2007ApJ...669..241M

Observational evidence has been mounting over the past decade that at least some luminous (∼2L*) galaxies have formed nearly all of their stars within a short period of time, only (1-2)x10⁹ yr after the big bang. These are examples of the first major episodes of star formation in the universe and provide insights into the formation of the earliest massive galaxies. We have examined in detail the stellar populations of six z∼1.5 galaxies that appear to be passively evolving, using both ground- and space-based photometry covering rest-frame UV to visible wavelengths. In addition, we have obtained medium-resolution spectroscopy for five of the six galaxies, covering the rest-frame UV portion of the spectrum. Spectral synthesis modeling for four of these galaxies favors a single burst of star formation more than 1 Gyr before the observed epoch. The other two exhibit slightly younger ages with a higher dust content and evidence for a small contribution from either recent star formation or active nuclei. The implied formation redshifts for the oldest of these sources are consistent with previous studies of passive galaxies at high redshift, and improved stellar modeling has shown these results to be quite robust. It now seems clear that any valid galaxy formation scenario must be able to account for these massive (∼2x10¹¹ M_☉) galaxies at very early times in the universe.