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.
