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

We study similar to 330 massive (M-* > 10(9.5) M-circle dot), newborn spheroidal galaxies (SGs) around the epoch of peak star formation (1 < z < 3) to explore the high-redshift origin of SGs and gain insight into when and how the old stellar populations that dominate today's Universe formed. The sample is drawn from the Hubble Space Telescope (HST)/WFC3 Early-Release Science programme, which provides deep 10-filter (0.2-1.7 mu m) HST imaging over one-third of the GOODS-South field. We find that the star formation episodes that built our SGs likely peaked in the redshift range 2 < z < 5 (with a median of z similar to 3) and have decay time-scales shorter than similar to 1.5Gyr. Starburst time-scales and ages show no trend with stellar mass in the range 10(9.5) < M-* < 10(10.5) M-circle dot. However, the time-scales show increased scatter towards lower values (< 0.3 Gyr) for M* > 10(10.5) M-circle dot, and an age trend becomes evident in this mass regime: SGs with M-* > 10(11.5) M-circle dot are similar to 2 Gyr older than their counterparts with M-* < 10(10.5) M-circle dot. Nevertheless, a smooth downsizing trend with galaxy mass is not observed, and the large scatter in starburst ages indicates 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 (similar to 40 per cent) in their specific star formation rates. Our empirical study indicates that processes other than major mergers (e. g. violent disc instability driven by cold streams and/or minor mergers) likely play a dominant role in building SGs, and creating a significant fraction of the old stellar populations that dominate today's Universe.