Search

We present a new empirical framework modeling the metallicity and star-formation history (SFH) dependence of X-ray luminous ($L > 10^{36}$ ergs s$^{-1}$) point-source population luminosity functions (XLFs) in normal galaxies. We expect the X-ray point-source populations are dominated by X-ray binaries (XRBs), with contributions from supernova remnants near the low luminosity end of our observations. Our framework is calibrated using the collective statistical power of 3,731 X-ray detected point-sources within 88 Chandra-observed galaxies at $D <$ 40 Mpc that span broad ranges of metallicity ($Z \approx$ 0.03-2 $Z_\odot$), SFH, and morphology (dwarf irregulars, late-types, and early-types). Our best-fitting models indicate that the XLF normalization per unit stellar mass declines by $\approx$2-3 dex from 10 Myr to 10 Gyr, with a slower age decline for low-metallicity populations. The shape of the XLF for luminous X-ray sources ($L < 10^{38}$ ergs s$^{-1}$) significantly steepens with increasing age and metallicity, while the lower-luminosity XLF appears to flatten with increasing age. Integration of our models provide predictions for X-ray scaling relations that agree very well with past results presented in the literature, including, e.g., the $L_{\rm X}$-SFR-$Z$ relation for high-mass XRBs (HMXBs) in young stellar populations as well as the $L_{\rm X}/M_\star$ ratio observed in early-type galaxies that harbor old populations of low-mass XRBs (LMXBs). The model framework and data sets presented in this paper further provide unique benchmarks that can be used for calibrating binary population synthesis models., Comment: Accepted for publication in ApJS; extended figures/materials available at https://lehmer.uark.edu/downloads/ ; python SED fitting code Lightning available at https://github.com/ebmonson/lightningpy

Supermassive black holes (SMBHs) are found in the centers of massive galaxies, and galaxy mergers should eventually lead to SMBH mergers. Quasar activity has long been associated with galaxy mergers, so here we investigate if supermassive black hole binaries (SMBHBs) are preferentially found in quasars. Our multimessenger investigation folds together a gravitational wave background signal from NANOGrav, a sample of periodic AGN candidates from the Catalina Real-Time Transient Survey, and a quasar mass function, to estimate an upper limit on the fraction of quasars which could host a SMBHB. We find at 95\% confidence that quasars are at most seven times as likely to host a SMBHB as a random galaxy. Quasars should therefore be prioritized as targets for SMBHB searches in pulsar timing arrays., Comment: 9 pages, 5 figures, 1 appendix

The supermassive black holes ($M_{\rm BH} \sim 10^{6}$$-$$10^{10}~M_\odot$) that power luminous active galactic nuclei (AGNs), i.e., quasars, generally show a correlation between thermal disk emission in the ultraviolet (UV) and coronal emission in hard X-rays. In contrast, some "massive" black holes (mBHs; $M_{\rm BH} \sim 10^{5}$$-$$10^{6}~M_\odot$) in low-mass galaxies present curious X-ray properties with coronal radiative output up to 100$\times$ weaker than expected. To examine this issue, we present a pilot study incorporating Very Large Array radio observations of a sample of 18 high-accretion-rate (Eddington ratios $L_{\rm bol}/L_{\rm Edd} > 0.1$), mBH-powered AGNs ($M_{\rm BH} \sim 10^{6}~M_\odot$) with Chandra X-ray coverage. Empirical correlations previously revealed in samples of radio-quiet, high-Eddington AGNs indicate that the radio$-$X-ray luminosity ratio, $L_{\rm R}/L_{\rm X}$, is approximately constant. Through multiwavelength analysis, we instead find that the X-ray-weaker mBHs in our sample tend toward larger values of $L_{\rm R}/L_{\rm X}$ even though they remain radio-quiet per their optical$-$UV properties. This trend results in a tentative but highly intriguing correlation between $L_{\rm R}/L_{\rm X}$ and X-ray weakness, which we argue is consistent with a scenario in which X-rays may be preferentially obscured from our line of sight by a "slim" accretion disk. We compare this observation to weak emission-line quasars (AGNs with exceptionally weak broad-line emission and a significant X-ray-weak fraction) and conclude by suggesting that our results may offer a new observational signature for finding high-accretion-rate AGNs., Comment: Accepted for publication in ApJ. 26 pages, 8 figures, 6 tables

