal Commission for the Exhibition of 1851 and a Senior Research Fellowship from Worcester College, Oxford (SK).- Publication:
-
American Astronomical Society Meeting Abstracts #215
- Pub Date:
- January 2010
- Bibcode:
- 2010AAS...21536301K