A ~50,000 solar mass black hole in the nucleus of RGG 118

Scaling relations between black hole (BH) masses and their host galaxy properties have been studied extensively over the last two decades, and point towards co-evolution of central massive BHs and their hosts. However, these relations remain poorly constrained for BH masses below $\sim10^{6}$ M_sun. Here we present optical and X-ray observations of the dwarf galaxy RGG 118 taken with the Magellan Echellette Spectrograph on the 6.5m Clay Telescope and Chandra X-ray Observatory. Based on Sloan Digital Sky Survey spectroscopy, RGG 118 was identified as possessing narrow emission line ratios indicative of photoionization partly due to an active galactic nucleus. Our higher resolution spectroscopy clearly reveals broad H$\alpha$ emission in the spectrum of RGG 118. Using virial BH mass estimate techniques, we calculate a BH mass of $\sim50,000$ \msun. We detect a nuclear X-ray point source in RGG 118, suggesting a total accretion powered luminosity of $L=4\times10^{40}~{\rm erg~s^{-1}}$, and an Eddington fraction of $\sim1$ per cent. The BH in RGG 118 is the smallest ever reported in a galaxy nucleus and we find that it lies on the extrapolation of the $M_{\rm BH}-\sigma_{\ast}$ relation to the lowest masses yet.Comment: Accepted to ApJL. 6 pages, 4 figure

Active Galactic Nuclei in Dwarf Galaxies

Supermassive Black Holes (BHs; typically with masses greater than 10^5 solar masses) are ubiquitous in the cores of all massive galaxies (stellar masses greater than 10^10 solar masses). However, BHs in dwarf galaxies (stellar masses less than 10^9.5 solar masses) are more difficult to detect, and thus have remained relatively elusive. Theoretical work suggests that clues to BH formation and growth can be found in present day dwarf galaxies, making this an important population to study. Searching for signs of BH accretion is one way of identifying BHs in dwarf galaxies more distant than the Local Group. Until recently, only a handful of dwarf galaxies had been identified with actively accreting central BHs, or Active Galactic Nuclei (AGNs). However, with the advent of large-scale surveys, it has been possible to search for signs of BH accretion in samples comprised of tens of thousands of galaxies. Recent works have identified roughly two hundred dwarf galaxies with AGN signatures, making it possible to begin to carry out demographic studies. This thesis describes an in-depth, multi-wavelength characterization of this relatively unexplored population. We determine which observational AGN identifiers used for massive galaxies also apply for dwarf galaxies, study the accretion properties of confirmed AGNs, and explore relationships between BH mass and host galaxy properties in these systems. We also present evidence for an active ~50, 000 solar mass BH in the center of the dwarf galaxy RGG 118, which at the time of writing, is the smallest BH yet reported in a galaxy center.PhDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/138472/1/vbaldas_1.pd

Hubble Space Telescope Imaging of the Active Dwarf Galaxy RGG 118

RGG 118 (SDSS 1523+1145) is a nearby ($z=0.0243$), dwarf disk galaxy ($M_{\ast}\approx2\times10^{9} M_{\odot}$) found to host an active $\sim50,000$ solar mass black hole at its core (Baldassare et al. 2015). RGG 118 is one of a growing collective sample of dwarf galaxies known to contain active galactic nuclei -- a group which, until recently, contained only a handful of objects. Here, we report on new \textit{Hubble Space Telescope} Wide Field Camera 3 UVIS and IR imaging of RGG 118, with the main goal of analyzing its structure. Using 2-D parametric modeling, we find that the morphology of RGG 118 is best described by an outer spiral disk, inner component consistent with a pseudobulge, and central PSF. The luminosity of the PSF is consistent with the central point source being dominated by the AGN. We measure the luminosity and mass of the "pseudobulge" and confirm that the central black hole in RGG 118 is under-massive with respect to the $M_{BH}-M_{\rm bulge}$ and $M_{BH}-L_{\rm bulge}$ relations. This result is consistent with a picture in which black holes in disk-dominated galaxies grow primarily through secular processes.Comment: Accepted to Astrophysical Journal. 11 pages, 8 figure

X-ray Analysis of AGN from the GALEX Time Domain Survey

We analyze the X-ray properties for a sample of 23 high probability AGN candidates with ultraviolet variability identified in Wasleske et al. (2022). Using data from the Chandra X-ray Observatory and the XMM-Newton Observatory, we find 11/23 nuclei are X-ray detected. We use SED modeling to compute star formation rates and show that the X-ray luminosities are typically in excess of the X-ray emission expected from star formation by at least an order of magnitude. Interestingly, this sample shows a diversity of optical spectroscopic properties. We explore possible reasons for why some objects lack optical spectroscopic signatures of black hole activity while still being UV variable and X-ray bright. We find that host galaxy stellar emission and obscuration from gas and dust are all potential factors. We study where this sample falls on relationships such as $\alpha_{\rm OX}-L_{2500}$ and $L_{X}-L_{IR}$ and find that some of the sample falls outside the typical scatter for these relations, indicating they differ from the standard quasar population. With the diversity of optical spectroscopic signatures and varying impacts of dust and stellar emissions on our sample, these results emphasizes the strength of variability in selecting the most complete set of AGN, regardless of other host galaxy properties.Comment: 19 pages, 10 figures. Accepted to The Astronomical Journa

