Swift UV/Optical Telescope Imaging of Star Forming Regions in M81 and Holmberg IX

TL;DR

This study uses Swift UVOT and ground-based optical data to analyze star formation and stellar populations in M81 and Holmberg IX, revealing recent starburst activity and galaxy properties.

Contribution

It introduces combined UV/optical imaging analysis methods, including pixel SED fitting and background modeling, to study galaxy star formation and evolution.

Findings

Evidence of star formation burst ~200 Myr ago in both galaxies

Holmberg IX's age consistent with tidal dwarf origin

Both galaxies fit Milky Way dust extinction law with 2175 Å bump

Abstract

We present Swift UV/Optical Telescope (UVOT) imaging of the galaxies M81 and Holmberg IX. We combine UVOT imaging in three near ultraviolet (NUV) filters (uvw2: 1928 {\AA}, uvm2: 2246 {\AA}, and uvw1: 2600 {\AA}) with ground based optical imaging from the Sloan Digital Sky Survey to constrain the stellar populations of both galaxies. Our analysis consists of three different methods. First we use the NUV imaging to identify UV star forming knots and then perform SED modeling on the UV/optical photometry of these sources. Second, we measure surface brightness profiles of the disk of M81 in the NUV and optical. Last we use SED fitting of individual pixels to map the properties of the two galaxies. In agreement with earlier studies we find evidence for a burst in star formation in both galaxies starting ~200 Myr ago coincident with the suggested time of an M81-M82 interaction. In line with theories of its origin as a tidal dwarf we find that the luminosity weighted age of Holmberg IX is a few hundred million years. Both galaxies are best fit by a Milky Way dust extinction law with a prominent 2175 {\AA} bump. In addition, we describe a stacked median filter technique for modeling the diffuse background light within a galaxy, and a Markov chain method for cleaning segment maps generated by SExtractor.