Star formation efficiency and flatten gradients in M31

We present and discuss results from a chemical evolution model for M31 based on a pronounced inside-out scenario for the galactic formation. The model has been built to reproduce three observational constraints of the M31 disk: the radial distributions of the total baryonic mass, the gas mass, and the O/H abundance. The model shows excellent agreement, throughout the galactic disk, with the observed radial distributions of the SFR and with gradients for fifteen chemical elements. From observations, we find that the gas mass profile of M31 presents a peak between 9 and 11 kpc and that the SFR diminishes for r>12 kpc. To obtain a consistent set of O/H values from H II regions, we correct those gaseous abundances due to the effect of temperature variations and O trapped in dust grains. From reproducing the radial distributions of the gas mass, we find that the star formation efficiency is variable in space, for the whole disk, and is constant in time for most of the evolution (t<12.8 Gyr). From reproducing the radial distributions of the SFR, we find that the efficiency decreases almost to zero for 12.8<t<13.0 Gyr and r>12 kpc . This reduction is supported by the star formation ring at aprox 10 kpc observed by Block et al. (2006), that probably originated 0.2 Gyr ago, due to an encounter between M31 and a satellite galaxy. The predicted radial distribution of each Xi/H ratio shows three different gradients, all of them flatten at different rates due to the r-dependence of the the star formation efficiency and the inside-out galactic formation. The inner gradient, r<8 kpc, starts being negative and becomes slightly positive. The central gradient, 8<r (kpc)<11, starts being negative, flattens slightly, but remains negative. The outer gradient, r>11 kpc, starts being positive and becomes slightly negative.

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Date added: Tue, 8 Oct 13