The Assembly History of Field Spheroidals: Evolution of Mass‐to‐Light Ratios and Signatures of Recent Star Formation

We present a comprehensive catalog of high signal-to-noise spectra obtained with the DEIMOS spectrograph on the Keck II telescope for a sample of F850LP=-0. 72^+0. 07-₀. ₀₅\0. 04. However, this evolution depends significantly on the dynamical mass, being slower for larger masses as reported in a previous letter. In addition, we separately show the intrinsic scatter of the FP increases with redshift as d (rms (M/L\ ₁) ) /dz=0. 040\0. 015. Although these trends are consistent with single burst populations which formed at zf>2 for high mass spheroidals and z₅~1. 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 depend strongly on galactic mass, ranging from <1% for masses above 10^11. 5 M\ to 20-40% below 10^11 M\. Independent support for recent activity is provided by spectroscopic (2 emission, H\) and photometric (blue cores and broad-band 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. Abridged