We present Keck/MOSFIRE K-band spectroscopy of the first mass-selected sample
of galaxies at z∼2.3. Targets are selected from the 3D-HST Treasury
survey. The six detected galaxies have a mean [NII]λ6584/Hα
ratio of 0.27±0.01, with a small standard deviation of 0.05. This mean
value is similar to that of UV-selected galaxies of the same mass. The mean
gas-phase oxygen abundance inferred from the [NII]/Hα ratios depends on
the calibration method, and ranges from 12+log(O/H)gas=8.57 for the
{Pettini} & {Pagel} (2004) calibration to 12+log(O/H)gas=8.87 for the
{Maiolino} {et~al.} (2008) calibration. Measurements of the stellar oxygen
abundance in nearby quiescent galaxies with the same number density indicate
12+log(O/H)stars=8.95, similar to the gas-phase abundances of the
z∼2.3 galaxies if the {Maiolino} {et~al.} (2008) calibration is used. This
suggests that these high-redshift star forming galaxies may be progenitors of
today's massive early-type galaxies. The main uncertainties are the absolute
calibration of the gas-phase oxygen abundance and the incompleteness of the
z∼2.3 sample: the galaxies with detected Hα tend to be larger and
have higher star formation rates than the galaxies without detected Hα,
and we may still be missing the most dust-obscured progenitors.Comment: 7 pages, 5 figures. Accepted for publication in ApJ