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 ?; uvm2: 2246 ?; uvw1: 2600 ?) 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 spectral energy distribution (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. Lastly, 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 ? 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.
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机译：我们介绍了星系M81和Holmberg IX的快速紫外/光学望远镜（UVOT）成像。我们将UVOT成像与三个近紫外（NUV）滤光片（uvw2：1928？; uvm2：2246？; uvw1：2600？）相结合，并结合了Sloan Digital Sky Survey的地面光学成像，以约束两个星系的恒星种群。我们的分析包括三种不同的方法。首先，我们使用NUV成像来识别形成UV星的结，然后对这些光源的UV /光学测光法执行光谱能量分布（SED）建模。其次，我们在NUV和光学系统中测量M81盘的表面亮度分布。最后，我们使用单个像素的SED拟合来映射两个星系的属性。与早期的研究相一致，我们发现证据表明两个星系在〜200年前就开始爆发，这与建议的M81-M82相互作用时间相吻合。根据潮汐源的理论，我们发现霍尔姆贝格九世的光度加权年龄为几亿年。这两个星系都最适合银河系消光定律，且突出2175？磕碰。此外，我们描述了一种用于对星系中的漫射背景光建模的堆叠中值滤波技术，以及用于清理由SExtractor生成的分段图的马尔可夫链方法。
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