Ben Rasmussen

Topic

The Cold Gas Content of Rapidly Dying Galaxies

Department of Physics and Astronomy

Date & location

• Thursday, August 13, 2026
• 9:30 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Sara Ellison, Department of Physics and Astronomy, University of Victoria (Supervisor)
• Dr. Helen Kirk, Department of Physics and Astronomy, UVic (Member)

External Examiner

• Dr. Decker French, Department of Astronomy, University of Illinois Urbana-Champaign

Chair of Oral Examination

• Dr. Annalee Lepp, Department of Gender Studies, UVic

Abstract

Observational studies of large samples of galaxies have shown that the vast majority of galaxies exist in one of two regimes: actively star forming, blue, gas-rich spirals, or quiescent, red, gas-poor ellipticals. The increasing fraction of quenched (not star forming) galaxies across cosmic time necessitates a transition from the blue, gas-rich population to the red and dead population. To gain insight into the quenching process, it is useful to identify galaxies that are actively, and rapidly, in the process of shutting down their star formation. Galaxies that are “caught in the act” of quenching, known as post-starburst (PSB) galaxies, can be found by selecting galaxies with a prominent and recently formed stellar population in combination with no active star formation at the time of observation. The principal fuel for star formation is the cold gas content, primarily in the form of the atomic Hydrogen gas phase (H_I) which cools to form the molecular Hydrogen gas phase (H₂; traced by the CO molecule). Thus, studying the cold gas in post-starburst galaxies provides important insights into the mechanisms that drive rapid quenching. Prior to this work, a multiphase (H_I + CO) assessment of both the atomic and molecular gas in PSBs did not exist in the literature.

In this thesis, I introduce the Ensemble of Multiphase Baryons Evolving in Rapidly-quenching Systems, or EMBERS survey, a homogeneously selected, nearly mass- and redshift-complete sample of the global atomic and molecular gas in a large population of PSBs. H_I measurements for 43 galaxies were obtained in 25 hours using the Five Hundred-metre Aperture Spherical Telescope (FAST), which were combined with 25 archival observations (total sample of 68) and presented by the EMBERS team in Ellison et al. (2025). Additionally, with 188.9 hours of observing with the Institut de Radioastronomie Millimétrique (IRAM) 30m telescope, with myself as primary observer, 52 new observations of CO(1–0) were obtained to trace the bulk molecular gas for PSBs in the EMBERS sample. These 52 new CO(1–0) measurements, which, combined with 9 archival observations, result in a total H₂ sample of 61, of which 58/61 have ancillary H_I measurements. EMBERS is thus the largest PSB sample to date in either gas phase and the only survey with access to both.

HI is detected in 57 out of 68 galaxies in EMBERS, with atomic gas fractions (𝑓_HI=M_HI/M_∗) ranging from 1.5 to 200 percent. The atomic gas properties of EMBERS galaxies are diverse, ranging from gas-poor, to categorically gas-rich compared to non-PSB galaxies. Conversely, I detect CO(1–0) in 34/61 galaxies, corresponding to molecular gas fractions (𝑓_H2=M_H2/M_∗) ranging from two to 250 percent. By comparing with a stellar-mass matched star-forming control sample from the literature, I find that PSBs on average are ∼ 0.3 − 0.6 dex depleted in H₂. I also find that the ratio between low-J CO transitions, 𝑟₂₁=L′_CO(2−1)/L′_CO(1−0), is significantly lower than in normal SF galaxies from the archive (0.44 compared to 0.73, respectively). When I consider both gas phases in tandem, the molecular-to-atomic gas ratio is consistent with that of regularly star forming galaxies, with a median of −0.39 dex. Considering both H_I and H₂ masses together, individual PSBs host diverse gas reservoirs ranging from gas-rich in both phases, elevated in one phase, or gas-poor, the latter of which is common at lower stellar mass. By stacking non-detected spectra in bins of stellar mass, I find a population of extremely gas poor, low-mass PSBs with gas fractions that are sub-0.5 percent. The existence of gas-normal and gas-depleted PSBs in both phases suggests that some PSBs may rejuvenate their star formation, but the rapid shutdown of star formation in others is likely terminal. Despite this diversity, the majority of EMBERS PSBs are gas-poor compared to SF controls, with the typical PSB hosting gas reservoirs intermediate to those found in star-forming and quenched galaxies.