Scholarship & Research
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Item The interplay between the central engine and the circumnuclear environment in Compton-thin AGN(Montana State University - Bozeman, College of Letters & Science, 2022) Chalise, Sulov; Chairperson, Graduate Committee: Anne Lohfink; This is a manuscript style paper that includes co-authored chapters.All massive galaxies harbor a supermassive black hole (SMBH) at their galactic center. If these SMBH are actively feeding then they are called Active galactic nuclei (AGN). Their accretion system contains a corona, an accretion disk and an axisymmetric dusty torus. The torus can be connected physically and dynamically to the circumnuclear disk of the galaxy which acts as a molecular gas reservoir for material to be accreted onto the SMBH. Further, AGN can emit radiation from radio up to the gamma rays. The AGN accretion disk emits photons mostly in the optical/UV band which are Compton up-scattered in the corona to generate X-rays. If present, a jet can produce additional high-energy and Synchrotron emission. In some AGN, a huge amount of material can be stripped away from the accretion disk creating an outowing wind. These --radiation pressure, jet, wind etc.--inject energy back into the host galaxy, regulating the SMBH growth. There exist a complex interplay between the AGN feeding and feedback. Understanding this interaction between the central engine and its circumnuclear environment is vital in context of galaxy evolution. My work aims to study this interaction in low to moderately obscured (or Compton-thin) AGN using their broadband multi-epoch X-ray spectra plus other emission bands whenever appropriate. From the spectral analysis of broad-line radio galaxy 3C 109, I was able to constrain its high-energy cutoff for the first time. In another Seyfert galaxy Mrk 926, I was able to explore the origin of its soft excess, and found that a warm coronal origin was slightly preferred. Finally, I performed a joint multi-wavelength analysis with a physical torus model of a sample of Polar-scattered Seyfert 1 galaxies. I utilized their multi-epoch broadband X-ray spectra along with their infrared spectral/photometric data, and was able to constrain their torus properties. Despite being a sample of similar moderately-inclined Compton-thin AGN, I found a complex and varied distribution of gas and dust in their torus.Item A multi-wavelength study of dwarf galaxies with active massive black holes(Montana State University - Bozeman, College of Letters & Science, 2023) Kimbrell, Seth Jordan; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.Dwarf galaxies which host massive black holes with M less than or equivalent to 106M circled dot give us an opportunity to better understand the formation mechanism behind the supermassive black holes that live in the center of galaxies. Studying how common massive black holes in dwarfs are is an important step in constraining the channels that led to those supermassive black holes. An important part of that study is understanding in what types of dwarf galaxies we can expect to find massive black holes. I present a multi-wavelength study of dwarf galaxies which attempts to find any trends in the morphologies of the hosts of active massive black holes. I begin by modeling the structures of a sample of galaxies which have been identified as black hole hosts; I then perform an identical modeling on a sample of galaxies which show no signs of hosting a massive black hole. I finish by describing an X-ray search for massive black holes among irregular/disturbed galaxies, including the discovery of a very bright X-ray source which is extremely likely to be a massive black hole in a dwarf-dwarf merger. This is one of the first active massive black holes discovered in such a late-stage merger, and it is also notable for radiating at nearly its upper limit.Item A deeper look into X-ray-selected AGN candidates in dwarf galaxies with Chandra(Montana State University - Bozeman, College of Letters & Science, 2023) Sanchez, Adonis Arismendy; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.The ability to accurately discern active massive black holes (BHs) in local dwarf galaxies is paramount to understanding the origins and processes of "seed" BHs in the early universe. We present Chandra X-ray Observatory observations of a sample of three dwarf galaxies (M* < or = 3 x 109 M circled dot, z < or = 0.15) pre-selected by Latimer et al. (2021a) as candidates for hosting active galactic nuclei (AGN). The galaxies were selected from the NASA-Sloan Atlas (NSA) with spatially coincident X-ray detections in the eROSITA Final Equatorial Depth Survey (eFEDS). Our new Chandra data reveal X-ray point sources in two of the three galaxies with luminosities of log(L 2-10keV/[erg s -1]) = 39.1 and 40.4. In the target galaxy with the nondetection, we calculate an upper limit on the luminosity for a potential source. We observed notably higher