Author ORCID Identifier

https://orcid.org/0000-0003-1551-1340

Semester

Summer

Date of Graduation

2024

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Sarah Burke-Spolaor

Committee Co-Chair

Loren Anderson

Committee Member

Loren Anderson

Committee Member

Julie Comerford

Committee Member

Emmanuel Fonseca

Abstract

Massive galaxy mergers are a fundamental consequence of the dynamic evolution of the Universe and play a central role in the evolution of galaxies. Because all massive galaxies harbor a central supermassive black hole (SMBH; M ≥ 106 M⊙), studying these merging systems with electromagnetic (EM) techniques constrains galaxy evolution models and provides a systematic means to examine the astrophysical mechanisms that facilitate the growth of SMBHs. These growing SMBHs are observable across the EM spectrum as Active Galactic Nuclei (AGN). Massive galaxy mergers are a natural formation mechanism for an SMBH pair, eventually evolving into an SMBH binary (SMBHB). As the sources of low-frequency gravitational waves detectable by pulsar timing arrays, robust EM identification of a population of SMBH pairs and binaries will greatly enhance the discoveries in the next age of multi-messenger astronomy. In this dissertation, I explore the radio population of post-merger galaxies using multi-waveband techniques and directly target late-stage SMBH pair evolution through investigations of their parsec-scale radio environment.

First, I explore the radio-emitting population of post-merger galaxies by synthesizing new, high-resolution Jansky Very Large Array observations with archival radio surveys. Spanning five orders of magnitude in spatial scale and two decades in radio frequency, I discover that the majority of post-mergers in this sample host low-luminosity, compact (< 1 kpc), steep-spectrum radio sources. Then, using multi-waveband techniques, I identify which of these sources is associated with an AGN and interpret this population with regard to AGN triggering mechanisms and feedback efficiency. I also predict the power of next-generation radio instruments in SMBHB studies using these radio AGN as a meta-sample.

Then, I directly investigate SMBHB evolution in a subset of these post-mergers by carrying out a targeted Very Long Baseline Array (VLBA) survey, totaling over 1300 hours of observations. Multi-frequency intensity maps of the milliarcsecond-scale radio emission reveal no secondary radio-emitting companion in any of these post-mergers. I then present compelling relative astrometric and morphological arguments that support a hypothesis in which SMBH pair evolution is highly efficient and that at least some SMBHBs can attain sub-pc orbital separations in astrophysical environments, overcoming the final parsec problem and entering the gravitational-wave regime. This work represents the most sensitive high-resolution survey to date targeting SMBHB evolutionary signatures, attaining μJy-level precision in our VLBA intensity maps and probing radio emission signatures down to L = 1020 W Hz-1. This survey illustrates the intensive observing campaign required to accurately assess the radio-emitting population of SMBHBs.

Lastly, I present the results of a targeted VLBA analysis to test the recoiling SMBH (rSMBH) hypothesis for the changing-look AGN Mrk 1018. rSMBHs are post-SMBHB coalescence systems that have so far remained observationally elusive. Through advanced astrometric calibration techniques, I place strict constraints on proper motion measurements of the milliarcsecond radio core, discovering low-significance proper motion in Right Ascension. Then, I interpret this result within the context of three distinct models: the secular evolution of a radio AGN; the orbital evolution of a SMBHB; or the kinematic signature of a rSMBH. When considering the apparent velocity of the dominant kinematic component of Mrk 1018, I conclude that this must be associated with an outflowing sub-component of the VLBI radio jet, although this does not rule out the rSMBH scenario. I then summarize future observational experiments that may be able to distinguish between the secular evolution of a radio AGN and the kinematics of a rSMBH.

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