X-ray and Ultraviolet Properties of AGN in Nearby Dwarf Galaxies

We present new Chandra X-ray Observatory and Hubble Space Telescope observations of eight optically selected broad-line AGN candidates in nearby dwarf galaxies ($z<0.055$). Including archival Chandra observations of three additional sources, our sample contains all ten galaxies from Reines et al. (2013) with both broad H$\alpha$ emission and narrow-line AGN ratios (6 AGNs, 4 Composites), as well as one low-metallicity dwarf galaxy with broad H$\alpha$ and narrow-line ratios characteristic of star formation. All eleven galaxies are detected in X-rays. Nuclear X-ray luminosities range from $L_{0.5-7 \rm{keV}}\approx5\times10^{39}$ to $1\times10^{42}$ $\rm{erg}\rm{s^{-1}}$. In all cases except for the star forming galaxy, the nuclear X-ray luminosities are significantly higher than would be expected from X-ray binaries, providing strong confirmation that AGN and composite dwarf galaxies do indeed host actively accreting BHs. Using our estimated BH masses (which range from $\sim7\times10^{4}-1\times10^{6}~M_{\odot}$), we find inferred Eddington fractions ranging from $\sim0.1-50\%$, i.e. comparable to massive broad-line quasars at higher redshift. We use the HST imaging to determine the ratio of ultraviolet to X-ray emission for these AGN, finding that they appear to be less X-ray luminous with respect to their UV emission than more massive quasars (i.e. $\alpha_{\rm OX}$ values an average of 0.36 lower than expected based on the relation between $\alpha_{\rm OX}$ and $2500{\rm \AA}$ luminosity). Finally, we discuss our results in the context of different accretion models onto nuclear BHs.Comment: 15 pages, 15 figures, 4 tables. Submitted to Ap

Intermediate-Mass Black Holes in Star Clusters and Dwarf Galaxies

Black holes (BHs) with masses between 100 to 100,000 times the mass of the Sun ($\rm{M}_{\odot}$) are classified as intermediate-mass black holes (IMBHs), potentially representing a crucial link between stellar-mass and supermassive BHs. Stellar-mass BHs are endpoints of the evolution of stars initially more massive than roughly 20 $\rm{M}_{\odot}$ and generally weigh about 10 to 100 $\rm{M}_{\odot}$. Supermassive BHs are found in the centre of many galaxies and weigh between $10^{6}$ to $10^{10} \ \rm{M}_{\odot}$. The origin of supermassive BHs remains an unresolved problem in astrophysics, with many viable pathways suggesting that they undergo an intermediate-mass phase. Whether IMBHs really stand as an independent category of BHs or rather they represent the heaviest stellar mass and the lightest supermassive BHs is still unclear, mostly owing to the lack of an observational smoking gun. The first part of this chapter discusses proposed formation channels of IMBHs and focuses on their formation and growth in dense stellar environments like globular and nuclear star clusters. It also highlights how the growth of IMBHs through mergers with other BHs is important from the point of view of gravitational waves and seeding of supermassive BHs in our Universe. The second part of the chapter focuses on the multi-wavelength observational constraints on IMBHs in dense star clusters and dwarf galactic nuclei. It also examines the potential insights that future gravitational wave detectors could offer in unraveling the mystery surrounding IMBHs.Comment: To appear as Chapter 2 of the book, "Black Holes in the Era of Gravitational Wave Astronomy", ed. Arca Sedda, Bortolas, Spera, pub. Elsevier. All authors equally contributed to the chapter. A. Askar is the author of part I of the chapter. V. F. Baldassare and M. Mezcua are authors of part II. Figures from other publications have been reproduced with permissio

Identifying AGNs in low-mass galaxies via long-term optical variability

We present an analysis of the nuclear variability of $\sim28,000$ nearby ($z<0.15$) galaxies with Sloan Digital Sky Survey (SDSS) spectroscopy in Stripe 82. We construct light curves using difference imaging of SDSS g-band images, which allows us to detect subtle variations in the central light output. We select variable AGN by assessing whether detected variability is well-described by a damped random walk model. We find 135 galaxies with AGN-like nuclear variability. While most of the variability-selected AGNs have narrow emission lines consistent with the presence of an AGN, a small fraction have narrow emission lines dominated by star formation. The star-forming systems with nuclear AGN-like variability tend to be low-mass ($M_{\ast}<10^{10}~M_{\odot}$), and may be AGNs missed by other selection techniques due to star formation dilution or low-metallicities. We explore the AGN fraction as a function of stellar mass, and find that the fraction of variable AGN increases with stellar mass, even after taking into account the fact that lower mass systems are fainter. There are several possible explanations for an observed decline in the fraction of variable AGN with decreasing stellar mass, including a drop in the supermassive black hole occupation fraction, a decrease in the ratio of black hole mass to galaxy stellar mass, or a change in the variability properties of lower-mass AGNs. We demonstrate that optical photometric variability is a promising avenue for detecting AGNs in low-mass, star formation-dominated galaxies, which has implications for the upcoming Large Synoptic Survey Telescope.Comment: Submitted to ApJ. Light curves and difference image videos posted at http://www.viviennebaldassare.com/diffi

X-ray Properties of Optically Variable Low-mass AGN Candidates

We present an X-ray analysis of fourteen nearby (z < 0.044) AGN in low mass galaxies (M_* <= 5*10^9 Msun) selected based on their optical variability (Baldassare et al. 2020). Comparing and contrasting different AGN selection techniques in low-mass galaxies is essential for obtaining an accurate estimate of the active fraction in this regime. We use both new and archival observations from the Chandra X-ray Observatory to search for X-ray point sources consistent with AGN. Four objects have detected nuclear X-ray emission with luminosities ranging from L_0.5-7 ~ 3*10^40 to 9*10^42 erg s^-1 with two more marginal detections. All of the detected galaxies have luminosities exceeding those anticipated from X-ray binaries, and all sources are nuclear, suggesting the X-ray emission in most sources is due to an AGN. These observations demonstrate the success of variability at identifying AGN in low-mass galaxies. We also explore emission line diagnostics and discuss the differences in the results of these methods for AGN selection, in particular regarding low-mass and low-metallicity systems