Tuesday

Andromeda and the Milky Way are the two large spirals in the local group. Studies of M31 are important as unlike the Milky Way we can obtain a complete census of all star-formation, gas and dust in the galaxy. The proximity of Andromeda provides the best spatial resolution, and it has a wealth of multi-wavelength data making it the ideal target for nearby-extragalactic studies.

I will start by presenting the results from our Herschel survey of Andromeda. We found that the dust temperatures in the bulge (∼30K) can be significantly higher than those in the star-forming disk (∼18K), and the first evidence that the far-infrared spectral index (β) varies within a galaxy from ∼1.8 in the outer ring to∼2.5. The change in β suggests the physical properties of the dust are varying between regions, but progress identifying the causes was difficult as our analysis did not have the resolution (~140pc at 500µm) to resolve individual GMCs. Variations in β have now been found in many nearby galaxies and in the JINGLE galaxy sample.

Recently, by applying PPMAP, a new dust SED fitting technique (Marsh2015) to our Herschel data, we can determine the column-density of dust in bins of temperature and emissivity index. The dust maps have a resolution of 40pc, ideal to compare to the dust extinction maps form the detailed Hubble measurements made by the PHAT team. We find that dust models over predict the ratio of optical-to-FIR opacity, and an anti-correlation exists between the dust extinction and our β values. Comparing with high-resolution CO suggests that local density is not the driving factor in β values. I will also present the latest results from HASHTAG, a new large program on the JCMT which observes the whole galaxy at 450 and 850µm, providing dust images at 20pc resolution.