(Cassiopeia – Summer / été 2022)
by Dr. Ryley Hill
Thesis defended on March 15, 2022
Department of Physics and Astronomy, University of British Columbia
Thesis advisor: Prof. Douglas Scott
The Lambda cold dark matter (Lambda CDM) model accurately reproduces many notable observations of our Universe, such as the existence of galaxy clusters embedded in a cosmic web. However, there remain many open questions about the physics governing baryons on galaxy cluster scales that the Lambda CDM model cannot address, such as how star-formation is triggered and quenched, and how feedback processes regulate structure growth. In order to investigate these questions we study SPT2349-56, a star-forming protocluster discovered at redshift 4.3, corresponding to a period when large-scale structure was actively forming. We use submillimetre observations to search for protocluster members, identifying 29 galaxies at z = 4.3. These galaxies are distributed into a central core 300 kpc in diameter, and a northern extension offset from the core by 400 kpc. We find three additional galaxies 1.5 Mpc from the main structure, suggesting the existence of other halos at the same redshift that are not covered by our data. An analysis of the velocity distribution of the central galaxies indicates that this region may be virialized with a mass of (9 ± 5)x1012 solar masses, while the two separated galaxy groups show significant velocity offsets from the central group. We estimate the average star-formation rate density of SPT2349-56 to be roughly 40,000 solar masses per year per cubic Megaparsec; this may be an order of magnitude greater than the most extreme examples seen in simulations. We carry out a suite of optical and near-infrared observations in order to characterize the stellar content of SPT2349-56. Using the submillimetre positions of the protocluster members, we identify counterparts and perform detailed source deblending, allowing us to fit spectral energy distributions and estimate stellar masses. We show that the galaxies in SPT2349-56 have stellar masses proportional to their star-formation rates, consistent with other protocluster galaxies and field submillimetre galaxies (SMGs) around redshift 4. However, the galaxies in SPT2349-56 have on average lower molecular gas-to-stellar mass fractions and depletion timescales than field SMGs, although with considerable scatter. Hydrodynamical simulations predict that the core galaxies will quickly merge into a brightest cluster galaxy, thus our observations provide a direct view of the early formation mechanisms of this class of object.