NH and Mg Index Trends in Elliptical Galaxies

TL;DR

This study investigates the near-UV spectral indices in elliptical galaxies, revealing that old, metal-poor stellar populations may mask nitrogen abundance trends, challenging previous assumptions about element correlations.

Contribution

It introduces two new spectral indices, Mg3334 and NH3375, and demonstrates that old, metal-poor stars can explain the flat trends observed in near-UV indices.

Findings

NH3360 correlates with nitrogen but anti-correlates with magnesium.

New indices Mg3334 and NH3375 are sensitive to Mg and N, respectively.

Old, metal-poor stellar populations can mask nitrogen abundance trends.

Abstract

We examine the spectrum in the vicinity of the NH3360 index of Davidge & Clark (1994), which was defined to measure the NH absorption around 3360 \AA and which shows almost no trend with velocity dispersion (Toloba et al. 2009), unlike other N- sensitive indices, which show a strong trend (Graves et al. 2007). Computing the effect of individual elements on the integrated spectrum with synthetic stellar population integrated spectra, we find that, while being well correlated with nitrogen abundance, NH3360 is almost equally well anti-correlated with Mg abundance. This prompts the definition of two new indices, Mg3334, which is mostly sensitive to magnesium, and NH3375, which is mostly sensitive to nitrogen. Rather surprisingly, we find that the new NH3375 index shows a trend versus optical absorption feature indices that is as shallow as the NH3360 index. We hypothesize that the lack of a strong index trend in these near-UV indices is due to the presence of an old metal-poor component of the galactic population. Comparison of observed index trends and those predicted by models shows that a modest fraction of an old, metal-poor stellar population could easily account for the observed flat trend in these near-UV indices, while still allowing substantial N abundance increase in the larger galaxies.