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 obtainedwith the DEIMOS spectrograph on the Keck II telescope for a sample ofF850LP<22.43 (AB) field spheroidal (E+S0s; 163) and bulge dominated disk (61)galaxies in the redshift range 0.2=-0.72^{+0.07}_{-0.05}\pm0.04. However, this evolution dependssignificantly on the dynamical mass, being slower for larger masses as reportedin a previous letter. In addition, we separately show the intrinsic scatter ofthe FP increases with redshift as d(rms(M/L_{\rm B}))/dz=0.040\pm0.015.Although these trends are consistent with single burst populations which formedat $z_f>2$ for high mass spheroidals and z_{f}~1.2 for lower mass systems, amore realistic picture is that most of the stellar mass formed in all systemsat z>2 with subsequent activity continuing to lower redshifts (z<1.2). Thefraction of stellar mass formed at recent times depend strongly on galacticmass, ranging from <1% for masses above 10^{11.5} M_{\odot} to 20-40% below10^{11} M_{\odot}. Independent support for recent activity is provided byspectroscopic ([\ion{O}{2}] emission, H\delta) and photometric (blue cores andbroad-band colors) diagnostics. Via the analysis of a large sample with manyindependent diagnostics, we are able to reconcile previously disparateinterpretations of the assembly history of field spheroidals. [Abridged]