Talk describing work presented in C J R Clark et al (2019), presented as a seminar given at University College London in April 2019, and at the East Asian Observatory in October 2019. An abridged version of this talk was presented at the following conferences:
"Cosmic Dust - Origin, Applications, and Implications" conference, Copenhagen, 2018;
"Linking the Milky Way and Nearby Galaxies" conference, Helsinki, 2019.