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 ratio spectra obtained with DEIMOS on the Keck II telescope for a sample of F850LP < 22.43 (AB) field spheroidal (E+S0 galaxies; 165) and bulge-dominated disk (61) galaxies in the redshift range 0.2 < z < 1.2, selected on the basis of visual morphology from the northern field of the Great Observatories Origins Deep Survey (GOODS-N). We discuss sample selection, photometric properties, and spectral reduction. We derive scale lengths, surface brightnesses, and photometric inhomogeneities from the ACS data and redshifts, stellar velocity dispersions, and [O II] and H(delta) equivalent widths from the Keck spectroscopy. Using the published 2 Ms Chandra Deep Field-North X-ray catalog, we identify active galactic nuclei (AGNs) to clarify the origin of emission lines seen in the Keck spectra. Only 2 of 13 [O II]-emitting early-type galaxies are identified as secure AGNs on the basis of their Xray emission. Contrary to earlier suggestions, we find that most spheroidals containing "blue cores" are not associated with nonthermal nuclear activity. We examine the zero point, tilt, and scatter of the fundamental plane (FP) as a function of redshift and morphological properties, carefully accounting for luminosity-dependent biases via Monte Carlo simulations. The evolution of the overall FP can be represented by a mean change in effective mass-to-light ratio given by (d log(M/LB)/dz) = -0.72… ± 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 that the intrinsic scatter of the FP increases with redshift as d(rms(M/LB))/dz = 0.040 ± 0.015. Although these trends are consistent with singleburst populations that formed at zf > 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(circled dot operator) to 20%-40% below 1011 M(circled dot operator). Independent support for recent activity is provided by spectroscopic ([O II] emission, H(delta)) 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.