We study the black hole mass $-$ host galaxy stellar mass relation, $M_{\rm{BH}}-M_{\ast}$, of a sample of $z<4$ optically-variable AGNs in the COSMOS field. The parent sample of 491 COSMOS AGNs were identified by optical variability from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) program. Using publicly-available catalogs and spectra, we consolidate their spectroscopic redshifts and estimate virial black hole masses using broad line widths and luminosities. We show that variability searches with deep, high precision photometry like the HSC-SSP can identity AGNs in low mass galaxies up to $z\sim1$. However, their black holes are more massive given their host galaxy stellar masses than predicted by the local relation for active galaxies. We report that $z\sim 0.5-4$ variability-selected AGNs are meanwhile more consistent with the $M_{\rm{BH}}-M_{\ast}$ relation for local inactive early-type galaxies. This result is consistent with most previous studies of the $M_{\rm{BH}}-M_{\ast}$ relation at similar redshifts and indicates that AGNs selected from variability are not intrinsically different from the broad-line Type 1 AGN population at similar luminosities. Our results demonstrate the need for robust black hole and stellar mass estimates for intermediate-mass black hole candidates in low-mass galaxies at similar redshifts. Assuming that these results do not reflect a selection bias, they appear to be consistent with self-regulated feedback models wherein the central black hole and stars in galaxies grow in tandem., Comment: 24 pages, 15 figures, accepted for publication in ApJ

Drawing from the Chandra archive and using a carefully selected set of nearby dwarf galaxies, we present a calibrated high-mass X-ray binary (HMXB) luminosity function in the low-mass galaxy regime and search for an already hinted at dependence on metallicity. Our study introduces a new sample of local dwarf galaxies (D < 12.5 Mpc and M* < 5 x 10^9 M_sun), expanding the specific star-formation rates (sSFR) and gas-phase metallicities probed in previous investigations. Our analysis of the observed X-ray luminosity function indicates a shallower power-law slope for the dwarf galaxy HMXB population. In our study, we focus on dwarf galaxies that are more representative in terms of sSFR compared to prior work. In this regime, the HMXB luminosity function exhibits significant stochastic sampling at high luminosities. This likely accounts for the pronounced scatter observed in the galaxy-integrated HMXB population's Lx/SFR versus metallicity for our galaxy sample. Our calibration is necessary to understand the AGN content of low mass galaxies identified in current and future X-ray survey fields and has implications for binary population synthesis models, as well as X-ray driven cosmic heating in the early universe., Comment: 19 pages, 7 figures

Active galactic nuclei (AGN) light curves observed with different wavebands show that the variability in longer wavelength bands lags the variability in shorter wavelength bands. Measuring these lags, or reverberation mapping, is used to measure the radial temperature profile and extent of AGN disks, typically with a reprocessing model that assumes X-rays are the main driver of the variability in other wavelength bands. To demonstrate how this reprocessing works with realistic accretion disk structures, we use 3D local shearing box multi-frequency radiation magnetohydrodynamic (MHD) simulations to model the UV-emitting region of an AGN disk, which is unstable to the magnetorotational instability (MRI) and convection. At the same time, we inject hard X-rays ($>1$~keV) into the simulation box to study the effects of X-ray irradiation on the local properties of the turbulence and the resulting variability of the emitted UV light curve. We find that disk turbulence is sufficient to drive intrinsic variability in emitted UV light curves and that a damped random walk (DRW) model is a good fit to this UV light curve for timescales $>5$~days. Meanwhile, the injected X-rays have almost no impact on the power spectrum of the emitted UV light curve. In addition, the injected X-ray and emitted UV light curves are only correlated if there is X-ray variability on timescales $>1$~day, in which case we find a correlation coefficient $r=0.52$. These results suggest that hard X-rays with scattering dominated opacity are likely not the main driver of the reverberation signals., Comment: 9 pages, 3 figures, submitted to ApJL

