Unravelling giant molecular clouds

Thesis

Giant molecular clouds (GMCs) provide the fuel for star formation and bear various feedbacks. Thus, they are key to our understanding of galaxy evolution and the baryon cycle. In this thesis, I study the cold molecular gas content in the nearby universe, including three megamaser-host galaxies, one early-type galaxy, and the central molecular zone (CMZ) of the Milky Way (MW). The spatial scales probed by the analyses range from kpc-scale gas discs (i.e. the environment GMCs reside in) to sub-pc clumps within GMCs.

I start with analysing the distribution and kinematics of the molecular gas of three megamaser galaxies: NGC 1194, NGC 3393, and NGC 5765B. I present Atacama Large Millimeter/sub-millimeter Array (ALMA) ¹²CO(2-1) observations with resolutions of about 100 pc. Every galaxy has an extended rotating molecular gas disc but all also have non-axisymmetric features. I explore the tentative correlation between the existence of megamasers and these features in the gas discs. I also examine the potential for using ¹²CO(2-1) as a dynamical tracer in these galaxies to measure the supermassive blackhole masses.

Next, I investigate resolved GMCs of the lenticular galaxy NGC 1387, exploiting high-resolution (14 pc) ¹²CO(2-1) observations from ALMA. I analysed the fundamental properties of 1285 individual GMCs. Unusually for an ETG, the GMCs of NGC 1387 follow scaling relations very similar to those of the MW disc, and most are virialised. A subset of GMCs have their angular momenta aligned with the large-scale galactic rotation.

Finally, in the MW CMZ, I study the gas-structure dendrogram across a large dynamical range (1–100 pc), primarily with ALMA HNCO(4-3) observations at 0.7 pc resolution. I discuss the dynamical state of the gas structures and identify galactic shear and magnetic fields as potential physical drivers of their morphologies.

Overall, this thesis has studied cold molecular gas (clouds) in special systems (megamaser hosts, the ETG, and the MW centre) with unprecedented physical resolutions. It enriches our understanding of molecular gas properties in the context of galaxy evolution.

Authors: Liang, F

• DOI: 10.5287/ora-mnw8yye9r
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: galaxy evolution; cold molecular gas
• Subjects: Astronomy