Variations of the dust properties of M82 with galactocentric distance

We use near ultraviolet and optical photometry to investigate the dust properties in the nearby starburst galaxy M82. By combining imaging from the Swift/UVOT instrument and optical data from the Sloan Digital Sky Survey, we derive the extinction curve parameterized by the standard Rv factor, and the strength of the NUV 2175 A feature - quantified by a parameter B -- out to projected galactocentric distances of 4 kpc. Our analysis is robust against possible degeneracies from the properties of the underlying stellar populations. Both B and Rv correlate with galactocentric distance, revealing a systematic trend of the dust properties. Our results confirm previous findings that dust in M82 is better fit by a Milky Way standard extinction curve (Hutton et al.), in contrast to a Calzetti law. We find a strong correlation between Rv and B, towards a stronger NUV bump in regions with higher Rv, possibly reflecting a distribution with larger dust grain sizes. The data we use were taken before SN2014J, and therefore can be used to probe the properties of the interstellar medium before the event. Our Rv values around the position of the supernova are significantly higher than recent measurements post-SN2014J (Rv~1.4). This result is consistent with a significant change in the dust properties after the supernova event, either from disruption of large grains or from the contribution by an intrinsic circumstellar component. Intrinsic variations among supernovae not accounted for could also give rise to this mismatch.