We present the mass-metallicity (MZ) and luminosity-metallicity (LZ)
relations at z ~ 0.8 from ~1350 galaxies in the Deep Extragalactic Evolutionary
Probe 2 (DEEP2) survey. We determine stellar masses by fitting the spectral
energy distribution inferred from photometry with current stellar population
synthesis models. This work raises the number of galaxies with metallicities at
z ~ 0.8 by more than an order of magnitude. We investigate the evolution in the
MZ and LZ relations in comparison with local MZ and LZ relations determined in
a consistent manner using ~21,000 galaxies in the Sloan Digital Sky Survey. We
show that high stellar mass galaxies (log(M/M_solar)~10.6) at z ~ 0.8 have
attained the chemical enrichment seen in the local universe, while lower
stellar mass galaxies (log(M/M_solar)~9.2) at z ~ 0.8 have lower metallicities
(Delta log(O/H)~0.15 dex) than galaxies at the same stellar mass in the local
universe. We find that the LZ relation evolves in both metallicity and B-band
luminosity between z ~ 0.8 and z~ 0, with the B-band luminosity evolving as a
function of stellar mass. We emphasize that the B-band luminosity should not be
used as a proxy for stellar mass in chemical evolution studies of star-forming
galaxies. Our study shows that both the metallicity evolution and the B-band
luminosity evolution for emission-line galaxies between the epochs are a
function of stellar mass, consistent with the cosmic downsizing scenario of
galaxy evolution.Comment: Accepted Version: 18 pages, 13 figure
