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 ∼0.3 dex, followed by C, Mg, and Si, with [ X/ Fe] ∼ 0.2 dex, and lastly Ti and Ca, mostly tracking Fe ([ X/ Fe] < 0.1 dex), 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.