Chemical abundance ratios for the bulge of M31

Abstract

We present abundance ratio estimates of individual elements, namely C, N, Na, and the so-called alpha elements, Mg, O, Si, Ca, and Ti, for the bulge of M31. The analysis is based on long-slit, high-quality spectroscopy of the bulge, taken with the OSIRIS spectrograph at the Gran Telescopio CANARIAS (GTC). Abundance ratios, [X/Fe]s, are inferred by comparing radially binned spectra of M31 with different state-of-the-art stellar population models, averaging out results from various methods, namely full-spectral, full-index, and line-strength fitting, respectively. For the bulk of the bulge, we find that O, N, and Na are significantly enhanced compared to Fe, with abundances of about 0.3dex, followed by C, Mg, and Si, with [X/Fe] about 0.2dex, and lastly, Ti and Ca, mostly tracking Fe ([X/Fe]<0.1dex), within the error bars. Performing the same analysis on SDSS stacked spectra of early-type galaxies with different velocity dispersion, we find that the abundance pattern of the M31 bulge is very similar to that of most massive galaxies, supporting a scenario where most of the bulge formed in a fast and intense episode of star-formation.