The environments of extreme star-forming galaxies across cosmic time

Cornish, Thomas and Wardlow, Julie (2023) The environments of extreme star-forming galaxies across cosmic time. PhD thesis, Lancaster University.

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Abstract

Unravelling the histories of massive elliptical galaxies in the local Universe is a monumental task. In the hierarchical paradigm which appears to govern structure formation in our Universe, these galaxies are thought to have complex histories rich with galaxy mergers. Furthermore, since they tend to reside in regions with high galaxy density such as galaxy clusters, their environments play a significant role in shaping their evolution. Significant progress has been made in recent years toward predicting how these galaxies evolve with cosmic time using both observations and simulations, but a definitive picture is yet to emerge. In this thesis we investigate the evolution of massive ellipticals through the environments of their expected progenitors at z > 1. Submillimetre galaxies (SMGs) are the sites of some of the most extreme star-forming activity in the Universe, and represent one such population of candidate progenitors. We thus begin with a wide-field narrowband survey of the environments of three spectroscopically-confirmed SMGs and search for signs of protocluster-like environments via overdensities of Hα and [OIII] emitters. We first present the VLT/HAWK-I observations used for this study and the bespoke data reduction pipeline written to process these data. After combining these data with archival observations covering the (observed-frame) UV-to-MIR parts of the spectrum, we fit spectral energy distributions to obtain photometric redshifts for our HAWK-I detections, from which we identify candidate companions for each target SMG and compare their densities with expected values for the coeval blank field. We find that two of the three SMGs reside in galaxy overdensities spanning scales of ~4 Mpc, whose present-day mass estimates are consistent with local galaxy clusters; these SMGs therefore reside in potential protoclusters, as expected of the progenitors of local massive ellipticals. The third SMG also resides in an overdensity, but on a smaller physical scale of ~1.6 Mpc, with a present-day mass estimate indicative of either a galaxy cluster or a galaxy group. We then use JCMT/SCUBA-2 submillimetre observations from the S2COSMOS survey to measure the density of SMGs in the environments of massive (M∗ > 1011 Msun), radio-quiet (L500 MHz < ~ 1025 W Hz-1) galaxies at z = 1–3. Observations in this wavelength regime trace dust-obscured star formation, which is expected to contribute significantly to the total star formation at these epochs. By searching for overdensities of SMGs we assess whether these massive radio-quiet galaxies reside in the most massive dark matter halos, as is the case for the radio-loud galaxies of similar mass which are the suspected progenitors of local brightest cluster galaxies (BCGs). SMG number counts reveal no significant overdensities in the environments of massive radio-quiet galaxies relative to blank-field expectations. Tentative signs of weak overdensities are seen on small (< ~0.5 Mpc radius) scales when counting peaks in the S2COSMOS SNR map. However our overall results suggest that massive radio-quiet galaxies do not reside in significant SMG overdensities and thus occupy less massive dark matter halos than their radio-loud counterparts. This implies that strong radio emission is intrinsically linked with the density of the surrounding environment. Overall we find that SMGs are good candidates for being a progenitor phase in the formation of local massive early-type galaxies, but massive, high-redshift radio-quiet galaxies likely occupy regions of lower SMG density.