Luminosity Dependence and Redshift Evolution of Strong Emission-Line Diagnostics in Star-Forming Galaxies

Abstract

We examine the redshift evolution of standard strong emission-line diagnostics for Hβ-selected star-forming galaxies using the local SDSS sample and a new z=0.2{--}2.3 sample obtained from Hubble Space Telescope WFC3 grism and Keck DEIMOS and MOSFIRE data. We use the SDSS galaxies to show that there is a systematic dependence of the strong emission-line properties on Balmer-line luminosity, which we interpret as showing that both the N/O abundance and the ionization parameter increase with increasing line luminosity. Allowing for the luminosity dependence tightens the diagnostic diagrams and the metallicity calibrations. The combined SDSS and high-redshift samples show that there is no redshift evolution in the line properties once the luminosity correction is applied, I.e., all galaxies with a given L({{H}}β ) have similar strong emission-line distributions at all the observed redshifts. We argue that the best metal diagnostic for the high-redshift galaxies may be a luminosity-adjusted version of the [N II]6584/Hα metallicity relation.

Based in part on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA and was made possible by the generous financial support of the W. M. Keck Foundation.