We present Keck/MOSFIRE K-band spectroscopy of the first mass-selected sample of galaxies at z \sim 2.3 .
Targets are selected from the 3D-HST Treasury survey .
The six detected galaxies have a mean [ NII ] \lambda 6584/H \alpha ratio of 0.27 \pm 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 ] /H \alpha ratios depends on the calibration method , and ranges from 12+log ( O/H ) _ { \mathrm { gas } } = 8.57 for the Pettini & Pagel ( 2004 ) calibration to 12+log ( O/H ) _ { \mathrm { 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 ) _ { \mathrm { stars } } = 8.95 , similar to the gas-phase abundances of the z \sim 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 \sim 2.3 sample : the galaxies with detected H \alpha tend to be larger and have higher star formation rates than the galaxies without detected H \alpha , and we may still be missing the most dust-obscured progenitors .