Elemental abundance ratios for the bulge of M31

TL;DR

This study maps radial metallicity $[Fe/H]$ and detailed abundance patterns $[X/Fe]$ in the M31 bulge out to ~0.6 kpc using OSIRIS/GTC long-slit spectra and compares to SDSS ETG stacks. It validates FSF, FIF, and IF abundance-determination methods against mocks and ETGs, then applies them with E-MILES and CvD18 models to derive element-by-element radial trends, correcting for the Milky Way pattern in oxygen where necessary. The authors interpret the results with revised chemical-evolution models, finding that a fast, intense star-formation episode best explains the bulk of the bulge but that decoupled central abundance patterns require additional processes such as differential winds or IMF variations. Overall, the M31 bulge exhibits abundance patterns akin to the most massive ETGs, with strong enhancements in O, N, and Na, and a complex radial behavior among elements that informs bulge formation scenarios.

Abstract

We present radial trends of metallicity ([Fe/H]) and abundance ratios ([X/Fe]) for several chemical elements -- including C, N, Na, and the so-called alpha-elements (O, Mg, Si, Ca, and Ti) -- in the bulge of M31, out to ~0.6kpc from the center. We estimated abundances using full-spectrum fitting, full-index fitting, and line-strength analysis, in combination with different stellar population models. We first tested these techniques on mock spectra and SDSS stacked spectra of ETGs, and then applied them to high-quality long-slit spectroscopy of the M31 bulge obtained with the OSIRIS spectrograph at the GTC. We find that O, N, and Na are significantly enhanced relative to Fe across the bulge, with typical abundances >0.3~dex. In particular, N and Na show steep central enhancements, reaching ~0.5dex. C, Mg, and Si exhibit intermediate enhancements of [X/Fe]~0.2dex, with C and Mg decreasing toward the center to <~0.1dex; while Ca, and to a lesser extent Ti, closely follow Fe, with [X/Fe]<0.1dex. Applying the same analysis to SDSS stacked spectra of ETGs revealed that the abundance pattern of the M31 bulge closely resembles that of the most massive galaxies, except for N, which is significantly more enhanced (by ~0.1dex) in the bulge. For the bulk of the bulge, chemical evolution models assuming high star-formation efficiency and a short gas infall timescale reproduce the overall trends in [Fe/H] and [X/Fe]. In the central region (<~100pc), the high metallicity content of the bulge can be explained by either an IMF flatter than Salpeter at high mass, or a prolonged star formation. Additional processes, such as differential galactic winds, appear necessary to account for the observed decoupling among alpha elements and the strong central N enhancement. Our results support a scenario whereby the bulk of the M31 bulge formed during a fast and intense episode of star formation.

Figures (15)

• Figure 1: Metallicity as a function of galactocentric distance, R, for the bulge of M31. Positive and negative values of R correspond to opposite sides of the slit. The top and bottom panels show $\rm [Fe/H]$ and $\rm [M/H]$ , respectively. Lines with different colors refer to E-MILES BaSTI (blue) and Padova00 (red), and Cvd18 models (green), respectively. Thick and thin lines indicate 1SSP and 2SSP models, respectively, while solid, dashed, and dotted lines plot results of different methods, i.e., full-spectral fitting, full-index fitting, and index fitting, respectively, as shown by the labels in the top panel. Median trends are plotted as black curves with black dots.
• Figure 2: Same as Fig. \ref{['fig:xh_m31']}, but plotting the metallicity profiles of ETGs stacked spectra as a function of velocity dispersion $\sigma$. The velocity dispersion of the M31 bulge ($\sigma \sim 150$ km s$^{-1}$ ) is marked with a vertical gray arrow in both panels. The gray triangles with error bars, at $\sigma \sim 150$ km s$^{-1}$ and $\sigma \sim 280$ km s$^{-1}$ show the estimates of $\rm [Fe/H]$ and $\rm [M/H]$ obtained by mimicking the Sloan fiber aperture for low- and high-$\sigma$ ETGs (see the text).
• Figure 3: Individual abundance ratios as a function of galactocentric distance, R, for the bulge of M31. Positive and negative values of R refer to different sides of the slit. From left to right, and top to bottom, $\alpha$ elements elements are shown first, namely O, Mg, Si, Ca, and Ti (in order of decreasing atomic number), followed by C, N, and Na. Lines with different colors refer to E-MILES BaSTI (blue) and Padova00 (red), and Cvd18 models (green), respectively. Thick and thin lines are for 1SSP and 2SSP models, respectively, while solid, dashed, and dotted lines plot results of different methods, i.e., full-spectral fitting, full-index fitting, and index fitting, respectively, as shown by the labels in the top--left panel.
• Figure 4: Median values of abundance ratios, obtained from different models and methods as a function of (left) galactocentric distance, R, for the bulge of M31, and (right) galaxy velocity dispersion for stacked spectra of ETGs. In both plots, different colors refer to different chemical elements (see labels in the left panel). Dashed blue curves show $\rm [O/Fe]$ abundances corrected for the Milky Way pattern (see the text). In each panel, error bars are plotted only for the end points. In the right panel, dots with error bars mark abundance estimates obtained by mimicking the Sloan fiber aperture (see the text) for the M31 bulge (see brown arrow). For clarity, small horizontal shifts have been applied to the symbols (error bars and dots) corresponding to different elements.
• Figure 5: Median difference in abundance ratios between the bulge of M31 and stacked spectra of ETGs for different elements, as shown on the x-axis. Median values and error bars are derived from different models and fitting methods (see the text). Different elements are color-coded as in Fig. \ref{['fig:xfe_rad_sig']}. Empty and filled symbols denote ETGs with low- ($\sigma = 150$ km s$^{-1}$ ) and high- ($\sigma > 260$ kms ) velocity dispersions, respectively, as indicated in the lower-left corner. The horizontal dashed line marks a difference of zero, while the vertical dotted line separates the $\alpha$-elements (left) from the remaining ones (right).