Star Clusters in M 31. IV. A Comparative Analysis of Absorption Line Indices in Old M 31 and Milky Way Clusters

TL;DR

This study compares absorption line indices in old globular clusters from M 31 and the Milky Way, finding no significant chemical composition differences and clarifying previous discrepancies due to calibration issues.

Contribution

It provides a detailed comparison of absorption line indices in M 31 and Galactic clusters, correcting prior calibration errors and challenging previous claims of chemical differences.

Findings

No significant chemical differences between M 31 and Milky Way clusters

CN indices are consistent across both cluster systems

Calibration uncertainties explained previous apparent differences

Abstract

We present absorption line indices measured in the integrated spectra of globular clusters both from the Galaxy and from M 31. Our samples include 41 Galactic globular clusters, and more than 300 clusters in M 31. The conversion of instrumental equivalent widths into the Lick system is described, and zero-point uncertainties are provided. Comparison of line indices of old M 31 clusters and Galactic globular clusters suggests an absence of important differences in chemical composition between the two cluster systems. In particular, CN indices in the spectra of M 31 and Galactic clusters are essentially consistent with each other, in disagreement with several previous works. We reanalyze some of the previous data, and conclude that reported CN differences between M 31 and Galactic clusters were mostly due to data calibration uncertainties. Our data support the conclusion that the chemical compositions of Milky Way and M 31 globular clusters are not substantially different, and that there is no need to resort to enhanced nitrogen abundances to account for the optical spectra of M 31 globular clusters.