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 parametrized by the standard R_V factor, and the strength of the NUV 2175 Å 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 R_V correlate with galactocentric distance, revealing a systematic trend of the dust properties. Our results confirm previous findings that dust in M82 is better fitted by a Milky Way standard extinction curve, in contrast to a Calzetti law. We find a strong correlation between R_V and B, towards a stronger NUV bump in regions with higher R_V, 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 R_V values around the position of the supernova are significantly higher than recent measurements post-SN2014J (R_V ≈ 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.

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