Constraining the coevolution of galaxies and black holes at low mass

Recent detections of massive black holes (BHs), with masses of roughly 106--109 M⊙ , in both our own Milky Way and in other galaxies suggest that central galactic BHs are common. In this work, we attempt to shed some light on the relatively new population of local intermediate-mass BHs (103--10 6 M⊙ ) that reside at the low-mass end of the central BH mass distribution. This population is important because it represents the closest analogue of primordial galactic BHs, not far removed from the first "seed" BHs. Understanding the characteristics of this population will help constrain any seed BH formation model.;We begin in Chapter 2 by looking at the properties of elliptical galaxies, nearly all of which likely host a massive BH. Although elliptical galaxies were thought to be a very homogeneous class, whose properties scaled simply with increasing mass, we show with the large sample available from the Sloan Digital Sky Survey (SDSS) that important deviations exist between low-mass and high-mass elliptical galaxies. Such differences arise due to slightly varied formation histories, predominantly involving the fraction of baryonic mass in gas (as opposed to stars) during merger events that build up elliptical galaxies. The gas fraction, higher in low-mass ellipticals, alters the structure of the remnant elliptical because gas is collisional and dissipative, whereas stars are dissipationless. Carefully understanding these host-galaxy properties will be key in future studies.;In Chapter 3, we present multiwavelength observations of NGC 4395, the least luminous Seyfert AGN known. NGC 4395 is special for two reasons: (a) the AGN resides in an essentially bulgeless host spiral, challenging the deep connection that is prevalent between massive BHs and host bulges; and (b) the central BH is thought to have a mass of only ∼105 M⊙ , placing it squarely in the intermediate-mass BH regime. We present results from reverberation mapping, a technique that is used to estimate the mass of the central BH (MBH). We estimate MBH ≈ 3 x 105 M⊙ , consistent with another reverberation result based on simultaneous UV observations.;In Chapter 4, we present an archival study of Chandra X-ray Telescope data, in which we search for AGN signatures in a population of bulgeless or nearly-bulgeless late-type spiral galaxies. Motivated by the case of NGC 4395, it is unclear how many BHs exist in bulgeless systems. Optical studies are notoriously poorly suited for this task, since BHs in such late-type spirals are likely of low mass (since the host galaxy is equally of low mass) and thus have very weak AGN signatures. In addition, late-type spirals have relatively large amounts of circumnuclear gas and dust, which easily obscures optical radiation. X-ray photons are largely unaffected (excluding exceptionally large columns of gas) and are therefore a more reliable indicator of central AGN activity (even weak, radiatively inefficient accretion). Unfortunately, stellar-mass BH binaries are equally capable of producing such X-ray emission, and we lack the depth, spacial resolution, and sensitivity to make a definitive identification. Based on nuclear stellar densities and the probability of finding an X-ray binary, we still conclude that roughly ∼20--25% of bulgeless galaxies host an AGN, implying that objects such as NGC 4395 are not as rare as once thought.;Finally, in Chapter 5, we present a comprehensive X-ray study of a large sample of intermediate-mass BHs discovered in SDSS via optical identification of broad emission lines. Such BHs, while sampling a mass regime that is poorly understood, are biased toward high accretion rates to make them optically identifiable. Interestingly, such high-accretion, low-mass BHs behave in many ways like narrow-line Seyfert 1 nuclei (a higher-mass cousin), except that in general the low-mass sample is X-ray bright. This result is surprising, and we suggest it may arise from a slim disk, an accretion-disk structure which differs from the usual geometrically thin and optically thick disk normally associated with high accretion rates. Slim disks are thought to arise in systems with Eddington or super-Eddington accretion. Although the precise physical explanation is still elusive, it is surprising that optically selected AGNs should exhibit such differing broader spectral energy distributions. (Abstract shortened by UMI.)...