(Figure 6)Hi, this is Rik. The actual caption is too long and cryptic to quote verbatim, but this figure shows the (oxygen) mass-metallicity relation for z~1-2 DEEP2 galaxies (colored points) and z=0 star-forming galaxies from SDSS (contours). Oxygen abundances are determined by emission line ratios; in the left panel a diagnostic based on the [NII]/Halpha ratio is used, while the abundances in the right panel are effectively found through ([OIII]/Hbeta)/([NII]/Halpha).
Regardless of the measure, there's a strong shift in the normalization: at a given mass, galaxies at lower redshift are more metal-rich than at high redshift. This is reassuring. The effect is stronger for the abundance measure on the right (i.e. the one incorporating both [OIII] and [NII]) than the one on the left. Later on, through other line ratios the authors find that gas in z~2 galaxies is more highly excited, and thus has stronger emission lines, than at z=0. Some of this may be due to shocks and AGN, but for the most part the high-z galaxies simply appear to have higher interstellar pressures. This makes z~2 galaxies appear more metal-rich than they actually are (this is stronger with the [NII] indicator than the [OIII]/[NII]); thus, characterizing these ionization effects is crucial to accurately measure the evolution of the M-Z relation.
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