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A Search for the Smallest Supermassive Black Holes

Ghosh, Himel

Abstract Details

2009, Doctor of Philosophy, Ohio State University, Astronomy.
Relations between supermassive black holes (SMBHs) and their hostgalaxies are now well known, but several questions remain: Do all galaxies harbor SMBHs? Do correlations between BH mass and host galaxy properties extend to lower mass BHs and all galaxy types? Is the galactic bulge or the dark matter halo the defining component for the nuclear BH? Answering these questions requires a study of low mass SMBHs, in particular those that reside in the latest-type spiral galaxies. While the presence of an active galactic nucleus (AGN) provides certain proof of the existence of an SMBH, galaxies that are not previously known to host AGNs may nevertheless have SMBHs at their centers. In other words, a galaxy may appear quiescent when in reality there is an accreting SMBH, if the accretion level is low enough. This thesis presents a search for such SMBHs by looking for the presence of low-level nuclear activity, as evidenced primarily by their x-ray emission, in a well-defined sample of nearby, optically quiescent spiral galaxies. This work demonstrates that traditional methods of identifying AGNs, developed over the course of studying luminous (> 1042 erg/s) AGNs, are inadequate for the population of low luminosity AGNs found in nearby galaxies, and develops the techniques that must be used instead. These techniques are then applied to an x-ray survey of nearby, face-on spiral galaxies. The survey includes new snapshot observations of 37 galaxies, which are combined with archival data for a further 18 galaxies. Where available, multi-wavelength data are used to help distinguish AGNs from other types of x-ray sources. These observations show the power of x-ray observations in detecting hidden AGNs, and also address the question of the prevalence of SMBHs in spiral galaxies that do not have bulges. This study has uncovered 14 previously unknown AGNs and strong AGN candidates, including two in galaxies of type Sd and Sdm. If the latter are confirmed as bona fide AGNs they will be only the fourth and fifth AGNs known to exist in bulgeless galaxies.
Smita Mathur, PhD (Advisor)
Paul Martini, PhD (Committee Member)
Andrew Gould, PhD (Committee Member)
170 p.

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Citations

  • Ghosh, H. (2009). A Search for the Smallest Supermassive Black Holes [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1253492291

    APA Style (7th edition)

  • Ghosh, Himel. A Search for the Smallest Supermassive Black Holes. 2009. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1253492291.

    MLA Style (8th edition)

  • Ghosh, Himel. "A Search for the Smallest Supermassive Black Holes." Doctoral dissertation, Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1253492291

    Chicago Manual of Style (17th edition)