Adventures in Andromeda: The Interplay of Interstellar Dust and Gas in our Big Neighbour

In this thesis, we examine the interplay of interstellar dust and gas in the Andromeda galaxy (M31). In Chapter 2, we use $^{12}$CO CARMA observations of M31 and dust mass surface density measurements from the Bayesian algorithm PPMAP to investigate whether the dust emissivity index ($\beta$) is different inside and outside molecular clouds. We find little difference between the average $\beta$ inside molecular clouds compared to outside molecular clouds, in disagreement with models which predict an increase of $\beta$ in dense environments. In Chapter 3, we use submillimetre observations of M31 at 450 and 850 $\mu$m from the HASHTAG large programme and present first results of the spatial distribution of dust temperature, $\beta$ and dust mass surface density from this survey. We search for any excess emission from dust at these wavelengths and do not find strong evidence for the presence of a sub-mm excess. In Chapter 4, we produce a new dust-selected cloud catalogue using archival Herschel observations and HASHTAG observations. We show that dust is a good tracer of the ISM gas mass at the scale of individual molecular clouds but with an offset from the combined CO-traced + HI gas masses. In Chapter 5, we measure the star formation efficiency (SFE) of individual clouds, for the clouds extracted in Chapter 4, by using FUV+24 $\mu$m emission to trace both the massive star formation and dust-obscured star formation. We do not find any systematic trends of SFE with radius but do find strong correlations of the star formation rate with atomic gas and molecular gas. We find that SFE is correlated with dust temperature and $\beta$. In Chapter 6, we use a simulated galaxy to test and optimise the Bayesian algorithm PPMAP for application on observations of external galaxies. We find an offset between the input simulation dust mass surface density values and the PPMAP output.