Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level





Bret Lehmer

Committee Member

Daniel Kennefick

Second Committee Member

Woodrow Shew


Active Galactic Nuclei, Galaxy Clusters, Galaxy Evolution, Galaxy Protoclusters


In this dissertation I present work done from 2018-2023 to investigate the growth of galaxies and supermassive black holes (SMBH) in high redshift overdensities (protoclusters) by studying the star-forming galaxy and active galactic nucleus (AGN) population in the SSA22 protocluster. I examined possible environmental sources of the enhanced star formation and AGN activity in the z = 3.09 SSA22 protocluster using Hubble WFC3 ∼ 1.6 μm observations of the SSA22 field, including new observations centered on eight X-ray selected protocluster AGN. To investigate the role of mergers in the observed AGN and star formation enhancement, quantitative and visual morphological classifications were applied to F160W images of protocluster Lyman break galaxies (LBGs) in the fields of the AGN and z ∼ 3 field LBGs in SSA22 and the GOODS-N field. No significant differences are found between the morphologies and merger fractions of protocluster and field LBGs, though this conclusion is limited by small number statistics in the protocluster. The UV-to-near-IR spectral energy distributions (SED) of F160W-detected protocluster and field LBGs were fit to characterize their stellar masses and star formation histories (SFH). These fits suggest that the mean protocluster LBG is a factor of ∼ 2 times more massive and more attenuated than the mean z ∼ 3 field LBG. These results suggest that ongoing mergers are no more common among protocluster LBGs than field LBGs, though protocluster LBGs appear to be more massive. The larger mass of the protocluster LBGs may contribute to the enhancement of SMBH mass and accretion rate in the protocluster, in turn driving the protocluster AGN enhancement. I also analyzed the physical properties of 8 X-ray selected AGN and one candidate proto- quasar system (ADF22A1) in the SSA22 protocluster by fitting their X-ray-to-IR SEDs. SFHs were recovered for 7 of these systems which are well-fit by stellar population and AGN models. The majority of the protocluster AGN systems, including ADF22A1, are consistent with obscured SMBH growth in star forming galaxies. The SFRs, SMBH accretion rates, and masses estimated from the SED fits for the 9 protocluster AGN systems are consistent with the distributions of SFR, accretion rate, SMBH masses, and stellar masses for a sample of X-ray detected AGN candidates in the Chandra Deep Fields (CDF). The ratio between the sample-averaged SSA22 SMBH mass and CDF SMBH mass is constrained to < 1.41. Furthermore, no statistically significant trends between the AGN or host galaxy properties and their location in the protocluster are found. The similarity of the protocluster and field AGN populations suggests that the observed protocluster AGN fraction enhancement may be driven by the SMBH mass distribution of the entire galaxy population, rather than only the AGN population, consistent with the interpretation of the results of the WFC3 F160W study of LBGs. The results presented here add to the existing body of knowledge on the growth of galaxies and SMBHs in protoclusters, suggesting that galaxies and their black holes grow by the same mechanisms in protoclusters and field environments, and that the observed AGN enhancement is driven instead by more frequent, secular, triggering of AGN episodes in star forming galaxies.