Constraining the Properties of the Interstellar Medium in Luminous Active Galaxies Through Observations of Carbon Monoxide and Atomic Carbon

This thesis aims to constrain the properties of the interstellar medium (ISM) in Hot Dust-Obscured galaxies (Hot DOGs): some of the most luminous known galaxies in the universe discovered by the Wide-field Infrared Survey Explorer (WISE). They are thought to harbour some of the most powerful active galactic nuclei (AGN) in the universe, making Hot DOGs ideal laboratories for the study of AGN-ISM interactions. I use imaging from the Atacama LargeMillimetre/Submillimetre Array (ALMA) to investigate the molecular and atomic gas phases of the ISM in the host galaxies to provide resolved detail, unseen in prior studies of these objects, while placing constraints on their size, mass, and gas kinematics. In Chapter 4, I present observations of mid-J (J = 4–3 or J = 5–4) carbon monoxide (CO) emission lines and continuum emission from a sample of ten of the most luminous (Lbol ¸ 1014 L¯) Hot DOGs with redshifts up to 4.6. I uncover large (¸ 400kms¡1) velocity dispersions and spectral linewidths in these objects, suggesting a turbulent molecular ISM may be ubiquitous in Hot DOGs. The CO(4–3) observations contrast with previous CO(1–0) studies of the same sources: the CO(4–3) to CO(1–0) luminosity ratios exceed 300 in each source, suggesting that the lowest excited states of CO are underluminous and plausibly thermalised by the cosmic microwave background (CMB). I further examine mid-J CO in a sample of six Hot DOGs in Chapter 5 to investigate the spatial extent of CO gas in these sources and to examine whether there are multiple molecular components. In addition to evidence of galactic rotations, I find evidence that four out of the six sources plausibly host additional components, including potential inflows, a merger, and a molecular halo. The molecular gas is extended by up to 60 per cent, and even more so in previous [CII] observations, consistent with the ionised gas tracing more extended regions. Additionally, the large extent of gas in Hot DOGs reinforces the rapid galaxy assembly required over relatively brief periods of cosmic time, suggesting that future galaxy evolution models require greater merger rates at high-redshift. Finally, I investigate high-J (J = 7–6) CO and atomic carbon [CI] in a sample of fiveHot DOGs in Chapter 6, all of which have at least one previous low-J and mid-J CO observation. The warm molecular gas is detected with relatively short integration times (· 2000 s), showing the potential to investigate high-J CO in systems located at z ¸ 3. The broad velocity dispersions of each emission line, in conjunction with the broad [CII] emissions from other works, paint a highly dynamic and turbulent picture across the different gas phases in the ISM. This multiphase analysis shows that the mechanisms which stir up the gas in Hot DOGs are powerful and pervasive, affecting both dense and extended gas. The comparable luminosities of CO(7–6) and [CI] reveal that the different gas phases in the ISM are likely to be similarly influenced by dynamical processes, plausibly by the extreme levels of AGN feedback.