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

TL;DR

This study investigates how strong emission-line diagnostics in star-forming galaxies depend on luminosity and evolve with redshift, revealing that luminosity correction aligns galaxy properties across cosmic time.

Contribution

The paper demonstrates that accounting for luminosity dependence removes redshift evolution in emission-line diagnostics, improving metallicity estimates in high-redshift galaxies.

Findings

Emission-line properties depend systematically on Balmer-line luminosity.

Luminosity correction tightens diagnostic diagrams and metallicity calibrations.

No redshift evolution in line properties after luminosity adjustment.

Abstract

We examine the redshift evolution of standard strong emission-line diagnostics for Hbeta-selected star-forming galaxies using the local SDSS sample and a new z = 0.2 - 2.3 sample obtained from HST 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(Hbeta) 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 [NII]6584/Halpha metallicity relation.