A NICMOS Search for High-Redshift Elliptical Galaxy Candidates

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).

Based on observations collected at ESO, La Silla, at the CTIO, NOAO, which is operated by AURA, Inc., under cooperative agreement with the NSF, and with the NASA/ESA Hubble Space Telescope, obtained at the STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555.