Star formation efficiency and flattened gradients in M31

TL;DR

This paper develops a chemical evolution model for M31, demonstrating how inside-out formation and variable star formation efficiency explain observed abundance and mass gradients across the galaxy.

Contribution

It introduces a detailed inside-out formation model for M31 that reproduces multiple observational constraints and reveals spatial variations in star formation efficiency.

Findings

Star formation efficiency varies across M31's disk.

The model matches observed abundance and mass distributions.

Efficiency decreases near the galaxy's outskirts.

Abstract

We present a chemical evolution model for M31 based on a pronounced inside-out formation scenario. The model reproduces the three main 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 good agreement with the observed: SFR(r), Mstars(r), the C/H, N/H, Mg/H, Si/H, S/H, Ar/H, Cr/H, Fe/H, and Z gradients. From reproducing the observed 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 observed SFR, we find that the efficiency decreases almost to zero for 12.8<t(Gyr)< 13.0 and r>12 kpc. All the predicted Xi/H(r) gradients show three different slopes, due to the r-dependence of the the star formation efficiency and the inside-out galactic formation.