The fraction of local dwarf galaxies that hosts massive black holes is arguably the cleanest diagnostic of the dominant seed formation mechanism of today's supermassive black holes. A 5 per cent constraint on this quantity can be achieved with AXIS observations of 3300 galaxies across the mass spectrum through a combination of serendipitous extra-galactic fields plus a dedicated 1 Msec GO program., Comment: This White Paper is part of a series commissioned for the AXIS Probe Concept Mission

The James Webb Space Telescope is now detecting early black holes (BHs) as they transition from "seeds" to supermassive BHs. Recently Bogdan et al. (2023) reported the detection of an X-ray luminous supermassive BH, UHZ-1, with a photometric redshift at $z > 10$. Such an extreme source at this very high redshift provides new insights on seeding and growth models for BHs given the short time available for formation and growth. Harnessing the exquisite sensitivity of JWST/NIRSpec, here we report the spectroscopic confirmation of UHZ-1 at $z = 10.073 \pm 0.002$. We find that the NIRSpec/Prism spectrum is typical of recently discovered z~10 galaxies, characterized primarily by star-formation features. We see no clear evidence of the powerful X-ray source in the rest-frame UV/optical spectrum, which may suggest heavy obscuration of the central BH, in line with the Compton-thick column density measured in the X-rays. We perform a stellar population fit simultaneously to the new NIRSpec spectroscopy and previously available photometry. The fit yields a stellar mass estimate for the host galaxy that is significantly better constrained than prior photometric estimates ($M_*\sim 1.4^{+0.3}_{-0.4} \times 10^8 M_\odot$). Given the predicted BH mass ($M_{\rm BH}\sim10^7-10^8 M_\odot$), the resulting ratio of $M_{\rm BH}/M_*$ remains two to three orders of magnitude higher than local values, thus lending support to the heavy seeding channel for the formation of supermassive BHs within the first billion years of cosmic evolution., Comment: 10 pages, 4 figures, ApJL in-press. Updated to accepted version incl. X-ray image

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.

With its capacity to observe $\sim 10^{5-6}$ faint active galactic nuclei (AGN) out to redshift $z\approx 6$, Roman is poised to reveal a population of $10^{4-6}\, {\rm M_\odot}$ black holes during an epoch of vigorous galaxy assembly. By measuring the light curves of a subset of these AGN and looking for periodicity, Roman can identify several hundred massive black hole binaries (MBHBs) with 5-12 day orbital periods, which emit copious gravitational radiation and will inevitably merge on timescales of $10^{3-5}$ years. During the last few months of their merger, such binaries are observable with the Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave mission set to launch in the mid-2030s. Roman can thus find LISA precursors, provide uniquely robust constraints on the LISA source population, help identify the host galaxies of LISA mergers, and unlock the potential of multi-messenger astrophysics with massive black hole binaries., Comment: White Paper for the Nancy Grace Roman Space Telescope's Core Community Surveys (https://roman.gsfc.nasa.gov/science/ccs_white_papers.html)

The variability of quasar light curves can be used to study the structure of quasar accretion disks. For example, continuum reverberation mapping uses delays between variability in short and long wavelength bands ("short" lags) to measure the radial extent and temperature profile of the disk. Recently, a potential reverse lag, where variations in shorter wavelength bands lag the longer wavelength bands at the much longer viscous timescale, was detected for Fairall 9. Inspired by this detection, we derive a timescale for these "long" negative lags from fluctuation propagation models and recent simulations. We use this timescale to forecast our ability to detect long lags using the Vera Rubin Legacy Survey of Space and Time (LSST). After exploring several methods, including the interpolated cross-correlation function, a Von-Neumann estimator, javelin, and a maximum-likelihood Fourier method, we find that our two main methods, javelin and the maximum-likelihood method, can together detect long lags of up to several hundred days in mock LSST light curves. Our methods work best on proposed LSST cadences with long season lengths, but can also work for the current baseline LSST cadence, especially if we add observations from other optical telescopes during seasonal gaps. We find that LSST has the potential to detect dozens to hundreds of additional long lags. Detecting these long lags can teach us about the vertical structure of quasar disks and how it scales with different quasar properties., Comment: 40 pages, 36 figures, accepted to ApJ

