Panchromatic Hubble Andromeda Treasury. XIX. The Ancient Star Formation History of the M31 Disk

TL;DR

This study maps the star formation history of M31's disk, revealing major epochs of star formation and significant mixing, and estimates the total stellar mass formed over 14 billion years.

Contribution

It provides a detailed, spatially-resolved star formation history of M31's disk using stellar evolution models fitted to color-magnitude diagrams, covering a large area outside the galaxy's core.

Findings

Most star formation occurred before 8 Gyr ago.

Significant mixing of stellar populations occurred after 2 Gyr ago.

Approximately 5×10^{10} solar masses of stars formed in the past 14 Gyr.

Abstract

We map the star formation history across M31 by fitting stellar evolution models to color-magnitude diagrams of each 83$''{\times}$83$''$ (0.3$\times$1.4 kpc, deprojected) region of the PHAT survey outside of the innermost 6$'{\times}$12$'$ portion. We find that most of the star formation occurred prior to $\sim$8 Gyr ago, followed by a relatively quiescent period until $\sim$4 Gyr ago, a subsequent star formation episode about 2 Gyr ago and a return to relative quiescence. There appears to be little, if any, structure visible for populations with ages older than 2 Gyr, suggesting significant mixing since that epoch. Finally, assuming a Kroupa IMF from 0.1$-$100 M$_{\odot}$, we find that the total amount of star formation over the past 14 Gyr in the area over which we have fit models is 5${\times}$10$^{10}$ M$_{\odot}$. Fitting the radial distribution of this star formation and assuming azimuthal symmetry, (1.5$\pm$0.2)${\times}$10$^{11}$ M$_{\odot}$ of stars have formed in the M31 disk as a whole, (9$\pm$2)${\times}$10$^{10}$ M$_{\odot}$ of which has likely survived to the present after accounting for evolutionary effects. This mass is about one fifth of the total dynamical mass of M31.