On the origin of M81 group extended dust emission
  Galactic cirrus emission at far-infrared wavelengths affects many
extragalactic observations. Separating this emission from that associated with
extragalactic objects is both important and difficult. In this paper we discuss
a particular case, the M81 group, and the identification of diffuse structures
prominent in the infrared, but also detected at optical wavelengths. The origin
of these structures has previously been controversial, ranging from them being
the result of a past interaction between M81 and M82 or due to more local
Galactic emission. We show that over of order a few arcminute scales the
far-infrared (Herschel 250 &\mu&m) emission correlates spatially very well with
a particular narrow velocity (2-3 km/s) component of the Galactic HI. We find
no evidence that any of the far-infrared emission associated with these
features actually originates in the M81 group. Thus we infer that the
associated diffuse optical emission must be due to galactic light back
scattered off dust in our galaxy. Ultra-violet observations pick out young
stellar associations around M81, but no detectable far-infrared emission. We
consider in detail one of the Galactic cirrus features, finding that the
far-infrared HI relation breaks down below arc minute scales and that at
smaller scales there can be quite large dust temperature variations.
