Down-sizing in galaxy formation at<i>z</i>∼ 1 in the Subaru/<i>XMM-Newton</i>Deep Survey (SXDS)

Abstract

We use the deep wide-field optical imaging data of the Subaru/XMM-Newton Deep\nSurvey (SXDS) to discuss the luminosity (mass) dependent galaxy colours down to\nz'=25.0 (5 x 10^9 h_{70}^{-2} Msun) for z~1 galaxies in colour-selected high\ndensity regions. We find an apparent absence of galaxies on the red\ncolour-magnitude sequence below z'~24.2, corresponding to ~M*+2 (~10^{10} Msun)\nwith respect to passively evolving galaxies at z~1. Galaxies brighter than\nM*-0.5 (8 x 10^{10} Msun), however, are predominantly red passively evolving\nsystems, with few blue star forming galaxies at these magnitudes.\n This apparent age gradient, where massive galaxies are dominated by old\nstellar populations while less massive galaxies have more extended star\nformation histories, supports the `down-sizing' idea where the mass of galaxies\nhosting star formation decreases as the Universe ages. Combined with the lack\nof evolution in the shape of the stellar mass function for massive galaxies\nsince at least z~1, it appears that galaxy formation processes (both star\nformation and mass assembly) should have occurred in an accelerated way in\nmassive systems in high density regions, while these processes should have been\nslower in smaller systems. This result provides an interesting challenge for\nmodern CDM-based galaxy formation theories which predict later formation epochs\nof massive systems, commonly referred to as ``bottom-up''.\n