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 < 22.43 ( AB ) field spheroidal ( E+S0s ; 163 ) 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 , [ O ii ] and H \delta equivalent widths from the Keck spectroscopy .
Using the published 2Ms Chandra X-ray catalog we identify active galactic nuclei to clarify the origin of emission lines seen in the Keck spectra .
Only 2/13 [ O ii ] emitting early-type galaxies are identified as secure AGN on the basis of their X-ray emission .
Contrary to earlier suggestions , we find that most spheroidals containing ‘ blue cores ’ are not associated with non-thermal 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 Montecarlo 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 / L _ { B } ) / dz > = -0.72 ^ { +0.07 } _ { -0.05 } \pm 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 _ { B } ) ) /dz= 0.040 \pm 0.015 .
Although these trends are consistent with single burst populations which formed at z _ { f } > 2 for high mass spheroidals and z _ { f } \sim 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 _ { \odot } to 20-40 % below 10 ^ { 11 } M _ { \odot } .
Independent support for recent activity is provided by spectroscopic ( [ O ii ] emission , H \delta ) 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 .
We discuss the implications of this measurement for the determination of the evolution of the number density of E+S0s galaxies , suggesting number density evolution of the morphologically selected population has occurred since z \sim 1.2 .