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Adventures in Andromeda: The interplay of interstellar dust and gas in our big neighbour

Eknath, Gayathri 2023. Adventures in Andromeda: The interplay of interstellar dust and gas in our big neighbour. PhD Thesis, Cardiff University.
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Past studies of dust in the Andromeda galaxy (M31) have shown radial variations in the dust emissivity index (β). Understanding the astrophysical reasons behind these radial variations may give clues about the chemical composition of dust grains, their physical structure, and the evolution of dust. In Chapter 2, we use 12CO(J=1-0) observations and dust mass surface density measurements derived from Herschel observations to produce two cloud catalogues. We use these catalogues to investigate whether there is evidence that β is different inside and outside molecular clouds. Our results confirm the radial variations of β seen in previous studies. We find little difference between the average β inside molecular clouds compared to outside molecular clouds, in disagreement with models which predict an increase of β in dense environments. We find some clouds traced by dust with very little CO which may be either clouds dominated by atomic gas or clouds of molecular gas that contain little CO. Finding excess emission at submillimetre wavelengths beyond 500 μm (coined "sub-mm excess") might imply that there is a lot of very cold dust in a galaxy or that the dust grains have unusual emission properties. In Chapter 3, we use new submillimetre observations of M31 at 450 and 850 μm to search for any excess emission from dust at these wavelengths. We do not find strong evidence for the presence of a sub-mm excess. We present the first results of the HASHTAG large programme showing the spatial distribution of dust temperature, β and dust mass surface density in M31. In Chapter 4, we produce a dust-selected cloud catalogue using archival Herschel observations and new JCMT observations of M31 from the HASHTAG large programme. We then examine how much CO-traced molecular gas and atomic hydrogen is in our dust-selected clouds. 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. We propose that the offset could be due to variations in the ratio of gas to dust, or due to missing CO-dark molecular gas which is not being traced. Star formation is an inefficient process. What is driving this inefficiency is still a mystery. In Chapter 5, we measure the star formation efficiency (SFE) of individual clouds for the clouds extracted in Chapter 4. We investigate if the SFE varies in clouds across different positions in the galaxy. We use FUV+24 μm emission to trace both the massive star formation and dust-obscured star formation. We also study whether any observational dust properties influence the SFE of clouds. We do not find any systematic trends of SFE with radius but do find strong correlations of the star formation rate with both atomic gas and CO-traced molecular gas. We find that SFE is also correlated with dust temperature and β. 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 that there is an offset between the input simulation dust mass surface density values and the PPMAP output.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: Astrophysics, Galaxies, Star Formation
Funders: STFC, Bell Burnell Graduate Scholarship Fund
Date of First Compliant Deposit: 29 February 2024
Last Modified: 05 Mar 2024 16:40

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