Dual active galactic nuclei within Romulus25
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Full Text E-thesis
Date
2025-09-30
Authors
Sheehan, Ben
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Publisher
University College Cork
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Abstract
Dual active galactic nuclei (DAGN) trace the growth of supermassive black holes (SMBHs) during galaxy interactions, while also being an important electromagnetic precursor of SMBH mergers which are important for missions such as LISA. In this thesis, we aim to study the properties of DAGN within the Romulus25 simulation, which combines sub-kpc resolution with physically motivated prescriptions for SMBH seeds, dynamics, and accretion. We find that many observed DAGNs may merge prior to a Hubble time, making them incredibly important to the LISA mission. DAGNs preferentially occupy the most massive galaxies at all redshifts and are comprised of mostly low mass ratio SMBH pairs. We also investigate if DAGN activity is a good tracer of galaxy merger enhanced star formation or black hole accretion, and find that they only serve as a tracer at redshift greater than 3. We further find DAGNs in massive galaxies display a universal distribution of duty cycles regardless of the epoch which the DAGN underwent its active phase, and that the subpopulation of DAGNs that merge prior to a Hubble time tend to inhabit the larger end of this duty cycle distribution. Important for informing current DAGN observations, we provide estimates on the time since galaxy merger and time until SMBH merger for DAGNs, and find that on average most observed systems are rarely in post-merger galaxies and are > 1 Gyr away from their SMBHs merging. We also identify a novel in-situ formation channel for DAGNs that does not require any galaxy merger, which is afforded to the Romulus25 simulation due to its unique SMBH seeding prescription, and quantify how much this channel effects the results. These results provide insights into many fronts of research into DAGNs, including providing insights into the temporal connection between DAGN activity and the galaxy mergers that formed them and the SMBH mergers they may undergo, whilst also guiding future multi-messenger astrophysics missions which focus on DAGNs, and providing a general overview on the demographics and origins of DAGNs.
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Keywords
Dual active galactic nuclei , Supermassive black hole evolution , Supermassive black hole physics
Citation
Sheehan, B. 2025. Dual active galactic nuclei within Romulus25. MRes Thesis, University College Cork.