Quasar feedback may regulate the growth of supermassive black holes, quench coeval star formation, and impact galaxy morphology and the circumgalactic medium. However, direct evidence for quasar feedback in action at the epoch of peak black hole accretion at z ~ 2 remains elusive. A good case in point is the z = 1.6 quasar WISEA J100211.29+013706.7 (XID 2028) where past analyses of the same ground-based data have come to different conclusions. Here we revisit this object with the integral field unit of the Near Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST) as part of Early Release Science program Q3D. The excellent angular resolution and sensitivity of the JWST data reveal new morphological and kinematic sub-structures in the outflowing gas plume. An analysis of the emission line ratios indicates that photoionization by the central quasar dominates the ionization state of the gas with no obvious sign for a major contribution from hot young stars anywhere in the host galaxy. Rest-frame near-ultraviolet emission aligned along the wide-angle cone of outflowing gas is interpreted as a scattering cone. The outflow has cleared a channel in the dusty host galaxy through which some of the quasar ionizing radiation is able to escape and heat the surrounding interstellar and circumgalactic media. The warm ionized outflow is not powerful enough to impact the host galaxy via mechanical feedback, but radiative feedback by the AGN, aided by the outflow, may help explain the unusually small molecular gas mass fraction in the galaxy host., Comment: 17 pages, 9 figures, accepted for publication in The Astrophysical Journal

Owing to their quiet evolutionary histories, nearby dwarf galaxies (stellar masses $M_\star \lesssim 3 \times 10^9 M_\odot$) have the potential to teach us about the mechanism(s) that 'seeded' the growth of supermassive black holes, and also how the first stellar mass black holes formed and interacted with their environments. Here, we present high spatial-resolution observations of three dwarf galaxies in the X-ray (Chandra), the optical/near-infrared (Hubble Space Telescope), and the radio (Karl G. Jansky Very Large Array). These three galaxies were previously identified as hosting candidate active galactic nuclei on the basis of lower resolution X-ray imaging. With our new observations, we find that X-ray sources in two galaxies (SDSS J121326.01+543631.6 and SDSS J122111.29+173819.1) are off nuclear and lack corresponding radio emission, implying they are likely luminous X-ray binaries. The third galaxy (Mrk 1434) contains two X-ray sources (each with $L_{\rm X} \approx 10^{40}$ erg s$^{-1}$) separated by 2".8, has a low metallicity (12 + log (O/H) = 7.8), and emits nebular \ion{He}{II} $\lambda$4686 line emission. The northern source has spatially coincident point-like radio emission at 9.0 GHz and extended radio emission at 5.5 GHz. We discuss X-ray binary interpretations (where an ultraluminous X-ray source blows a 'radio bubble') and active galactic nucleus interpretations (where a $\approx 4\times10^5 M_\odot$ black hole launches a jet). In either case, we find that the \ion{He}{II} emission cannot be photoionised by the X-ray source, unless the source was $\approx$30-90 times more luminous several hundred years ago., Comment: Accepted for publication in MNRAS. 11 pages, 1 figure, 6 tables

