Carbon and Nitrogen Abundances in Early-type Galaxies

TL;DR

This study systematically examines nitrogen abundances in 35 early-type galaxies using the nitrogen-sensitive NH3360 feature, revealing that nitrogen levels are mildly enhanced and independent of galaxy mass or environment, contrasting previous carbon-based findings.

Contribution

First to utilize NH3360 for nitrogen abundance in early-type galaxies, breaking degeneracies and providing new insights into chemical content and environmental effects.

Findings

Nitrogen abundance shows little dependence on galaxy mass.

N/Fe ratios are mildly enhanced relative to solar.

No significant environmental dependence for nitrogen abundance.

Abstract

For the first time, we undertake a systematic examination of the nitrogen abundances for a sample of 35 early-type galaxies spanning a range of masses and local environment. The nitrogen-sensitive molecular feature at 3360\AA has been employed in conjunction with a suite of atomic- and molecular-sensitive indices to provide unique and definitive constraints on the chemical content of these systems. By employing NH3360, we are now able to break the carbon, nitrogen, and oxygen degeneracies inherent to the use of the CN-index. We demonstrate that the NH3360 feature shows little dependency upon the velocity dispersion (our proxy for mass) of the galaxies, contrary to what is seen for carbon- and magnesium-sensitive indices. At face value, these results are at odds with conclusions drawn previously using indices sensitive to both carbon and nitrogen, such as cyanogen (CN). With the aid of stellar population models, we find that the N/Fe ratios in these galaxies are consistent with being mildly-enhanced with respect to the solar ratio. We also explore the dependence of these findings upon environment, by analyzing the co-added spectra of galaxies in the field and the Coma cluster. We confirm the previously found differences in carbon abundances between galaxies in low- and high-density environments, while showing that these differences do not seem to exist for nitrogen. We discuss the implications of these findings for the derivation of the star formation histories in early-type galaxies, and for the origin of carbon and nitrogen, themselves.