The ACS Nearby Galaxy Survey Treasury I. The Star Formation History of the M81 Outer Disk

TL;DR

This study uses deep Hubble imaging to analyze the star formation history and metallicity evolution of the outer disk of galaxy M81, revealing most stars formed by redshift ~1 and highlighting the disk's rapid enrichment.

Contribution

First detailed CMD-based analysis of the M81 outer disk's stellar populations, providing insights into its formation history and evolution.

Findings

Most stars formed by redshift ~1

Outer disk metallicity ranges from -1 to 0

The spiral arm structure persists for over 100 Myr

Abstract

The ACS Nearby Galaxy Survey Treasury (ANGST) is a large Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) treasury program to obtain resolved stellar photometry for a volume-limited sample of galaxies out to 4 Mpc. As part of this program, we have obtained deep ACS imaging of a field in the outer disk of the large spiral galaxy M81. The field contains the outskirts of a spiral arm as well as an area containing no current star formation. Our imaging results in a color-magnitude diagram (CMD) reaching to F814W = 28.8 and F606W = 29.5, one magnitude fainter than the red clump. Through detailed modeling of the full CMD, we quantify the age and metallicity distribution of the stellar populations contained in the field. The mean metallicity in the field is -1<[M/H]<0 and only a small fraction of stars have ages <~1 Gyr. The results show that most of the stars in this outer disk field were formed by z~1 and that the arm structure at this radius has a lifetime of >~100 Myr. We discuss the measured evolution of the M81 disk in the context of surveys of high-redshift disk galaxies and deep stellar photometry of other nearby galaxies. All of these indicate that massive spiral disks are mostly formed by z~1 and that they have experienced rapid metal enrichment.