We report the detection of a long-timescale negative lag, where the blue bands lag the red bands, in the nearby Seyfert 1 galaxy Fairall 9. Active Galactic Nuclei (AGN) light curves show variability over a wide range of timescales. By measuring time lags between different wavelengths, the otherwise inaccessible structure and kinematics of the accretion disk can be studied. One common approach, reverberation mapping, quantifies the continuum and line lags moving outwards through the disk at the light-travel time, revealing the size and temperature profile of the disk. Inspired by numerical simulations, we expect longer lags to exist in AGN light curves that travel inward on longer timescales, tracing the accretion process itself. By analyzing AGN light curves in both temporal and frequency space, we report the detection of long-timescale lags ($\sim -70$ days) in Fairall 9 which propagate in the opposite direction to the reverberation lag. The short continuum lag ($<10$ days) is also detected and is consistent with reverberation lags reported in the literature. When fitting the longer lag as a function of frequency with a model motivated by the thin disk model, we find that the disk scale height likely increases outward in the disk. This detection raises the exciting prospect of mapping accretion disk structures across a wide range of AGN parameters., Comment: 24 pages, 14 figures, submitted to ApJ

We present new 5 GHz VLA observations of a sample of 8 active intermediate-mass black holes with masses $10^{4.9} < M < 10^{6.1}\ M_{\odot}$ found in galaxies with stellar masses $M_{*} < 3 \times 10^{9}\ M_{\odot}$. We detected 5 of the 8 sources at high significance. Of the detections, 4 were consistent with a point source, and one (SDSS J095418.15+471725.1, with black hole mass $M < 10^{5}\ M_{\odot}$) clearly shows extended emission that has a jet morphology. Combining our new radio data with the black hole masses and literature X-ray measurements, we put the sources on the fundamental plane of black hole accretion. We find that the extent to which the sources agree with the fundamental plane depends on their star-forming/composite/AGN classification based on optical narrow emission line ratios. he single star-forming source is inconsistent with the fundamental plane. The three composite sources are consistent, and three of the four AGN sources are inconsistent with the fundamental plane. We argue that this inconsistency is genuine and not a result of misattributing star-formation to black hole activity. Instead, we identify the sources in our sample that have AGN-like optical emission line ratios as not following the fundamental plane and thus caution the use of the fundamental plane to estimate masses without additional constraints, such as radio spectral index, radiative efficiency, or the Eddington fraction., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society. 9 pages, 2 figures. Images can be accessed in fits format from https://doi.org/10.7302/3100-6e62

The James Webb Space Telescope (JWST) will have the sensitivity to detect early low-mass black holes as they transition from "seeds" to supermassive black holes (BHs). Based on the JAGUAR mock catalog of galaxies, we present a clean color selection that takes advantage of the unique UV slope of accreting supermassive black holes with relatively low mass and high accretion rates. We show for a range of host stellar masses that those galaxies hosting $\sim 10^{6} M_{\odot}$ BHs radiating at >10% of their Eddington luminosity separate in color space from inactive systems. Here we propose a set of 3-band, 2-color selection boxes (with 90% completeness; 90% purity; balanced purity/completeness) with JWST/NIRCam to identify the most promising growing BH candidates at $z \sim 7-10$., Comment: Updated to accepted version. 11 pages, 4 figures, 1 table. Published in ApJL

The nanohertz gravitational wave background (GWB) is believed to be dominated by GW emission from supermassive black hole binaries (SMBHBs). Observations of several dual active galactic nuclei (AGN) strongly suggest a link between AGN and SMBHBs, given that these dual AGN systems will eventually form bound binary pairs. Here we develop an exploratory SMBHB population model based on empirically constrained quasar populations, allowing us to decompose the GWB amplitude into an underlying distribution of SMBH masses, SMBHB number density, and volume enclosing the GWB. Our approach also allows us to self-consistently predict the number of local SMBHB systems from the GWB amplitude. Interestingly, we find the local number density of SMBHBs implied by the common-process signal in the NANOGrav 12.5-yr dataset to be roughly five times larger than previously predicted by other models. We also find that at most $\sim 25 \%$ of SMBHBs can be associated with quasars. Furthermore, our quasar-based approach predicts $\gtrsim 95\%$ of the GWB signal comes from $z \lesssim 2.5$, and that SMBHBs contributing to the GWB have masses $\gtrsim 10^8 M_\odot$. We also explore how different empirical galaxy-black hole scaling relations affect the local number density of GW sources, and find that relations predicting more massive black holes decrease the local number density of SMBHBs. Overall, our results point to the important role that a measurement of the GWB will play in directly constraining the cosmic population of SMBHBs, as well as their connections to quasars and galaxy mergers., Comment: 12 pages, 6 figures, 1 appendix, accepted for publication in ApJ

