Keck Spectroscopy of z>1 Field Spheroidals: Dynamical Constraints on the
  Growth Rate of Red "Nuggets"
  We present deep Keck spectroscopy for 17 morphologically-selected field
spheroidals in the redshift range 1.05<z<1.60 in order to investigate the
continuity in physical properties between the claimed massive compact red
galaxies ("nuggets") at z~2 and well-established data for massive spheroidal
galaxies below z~1. By combining Keck-based stellar velocity dispersions with
HST-based sizes, we find that the most massive systems (Mdyn > 10^11 Msol) grew
in size over 0<z<1.6 as (1+z)^(-0.75 +- 0.10) (i.e., x2 since z=1.5) whereas
intermediate mass systems (10^11 Msol > Mdyn > 10^10 Msol) did not grow
significantly. These trends are consistent with a picture in which more massive
spheroidals formed at higher redshift via "wetter" mergers involving greater
dissipation. To examine growth under the favored "dry" merger hypothesis, we
also examine size growth at a fixed velocity dispersion. This test, uniquely
possible with our dynamical data, allows us to consider the effects of
"progenitor bias." Above our completeness limit (sigma > 200 km/s), we find
size growth consistent with that inferred for the mass-selected sample, thus
ruling out strong progenitor bias. To maintain continuity in the growth of
massive galaxies over the past 10 Gyr, our new results imply that size
evolution over 1.3<z<2.3, a period of 1.9 Gyr, must have been even more
dramatic than hitherto claimed if the red sources at z>2 are truly massive and
compact.
