Chemical abundances of DEEP2 star-forming galaxies at z∼1.0-1.5

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. H beta, [O III], H alpha, and [N II] emission line fluxes are measured for these galaxies. Application of the O3N2 and N2 strong-line abundance indicators implies average gasphase 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 M-B = -20.3 to -23.1 in rest-frame optical luminosity and from 4 x 10(9) to 2 x 10(11) M-circle dot in stellar mass. At fixed oxygen abundance, these relationships are displaced from the local ones by several magnitudes toward brighter absolute B-band luminosity and more than an order of magnitude toward 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 similar to 1 and z similar to 0 (Willmer et al. 2005), they will fade on average by similar to 1.3 mag in M-B. To fall on local L-Z and M-Z relationships, these galaxies must increase by a factor of 6 - 7 in M/L-B between z similar to 1 and z similar to 0 and by factor of 2 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 similar to 2 star-forming galaxies have similar [N II]/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 similar to 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.