A NICMOS Search for High-Redshift Elliptical Galaxy Candidates

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Abstract
We have collected optical follow-up observations from the ground and the Hubble Space Telescope (HST) for 13.74 arcmin2 of archival images taken with the HST Near-Infrared Camera and Multiobject Spectrograph (NICMOS). We use two criteria to select E/S0 galaxy candidates at z 1 and z 1.5 (high-redshift elliptical candidates [HizECs]) based on colors and infrared morphology. The observed surface density of HizECs is significantly smaller than what is predicted assuming the local luminosity function (LF) of E/S0, constant comoving density, and pure passive evolution with a high redshift of formation of the stellar populations (z 5). On the other hand, assuming a low redshift (z 2) of formation, not enough HizECs are predicted. The data are very well matched assuming that a substantial fraction of present-day E/S0's (10%-66%, depending on cosmology and on the local LF of E/S0's) formed at z = 3 or higher and then evolved passively. The rest of the current E/S0 population may have formed at lower redshift or may have been rejected by the selection criteria because of interactions or more recent episodes of star formation (disturbed morphology and/or bluer colors).