The ACS Nearby Galaxy Survey Treasury II. Young Stars and their Relation to Halpha and UV Emission Timescales in the M81 Outer Disk

TL;DR

This study uses Hubble Space Telescope data to analyze the recent star formation history in the outer disk of M81, revealing age differences between HII and UV-bright regions and discussing implications for star formation indicators.

Contribution

First detailed stellar population analysis of M81's outer disk regions linking UV and Halpha emission to star formation history.

Findings

HII regions contain massive stars aged <10 Myr

UV-bright/Halpha-faint regions have older stellar populations (>16 Myr)

Star formation in these regions occurred over the past 65 Myr

Abstract

We have obtained resolved stellar photometry from Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) observations of a field in the outer disk of M81 as part of the ACS Nearby Galaxy Survey Treasury (ANGST). Motivated by the recent discovery of extended UV (XUV) disks around many nearby spiral galaxies, we use the observed stellar population to derive the recent star formation histories of five ~0.5 kpc-sized regions within this field. These regions were selected on the basis of their UV luminosity from GALEX and include two HII regions, two regions which are UV-bright but Halpha-faint, and one "control" region faint in both UV and Halpha. We estimate our effective SFR detection limit at ~2 x 10^-4 Msun/yr, which is lower than that of GALEX for regions of this size. As expected, the HII regions contain massive main sequence stars (in the mass range 18-27 Msun, based on our best extinction estimates), while similar massive main sequence stars are lacking in the UV-bright/Halpha-faint regions. The observations are consistent with stellar ages <10 Myr in the HII regions, and >16 Myr in the UV-bright/Halpha-faint regions. All regions but the control have formed ~10^4 Msun of stars over the past ~65 Myr. Thus, our results, for at least one small area in the outer disk of M81, are consistent with an age difference being sufficient to explain the observed discrepancy between star-forming regions detected in Halpha and those detected exclusively in UV. However, our data cannot conclusively rule out other explanations, such as a strongly truncated initial mass function (IMF).