Abstract
We present a comprehensive catalog of high signal-to-noise ratio spectra obtained with DEIMOS on the Keck II telescope for a sample of F850LP 2 for high-mass spheroidals and zf ~ 1.2 for lower mass systems, a more realistic picture is that most of the stellar mass formed in all systems at z > 2 with subsequent activity continuing to lower redshifts (z < 1.2). The fraction of stellar mass formed at recent times depends strongly on galactic mass, ranging from <1% for masses above 1011.5 M☉ to 20%-40% below 1011 M☉. Independent support for recent activity is provided by spectroscopic ([O II] emission, Hδ) and photometric (blue cores and broadband colors) diagnostics. Via the analysis of a large sample with many independent diagnostics, we are able to reconcile previously disparate interpretations of the assembly history of field spheroidals. We discuss the implications of this measurement for the determination of the evolution of the number density of E+S0 galaxies, suggesting that number density evolution of the morphologically selected population has occurred since z ~ 1.2.
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