Chemical Abundances of Deep2 Star-forming Galaxies at 1.0 < Z < 1.5

We present the results of near-infrared spectroscopic observations for a sample of 12 star-forming galaxies at 1.0 < z < 1.5, drawn from the DEEP2 Galaxy Redshift Survey. Hβ, [OIII], Hα, and [NII] emission-line fluxes are measured for these galaxies. Application of the O3N2 and N2 strongline abundance indicators implies average gas-phase oxygen abundances of 50 − 80% solar. We find preliminary evidence of luminosity-metallicity (L − Z) and mass-metallicity (M − Z) relationships within our sample, which spans from MB = −20.3 to −23.1 in rest-frame optical luminosity, and from 4× 10 to 2 × 10M⊙ in stellar mass. At fixed oxygen abundance, these relationships are displaced from the local ones by several magnitudes towards brighter absolute B-band luminosity and more than an order of magnitude towards larger stellar mass. If individual DEEP2 galaxies in our sample follow the observed global evolution in the B-band luminosity function of blue galaxies between z ∼ 1 and z ∼ 0 (Willmer et al. 2005), they will fade on average by ∼ 1.3 magnitudes in MB. To fall on local (L−Z) and (M −Z) relationships, these galaxies must increase by a factor of 6−7 in M/LB between z ∼ 1 and z ∼ 0, and by factor of two in both stellar mass and metallicity. Such concurrent increases in stellar mass and metallicity are consistent with the expectations of a “closed-box” chemical evolution model, in which the effects of feedback and large-scale outflows are not important. While Ks < 20.0 z ∼ 2 star-forming galaxies have similar [NII]/Ha ratios and rest-frame optical luminosities to those of the DEEP2 galaxies presented here, their higher M/LB ratios and clustering strengths indicate that they will experience different evolutionary paths to z ∼ 0. Finally, emission line diagnostic ratios indicate that the z > 1 DEEP2 galaxies in our sample are significantly offset from the excitation sequence observed in nearby H II regions and SDSS emission-line galaxies. This offset implies that physical conditions are different in the H II regions of distant galaxies hosting intense star formation, and may affect the chemical abundances derived from strong-line ratios for such objects. Subject headings: galaxies: evolution — galaxies: high-redshift – galaxies: abundances