Using spectroscopic data from the Deep Extragalactic Evolutionary Probe ( DEEP ) Groth Strip survey ( DGSS ) , we analyze the gas-phase oxygen abundances for 56 emission-line field galaxies in the redshift range 0.26 < z < 0.82 .
These galaxies comprise a small subset selected from among 693 objects in the DGSS .
They are chosen for chemical analysis because they exhibit the strongest emission lines , and thus have the highest star formation rates per unit luminosity .
Oxygen abundances relative to hydrogen range between 8.4 < 12 + log ( O / H ) < 9.0 
with typical uncertainties of 0.17 dex .
The 56 DGSS galaxies collectively exhibit a correlation between B-band luminosity and metallicity , i.e. , an L-Z relation .
Using the DGSS sample and local galaxy samples for comparison , we searched for a “ second parameter ” which might explain some of the dispersion in the L-Z relation .
Parameters such as galaxy color , emission line equivalent width , and effective radius were explored but found to be uncorrelated with residuals from the the mean L-Z relation .
The source of the dispersion in the L-Z relation is either intrinsic or depends upon a yet-unidentified combination of parameters .

Subsets of DGSS galaxies binned by redshift also exhibit L-Z correlations but with different zero points .
Galaxies in the highest redshift bin ( z = 0.6 - 0.82 ) are brighter by \sim 1 mag compared to the lowest redshift bin ( z = 0.26 - 0.40 ) and brighter by \sim 1 - 2 mag compared to local ( z < 0.1 ) field galaxies .
This offset from the local L-Z relation is greatest for objects at the low-luminosity ( M _ { B } \sim - 19 ) end of the sample , and vanishingly small for objects at the high-luminosity end of the sample ( M _ { B } \sim - 22 ) .
Thus , both the slope and zero point of the L-Z relation appear to evolve .
Based on the DGSS galaxies , we provide an approximate analytic expression for the mean gas-phase metallicity of galaxies as a function of luminosity and redshift—a recipe that may aid in modeling the properties of cosmologically distant galaxies .
Emission-line field galaxies have undergone moderate amounts of evolution in the past \sim 7 Gyr since z \sim 0.8 , and the evolution is most significant for galaxies of lower luminosity .
Either the least-luminous DGSS field galaxies have faded by 1–2 mag due to decreasing levels of star formation , or they have experienced an increase in the mean metallicity of the interstellar medium by factors of 1.3–2 ( 0.1-0.3 dex ) .
Plausible evolutionary models suggest that a combination of the two processes is likely .
The relatively greater degree of luminosity and metallicity evolution seen among the lower luminosity ( sub L* ) galaxies in the last 8 Gyr implies either a more protracted assembly process , or a more recent formation epoch compared to more luminous L* galaxies .