The Number Density of Old Passively-Evolving Galaxies at z=1 in the Subaru/XMM-Newton Deep Survey Field

Abstract

We obtained the number counts and the rest-frame B-band luminosity function of the color-selected old passively-evolving galaxies (OPEGs) at z=1 with very high statistical accuracy using a large and homogeneous sample of about 4000 such objects with z&#39; <25 detected in the area of 1.03 deg^2 in the Subaru/XMM-Newton Deep Survey (SXDS) field. Our selection criteria are defined on the i&#39;-z&#39; and R-z&#39; color-magnitude plane so that OPEGs at z=0.9-1.1 with formation redshift z_f=2-10 are properly sampled. The limiting magnitude corresponds to the luminosity of galaxies with M_*+3 at z=0. We made a pilot redshift observations for 99 OPEG candidates with 19 < z&#39; < 22 and found that at least 78% (73/93) of the entire sample, or 95% (73/77) of these whose redshifts were obtained are indeed lie between z=0.87 and 1.12 and the most of their spectra show the continuum break and strong Ca H and K lines, indicating that these objects are indeed dominated by the old stellar populations. We then compare our results with the luminosity functions of the color- or the morphologically-selected early type galaxies at z=0 taking the evolutionary factor into account and found that the number density of old passive galaxies with sim M_* magnitude at z~1 averaged over the SXDS area is 40-60% of the equivalently red galaxies and 60-85% of the morphologically-selected E/S0 galaxies at z=0 depending on their luminosity evolution. It is revealed that more than half, but not all, of the present-day early-type galaxies had already been formed into quiescent passive galaxies at z=1.