Massive Galaxies Near and Far: The Evolution of an Unbiased Population Over the Last Ten Billion Years
Abstract
Recent deep near-infrared surveys have extended the study of the buildup of stellar mass in galaxies to ever-increasing redshifts. We present a new photometric redshift code, EAZY, which provides precise redshift estimates that allow us to exploit the full depth of surveys that frequently reach much fainter than is currently feasible for large spectroscopic campaigns. EAZY includes a carefully-determined template set and a novel "template error function" that accounts for systematic differences between the templates and true galaxy SEDs. We present results from the NEWFIRM Medium-Band Survey, a 75-night program recently undertaken with the Mayall 4-m, that uses 5 medium-width NIR filters designed to measure precise photometric redshifts and rest-frame colors at z > 1.5. We find that the color bimodality between "red and dead" and blue star-forming galaxies, prominent at lower redshifts, persists to at least z ∼ 2.5. The separation of the two populations is greatly improved after accounting for the effects of dust-reddening of star-forming galaxies. The existence of apparently "dead" galaxies at these high redshifts is remarkable, given the large gas reservoir available at early times and the fact that the average star formation rate in the Universe was much higher than it is today. The mass density of quiescent galaxies with M > 10^11 M⊙ increases by ∼1 dex from z = 2.2 to the present day, whereas the mass density in star-forming galaxies is flat or decreases over the same time periods. Using a simple toy model, we show that modest mass growth (i.e., a factor of two) of individual galaxies can explain much of the strong density evolution of quiescent galaxies, due to the steepness of the exponential end of the mass function. We argue that mergers are a primary mechanism for building up the massive quiescent galaxy population, while less massive quiescent galaxies are continuously formed by transforming galaxies from the star-forming population. We conclude with a case study of a M = 4 × 10^11 M⊙ quiescent galaxy at z = 1.902 using spatially-resolved near-infrared spectroscopy with the WFC3 grism on the Hubble Space Telescope , demonstrating the promise of this technique for further bridging the gap between photometric and spectroscopic surveys.
- Publication:
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Ph.D. Thesis
- Pub Date:
- August 2010
- Bibcode:
- 2010PhDT.......229B