Oxygen Metallicity Determinations from Optical Emission Lines in Early-type Galaxies

TL;DR

This study measures oxygen abundances in the warm gas of seven early-type galaxies, finding near-solar metallicity levels that suggest an internal origin for the gas, using emission line analysis and photoionization models.

Contribution

It provides the first detailed oxygen metallicity measurements of warm gas in early-type galaxies using emission line diagnostics and photoionization modeling.

Findings

Mean oxygen metallicity of 1.01±0.50 solar in warm ISM

Warm ISM metallicity consistent with stellar population metallicity

Supports internal origin hypothesis for warm ISM

Abstract

We measured the oxygen abundances of the warm (T$\sim 10^{4}K$) phase of gas in seven early-type galaxies through long-slit observations. A template spectra was constructed from galaxies void of warm gas and subtracted from the emission-line galaxies, allowing for a clean measurement of the nebular lines. The ratios of the emission lines are consistent with photoionization, which likely originates from the UV flux of post-asymototic giant branch (PAGB) stars. We employ H II region photoionization models to determine a mean oxygen metallicity of $1.01\pm0.50$ solar for the warm interstellar medium (ISM) in this sample. This warm ISM 0.5 to 1.5 solar metallicity is consistent with modern determinations of the metallicity in the hot (T$\sim 10^{6}-10^{7}K$) ISM and the upper range of this warm ISM metallicity is consistent with stellar population metallicity determinations. A solar metallicity of the warm ISM favors an internal origin for the warm ISM such as AGB mass loss within the galaxy.