1206.2360 (S. Kaviraj et al.)

Newborn spheroids at high redshift: when and how did the dominant, old stars in today's massive galaxies form?    [PDF]

S. Kaviraj, S. Cohen, R. S. Ellis, S. Peirani, R. A. Windhorst, R. W. O'Connell, J. Silk, B. C. Whitmore, N. P. Hathi, R. E. Ryan Jr, M. A. Dopita, J. A. Frogel, A. Dekel

We study ~330 massive (M* > 10^9.5 MSun), newborn spheroidal galaxies (SGs) around the epoch of peak star formation (1 10^10.5 MSun, and an age trend becomes evident in this mass regime: SGs with M* > 10^11.5 MSun are ~2 Gyrs older than their counterparts with M* < 10^10.5 MSun. Nevertheless, a smooth downsizing trend with galaxy mass is not observed, and the large scatter in starburst ages indicate that SGs are not a particularly coeval population. Around half of the blue SGs appear not to drive their star formation via major mergers, and those that have experienced a recent major merger, show only modest enhancements (~40%) in their specific star formation rates. Our empirical study indicates that processes other than major mergers (e.g. violent disk instability driven by cold streams and/or minor mergers) likely play a dominant role in building SGs, and creating the old stellar populations that dominate today's Universe.

View original: http://arxiv.org/abs/1206.2360