Deep Hubble Space Telescope imaging surveys and the formation of spheroidal galaxies

We have extended our previous analysis of morphologically selected elliptical and S0 galaxies in the Hubble Deep Field (HDF) North to include Hubble Space Telescope ( HST ) data in the HDF South and the HDFS–NICMOS areas. Our final sample amounts to 69 E/S0 galaxies with K<20.15 over an area of 11 arcmin 2 . Although a moderately small number over a modest sky area, this sample benefits from the best imaging and photometric data available on high‐redshift galaxies. Multi‐waveband photometry allows us to estimate with good accuracy the redshifts for the majority of these galaxies, which lack a spectroscopic measure. We confirm our previous findings that massive E/S0s tend to disappear from flux‐limited samples at z>1.4. This adds to the evidence that the rest‐frame colours and spectral energy distributions (SEDs) of the numerous objects found at 0.8<z<1.2 are inconsistent with a very high redshift of formation for the bulk of stars, while they are more consistent with protracted (either continuous or episodic) star formation down to z≤1. These results based on high‐quality imaging on a small field can be complemented with data from colour‐selected extremely red objects (EROs) on much larger sky areas: our claimed demise of E/S0s going from z=1 to z=1.5 is paralleled by a similarly fast decrease in the areal density of EROs when the colour limit is changed from (R−K)=5 to (R−K)=6 (corresponding to z≃1 and z≃1.3 respectively). Altogether, the redshift interval from 1 to 2 seems to correspond to a very active phase for the assembly of massive E/S0 galaxies in the field, and also probably one where a substantial fraction of their stars are formed.