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We study ~330 massive (M* > 10^9.5 MSun), newborn spheroidal galaxies (SGs)around the epoch of peak star formation (1<z<3), to explore the high-redshiftorigin of SGs and gain insight into when and how the old stellar populationsthat dominate today's Universe formed. The sample is drawn from the HST/WFC3Early-Release Science programme, which provides deep 10-filter (0.2 - 1.7micron) HST imaging over a third of the GOODS-South field. We find that thestar formation episodes that built the SGs likely peaked in the redshift range2<z<5 (with a median of z~3) and have decay timescales shorter than ~1.5 Gyr.Starburst timescales and ages show no trend with stellar mass in the range10^9.5 < M* < 10^10.5 MSun. However, the timescales show increased scattertowards lower values (<0.3 Gyr) for M* > 10^10.5 MSun, and an age trend becomesevident in this mass regime: SGs with M* > 10^11.5 MSun are ~2 Gyrs older thantheir counterparts with M* < 10^10.5 MSun. Nevertheless, a smooth downsizingtrend with galaxy mass is not observed, and the large scatter in starburst agesindicate that SGs are not a particularly coeval population. Around half of theblue SGs appear not to drive their star formation via major mergers, and thosethat have experienced a recent major merger, show only modest enhancements(~40%) in their specific star formation rates. Our empirical study indicatesthat processes other than major mergers (e.g. violent disk instability drivenby cold streams and/or minor mergers) likely play a dominant role in buildingSGs, and creating a significant fraction of the old stellar populations thatdominate today's Universe.

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