Physics and Astronomy

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https://hdl.handle.net/10315/28582

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Open Access
Investigation into the Spatial Distribution of AGN Companion Galaxies in Gravitationally Isolated Environments
(2018-11-21) Sivasubramanium, Janakan; Robertis, Michael De
Active galaxies are an important subclass of galaxies, distinguished by an energetic core driven by enhanced accretion onto a central supermassive black hole. However, this accretion mechanism inevitably opens up a problem: what source provides the gaseous fuel for black hole accretion? In this research project, we examine the possibility that these active galaxies have engaged in some form of galactic cannibalism of their neighbouring galaxies to acquire a fuel supply to power their cores. By using data from the Sloan Digital Sky Survey (SDSS), we conduct an environmental survey around active/non-active galaxies and map out the spatial distribution of their companions. Our results show that, in gravitationally isolated environments, the local environment (less than 0.5 Mpc) around active galaxies are seen to have a scarcity of neighbouring galaxies relative to the non-active control sample a possible indication of a history of mergers and consumptions.
Open Access
Monitoring Broad Absorption-Line Quasar Variability
(2016-09-20) Rogerson, Jesse Aaron; Hall, Patrick B.
Winds generated by an accreting super massive black hole may provide feedback to the host galaxy and offer an explanation for the co-evolution of galaxies with their super massive black holes that has been reported in the literature. Some outflows are manifested as broad absorption line (BAL) troughs in quasar spectra, and are measured at velocities as high as 60, 000 km s 1 at ultra-violet wavelengths. These BAL troughs have been observed to vary on both long (years) and short (weeks) rest-frame time-scales and can emerge in a quasar that had none, or disappear completely. By monitoring the variability of absorption in BAL quasars, constraints can be placed on outflow models and the structure of quasars in general. In this study, we isolate a set of quasars that exhibit emergent C iv BALs in their spectra, by comparing archival data in the SDSS Data Release 7 to the BOSS Data Release 9 and 10. After visually defining a set of emergent BALs, follow-up observations were obtained with the Gemini Observatory for 105 quasars. BALs were formally detected in all but two of the quasars in the dataset, and we report 219 absorption complexes in the entire set. After a BAL has emerged, we find it is equally likely to continue increasing as it is to start decreasing in a subsequent observation. Based on the range of time between our observations, this indicates the coherence time-scale of BALs is less than 100 days. There is a strong signal of coordinated variability among two troughs in the same quasar. Further, coordination is stronger if the velocity separation between the two troughs is smaller. We conclude the variability is likely due to changes in the ionizing flux incident on the absorbing cloud, which agrees with the results of Filiz Ak et al. (2013). In this work we also test two competing models of BAL variability (bulk motion and ionization changes) in the context of a case study of the quasar SDSS J023011.28+005913.6, which had two high-velocity emergent troughs. Both models yield plausible results.
Open Access
Self-Similar Models of Quasar Outflow Shock Structures
(2024-03-16) Weiss, Erik Adam; Hall, Patrick B.
Supersonic winds of outflowing material are observed and/or predicted in a number of astrophysical systems; such winds may drive an expanding shock structure that shocks and pushes the ambient interstellar medium (ISM) outward. This thesis analyses, combines and expands on existing analytic self-similar models of these structures in the context of quasar accretion disk winds; we then search for the bulk acceleration (positive or negative) of low-velocity Ca II in the quasar SDSS J030000.0+004828.0 (J0300) and compare our results to model predictions. We find a strong upper limit on the acceleration magnitude, yielding plausible model constraints, but the observed range of gas velocities implies that the Ca II velocity profile does not coincide with that of the shocked ISM. We conclude with a discussion of recent results regarding J0300's outflow properties and briefly investigate the possibility of pre-existing ISM cloud disruption as an explanation for the observed Ca II velocity profile.
Open Access
Stars Versus Quasar Accretion Disks
(2023-08-04) Seaton, Lucas Matthew; Hall, Patrick B.
This thesis investigates the astrophysical effects of stars colliding with the accretion disks of supermassive black holes (SMBH) in active galactic nuclei (AGN). In this work, we model the AGN to have a thin accretion disk surrounded by a stellar nuclear cluster (SNC) containing stars on various orbital eccentricities and inclination angles striking the disk at periapse. An impacting star carves a tilted cylindrical tunnel out of the disk material and loses up to $6\times10^{-4}\%$ of its stellar mass. The time-averaged collisional luminosity is at most $10^{-2}$ times the entire disk's expected luminosity, while the emission profile of an impact site initially flares before quickly dimming to the brightness of the local disk. The parameters of the SMBH, accretion disk, SNC, and impacting stars are altered to reveal that star-disk collisions from a densely populated SNC can outshine the disk luminosity.

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