Star formation laws in the Andromeda galaxy: gas, stars, metals and the surface density of star formation

Abstract

We use hierarchical Bayesian regression analysis to investigate star formation laws in the Andromeda galaxy (M31) in both local (30, 155, and 750pc) and global cases. We study and compare the well-known Kennicutt-Schmidt law, the extended Schmidt law and the metallicity/star formation correlation. Using a combination of H$\alpha$ and 24 $\mu$m emission, a combination of far-ultraviolet and 24$\mu$m, and the total infrared emission, we estimate the total star formation rate (SFR) in M31 to be between $0.35\pm 0.04$M$_{\odot}$yr$^{-1}$ and $0.4\pm 0.04$M$_{\odot}$yr$^{-1}$. We produce a stellar mass surface density map using IRAC 3.6$\mu$m emission and measured the total stellar mass to be $6.9 \times 10^{10}$M$_{\odot}$. For the Kennicutt-Schmidt law in M31, we find the power-law index $N$ to be between 0.49 and 1.18, for all the laws, the power-law index varies more with changing gas tracer than with SFR tracer. The power-law index also changes with distance from the centre of the galaxy. We also applied the commonly-used ordinary least squares fitting method and showed that using different fitting methods leads to different power-law indices. There is a correlation between the surface density of SFR and the stellar mass surface density, which confirms that the Kennicutt-Schmidt law needs to be extended to consider the other physical properties of galaxies. We found a weak correlation between metallicity, the SFR and the stellar mass surface density.