fluxes and luminosities from the two detected X-ray sources compared to their original eFEDS observations, pointing to possible X-ray variability on the scale of a few years. We plot and fit the spectra of the X-ray sources with a power-law model, finding the likely presence of intrinsic absorption. The X-ray luminosities are above that expected from XRBs, but we cannot definitively rule out stellar-mass compact objects with the data on hand. Assuming the X-ray sources are accreting massive BHs with masses that scale with the stellar mass of the host galaxies, the Eddington ratios are on the order of a few x 10 -3.Item Hundreds of Low-mass Active Galaxies in the Galaxy And Mass Assembly (GAMA) Survey(American Astronomical Society, 2022-09) Salehirad, Sheyda; Reines, Amy E.; Molina, MalloryWe present an entirely new sample of 388 low-mass galaxies (M ⋆ ≤ 1010 M ⊙) that have spectroscopic signatures indicating the presence of massive black holes (BHs) in the form of active galactic nuclei (AGNs) or tidal disruption events. Of these, 70 have stellar masses in the dwarf galaxy regime with 108 ≲ M ⋆/M ⊙ ≲ 109.5. We identify the active galaxies by analyzing optical spectra of a parent sample of ∼23,000 low-mass emission-line galaxies in the Galaxy and Mass Assembly (GAMA) Survey Data Release 4, and employing four different diagnostics based on narrow emission-line ratios and the detection of high-ionization coronal lines. We find that 47 of the 388 low-mass active galaxies exhibit broad Hα in their spectra, corresponding to virial BH masses in the range M BH ∼ 105.0–7.7 M ⊙ with a median BH mass of 〈M BH〉 ∼ 106.2 M ⊙. Our sample extends to higher redshifts (z ≤ 0.3; 〈z〉 = 0.13) than previous samples of AGNs in low-mass/dwarf galaxies based on Sloan Digital Sky Survey spectroscopy, which can be attributed to the spectroscopic limit of GAMA being ∼2 mag deeper. Moreover, our multi-diagnostic approach has revealed low-mass active galaxies spanning a wide range of properties, from blue star-forming dwarfs to luminous “miniquasars” powered by low-mass BHs. As such, this work has implications for BH seeding and AGN feedback at low masses.Item Black hole scaling relations and feedback in dwarf galaxies(Montana State University - Bozeman, College of Letters & Science, 2022) Schutte, Zachary Willis; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.Over the past decade the discovery of a population of dwarf galaxies which host massive black holes (BH) has prompted the study of how these systems interact, grow and evolve. To address these questions I first present a new relationship between central black hole mass and host galaxy stellar bulge mass extending to the lowest BH masses known in dwarf galaxies (MBH < or ~ 10 5 M circle dot, M* ~ 10 9 M circle dot). I have obtained Hubble Space Telescope imaging of seven dwarf galaxies with optically selected broad-line active galactic nuclei (AGNs) and BH mass estimates from single-epoch spectroscopy. I perform modeling to decompose the structure of these galaxies and find that the majority have an inner bulge/pseudo-bulge component with an exponential disk that dominates the total stellar mass. Using the modeling results, I determine the stellar mass of each photometric component in each galaxy. I determine the M BH - M bulge relation using a total of 12 dwarf galaxies hosting broad-line AGNs, along with a comparison sample of 125 higher-mass galaxies. I find a strong correlation between BH mass and bulge mass which is in good agreement with correlations found previously when only considering higher-mass systems. Second, I investigated the role of AGN feedback in dwarf galaxies by studying Henize 2-10, a dwarf starburst galaxy previously reported to have a central massive black hole. At a distance of ~9 Mpc, it presents an opportunity to resolve the central region and determine if there is evidence for a black hole outflow impacting star formation. I found a ~150 pc long ionized filament connecting the region of the black hole with a site of recent star formation. Spectroscopy reveals a sinusoid-like position-velocity structure that is well described by a simple precessing bipolar outflow. I conclude that this black hole outflow triggered the star formation. The results from the work presented in this thesis show that the coevolution of massive BHs and their host galaxies extends into the lowest mass regime and that AGN feedback plays an important and complex role in dwarf galaxies.Item Searching for signatures of accreting massive black holes in dwarf galaxies(Montana State University - Bozeman, College of Letters & Science, 2022) Latimer, Lilikoi Jean; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.Identifying and analyzing massive black holes (BHs) in dwarf galaxies can advance our understanding of the formation and development of supermassive BHs. Searching for these objects can prove troublesome, however. Massive BHs, with masses of M BH < or ~ 10 6 M circled dot, are smaller than their supermassive cousins (M BH ? 