which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555. The Evolution of the Global Stellar Mass Density at 0 < z < 3**Based on observations taken with the NASA/ESA Hubble Space Telescope (HST)

The buildup of stellar mass in galaxies is the consequence of their past star formation and merging histories. Here we report measurements of rest-frame optical light and calculations of stellar mass at high redshift based on an infrared-selected sample of galaxies from the Hubble Deep Field-North. The bright envelope of rest-frame B-band galaxy luminosities is similar in the range 0 < z < 3, and the comoving luminosity density is constant to within a factor of 3 over that redshift range. However, galaxies at higher redshifts are bluer, and stellar population modeling indicates that they had significantly lower mass-to-light ratios than those of present-day L* galaxies. This leads to a global stellar mass density, * (z), that rises with time from z = 3 to the present. This measurement essentially traces the integral of the cosmic star formation history that has been the subject of previous investigations. Between 50% and 75% of the present-day stellar mass density had formed by z 1, but at z 2.7 we find that only 3%-14% of today's stars were present. This increase in * with time is broadly consistent with observations of the evolving global star formation rate, once dust extinction is taken into account, but is steeper at 1 < z < 3 than predicted by some recent semianalytic models of galaxy formation. The observations appear to be inconsistent with scenarios in which the bulk of stars in present-day galactic spheroids formed at z 2.

The Evolution of the Global Stellar Mass Density at 0 < z < 3**Based on observations taken with the NASA/ESA Hubble Space Telescope (HST), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.