The Trace of the Assembly of Massive E-S0 Galaxies at 0.8 < z < 1.5 in Galaxy Number Counts

摘要

K-band galaxy number counts (GNCs) exhibit a slope change at K ~ 17.5 mag not present in optical bands. To unveil the nature of this feature we have derived the contribution of different galaxy types to the total K-band GNCs at 0.3 < z < 1.5 and by redshift bins, and compared the results with the expectations of different galaxy evolutionary models, using deep multi-wavelength broad-band data in the Groth Strip region. We find that the slope change is due to a sudden swap of the galaxy population at z < 1.5 that numerically dominates the total GNCs at z < 1.5 (from quiescent E-SOs at K < 17.5 mag to blue star-forming disks at fainter magnitudes）. Models in which the bulk of massive E-SOs were formed at high redshifts (z > 2) and evolved passively since then cannot predict the slope change, whereas models that impose a late definitive assembly for them (at z < 1.5) can reproduce it. We conclude that the slope change in total K-band GNCs is thus nothing other than a vestige of the definitive assembly of a substantial fraction (~ 50%) of present-day massive E-SOs at 0.8 < z < 1.5. IR MOS spectroscopy for a significant sample of red galaxies at z < 1.5 is necessary to confirm the late arrival of the bulk of massive galaxies to the Red Sequence.