10 6-10 9 M circled dot), and the dwarf galaxies they reside in can yield very different environments than what one might find in a more standard massive galaxy, resulting in a different set of challenges than searching out active galactic nuclei (AGN) powered by supermassive BHs. Here we describe several methods for both identifying AGN candidates powered by massive BHs and searching for evidence to confirm the presence or absence of AGN candidates in dwarf galaxies. For the latter, we use different combinations of radio, X-ray, and mid-IR observations to search for AGN signatures, elucidating the various benefits and difficulties that lie with each one, and reporting the results when applied to our varying samples. For the former, we analyze the efficacy of a mid-IR color-color AGN selection method when applied to dwarf galaxies instead of massive galaxies, and use X-ray observations from a newly-released catalog to search for new AGN candidates in dwarf galaxies.Item X-ray analysis and modeling of NGC 3227(Montana State University - Bozeman, College of Letters & Science, 2022) Newman, Jared James; Chairperson, Graduate Committee: Yves U. Idzerda and Sachiko Tsuruta (co-chair)The 1.5 Seyfert galaxy NGC 3227 has been observed by several X-ray missions. We carried out combined analysis of the data from a total of twenty one observations. Six of the observations were preformed by Suzaku, eight by XMM-Newton, and seven by NuStar. A unified model was constructed which is consistent with all twenty one of the observations by the three satellites with large intensity and spectral changes. The model consists of a hard power law which is interpreted as the Comptonized emission from the corona above an accretion disk. In the high flux states an additional soft excess component dominates, which is consistent with a model with either a steeper power law or a warm Comptonization component. These emissions from the central engine are absorbed by multiple distinct absorbers at various ionization levels. A reflection component and several emission lines are also present. This is the most robust model of this source to date.Item Theoretical and observational studies of the central engines of active galactic nuclei(Montana State University - Bozeman, College of Letters & Science, 1995) Sivron, RanItem X-ray observations and theoretical modeling of Active Galactic Nuclei(Montana State University - Bozeman, College of Letters & Science, 1997) Kellen, Michael JamesItem Relativistic accretion flows onto supermassive black holes : shock formation and iron fluorescent emission lines in active galactic nuclei(Montana State University - Bozeman, College of Letters & Science, 2005) Fukumura, Keigo; Chairperson, Graduate Committee: Sachiko TsurutaOne of the exciting discoveries from the recent X-ray spectroscopic studies of active galactic nuclei (AGNs) is the so called βrelativistically-broadened iron fluorescent emission lineγ often detected in the hard X-ray spectra. It is generally believed to originate from the inner part of the accretion disk surrounding a supermassive black hole (BH) at the center. Although we have begun to obtain some physical insight regarding such emission lines supported by theoretical models (e.g., disk-corona model), exactly how and where the observed fluorescence may take place is still disputable. Here, an X-ray data with XMM-Newton Observatory of a typical narrow-line Seyfert 1 galaxy, NGC 4051, is analyzed based on a partial covering model to consistently explain the observed time-resolved temporal/spectral variations. This model implies that the intrinsic emission varies significantly in the presence of the covering cloud. We often detect a hard X-ray continuum originating from a hot region close to the central engines of AGNs. As a promising X-ray source candidate, relativistic hydrodynamic (HD) shocks are investigated systematically and then extended to the magnetohydrodynamic (MHD) shocks, given the widely accepted suggestion that the presence of the magnetic fields could play an important role in the accreting flows. I show that both HD and MHD shocks can form in the vicinity of the BH, perhaps responsible for creating such a hightemperature region where hard X-rays are produced. Particularly in the MHD shocked plasma, the hydro/magneto-dominated states are found. Considering the effect of such magnetic fields in the accretion disk, I calculate nonstandard iron fluorescent line profiles in the presence of spiral density waves and find multiple sharp sub-peak structures in extremely skewed line profiles, which will be detectable with upcoming X-ray satellites such as Astro-E2 XRS for testing the model. This dissertation is the result of my own work and also includes some work done in collaboration. Parts of this dissertation have been either already published in or submitted to the Astrophysical Journal and presented at conferences, while some are still in progress.