THE NUMBER DENSITY OF OLD PASSIVELY EVOLVING GALAXIES AT z = 1 IN THE SUBARU/XMM-NEWTON DEEP SURVEY FIELD

摘要

We obtained the number counts and the rest-frame B-band luminosity function of 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' < 25 detected in an area of 1.03 deg~2 in the Subaru/XMM-Newton Deep Survey (SXDS) field. Our selection criteria are defined on the i' — z' and R— z' color-magnitude plane so that OPEGs at z = 0.9-1.1 with formation redshift z_f = 2-10 are properly sampled with minimum contamination by other populations. The limiting magnitude corresponds to the luminosity of galaxies with M_* + 3 at z = 0. We made pilot redshift observations for 99 OPEG candidates with 19 < z' < 22 and found that at least 78% (73 of 93) of the entire sample, or 95% (73 of 77) of those whose redshifts were obtained, indeed lie between z = 0.87 and 1.12, and most of their spectra show a continuum break and strong Ca H and K lines, indicating that these objects are indeed dominated by the old stellar populations. The relationship between the observed redshift and the color closely follows the models used in defining the selection criteria in a consistent manner. We found that the surface number density of OPEGs varies by 10%-30% of the average of the entire sample from field to field even at a 30' scale and that the poorest field, SXDS-South, has only 65% of that of the richest one, SXDS-East. We then compared our results with the luminosity functions of the color- or 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 ～M_* magnitude at z ～ 1 averaged over the SXDS area is 40%-60% that of equivalently red galaxies and 60%-85% that of 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 evolved into quiescent passive galaxies at z = 1.