The Mass Assembly and Star Formation Characteristics of Field Galaxies of Known Morphology

Abstract

We discuss a new method for inferring the stellar mass of a distant galaxy of known redshift based on the combination of a near-infrared luminosity and multi-band optical photometry. The typical uncertainty for field galaxies with I<22 in the redshift range 0<z<1 is a factor 2. We apply this method to a newly-constructed sample of 321 field galaxies with redshifts and Hubble Space Telescope morphologies enabling us to construct the stellar mass density associated with various morphologies as a function of redshift. We find a marked decline with time in the stellar mass associated with peculiar galaxies accompanied by a modest rise in that observed for ellipticals. The result suggests that peculiars decline in abundance because they transform and merge into regular systems. The star formation rate per unit stellar mass indicates that massive systems completed the bulk of their star formation before redshift one, whereas dwarfs continue to undergo major episodes of activity until the present epoch.