Carbon and nitrogen abundances in early-type galaxies

Abstract

For the first time, we undertake a systematic examination of the nitrogen abundances using the NH3360 feature for a sample of 35 early-type galaxies spanning a range of masses and local environment. The nitrogen-sensitive molecular feature at 3360Å 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 cyanogen (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 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.