University of Tasmania

File(s) under permanent embargo

The ACS nearby galaxy survey treasury. II. Young stars and their relation to Hα and UV Emission timescales in the M81 Outer Disk

journal contribution
posted on 2023-05-16, 23:30 authored by Gogarten, SM, Dalcanton, JJ, Williams, BF, Seth, AC, Dolphin, A, Weisz, D, Skillman, E, Holtzman, J, Andrew ColeAndrew Cole, Girardi, L, de Jong, RS, Karachentsev, ID, Olsen, K, Rosema, K
We have obtained resolved stellar photometry from Hubble Space Telescope Advanced Camera for Surveys observations of a field in the outer disk of M81 as part of ANGST. Motivated by the recent discovery of extended UV 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 H II regions, two regions that are UV-bright but Hα-faint, and one "control" region faint in both UV and Hα. We estimate our effective star formation rate detection limit at ~2 x 10–4 M yr–1, which is lower than that of GALEX for regions of this size. As expected, the H II regions contain massive main-sequence stars (in the mass range 18-27 M, based on our best extinction estimates), while similar massive main-sequence stars are lacking in the UV-bright/Hα-faint regions. The observations are consistent with stellar ages ≲ 10 Myr in the H II regions, and ≳ 16 Myr in the UV-bright/Hα-faint regions. All regions but the control have formed ~ 104 M 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 Hα and those detected exclusively in UV. However, our data cannot conclusively rule out other explanations, such as a strongly truncated initial mass function.


Publication title

The Astrophysical Journal








School of Natural Sciences


University of Chicago Press

Place of publication

Chicago, USA

Rights statement

Copyright © 2009 The American Astronomical Society

Repository Status

  • Restricted

Socio-economic Objectives

Expanding knowledge in the physical sciences

Usage metrics

    University Of Tasmania


    Ref. manager