We present an analysis of 12 optically selected dual AGN candidates at $z < 0.34$. Each candidate was originally identified via double-peaked [O III] $\lambda$5007 emission lines, and have received follow-up $Chandra$ and $HST$ observations. Because the X-ray data are low-count ($<100$ counts) with small separations ($<1$"), a robust analysis is necessary for classifying each source. Pairing long-slit [O III] observations with existing $Chandra$ observations, we re-analyze the X-ray observations with ${\tt BAYMAX}$ to determine whether the X-ray emission from each system is more likely a single or dual point source. We find that 4 of the 12 sources are likely dual X-ray point source systems. We examine each point source's spectra via a Monte Carlo method that probabilistically identifies the likely origin of each photon. When doing so, we find that (i) the secondary X-ray point sources in 2 of the systems have $L_{\mathrm{X}}<10^{40}$ erg s$^{-1}$, such that we cannot rule out a non-AGN origin, (ii) one source has a secondary with $L_{\mathrm{X}}>10^{40}$ erg s$^{-1}$ but a spectrum that is too soft to definitively preclude being X-ray emitting diffuse gas that was photoionized by the primary AGN, and (iii) one system (SDSS J1126+2944) is a dual AGN. Additionally, using complementary $HST$ observations, we analyze a sub-sample of systems that are visually identified as merging. Our results suggest that dual AGNs may preferentially reside in mergers with small separations, consistent with both simulations and observations., Comment: 26 pages, 9 figures, accepted to ApJ

To gain insights into long-term Active Galactic Nuclei (AGN) variability, we analyze an AGN sample from the Sloan Digital Sky Survey (SDSS) and compare their photometry with observations from the Hyper Suprime-Cam survey (HSC) observed $\langle 14.85 \rangle$ years after SDSS. On average, the AGN are fainter in HSC than SDSS. We demonstrate that the difference is not due to subtle differences in the SDSS versus HSC filters or photometry. The decrease in mean brightness is redshift dependent, consistent with expectations for a change that is a function of the rest-frame time separation between observations. At a given redshift, the mean decrease in brightness is stronger for more luminous AGN and for objects with longer time separation between measurements. We demonstrate that the dependence on redshift and luminosity of measured mean brightness decrease is consistent with simple models of Eddington ratio variability in AGN on long (Myr, Gyr) timescales. We show how our results can be used to constrain the variability and demographic properties of AGN populations., Comment: 8 pages, 5 figures, accepted to ApJL; The associated data and code are available at https://github.com/nevencaplar/AGN-Going-Down

We present an analysis of the long-term optical variability for $\sim50,000$ nearby (z<0.055) galaxies from the NASA-Sloan Atlas, $35,000$ of which are low-mass ($M_{\ast}<10^{10}~M_{\odot}$). We use difference imaging of Palomar Transient Factory (PTF) R-band observations to construct light curves with typical baselines of several years. We then search for subtle variations in the nuclear light output. We determine whether detected variability is AGN-like by assessing the fit quality to a damped random walk model. We identify 424 variability-selected AGN, including 244 with stellar masses between $10^{7}$ and $10^{10}~M_{\odot}$. 75% of low-mass galaxies with AGN-like variability have narrow emission lines dominated by star formation. After controlling for nucleus magnitude, the fraction of variable AGN is constant down to $M_{\ast}=10^{9}~M_{\odot}$, suggesting no drastic decline in the BH occupation fraction down to this stellar mass regime. Combining our NASA-Sloan Atlas sample with samples of nearby galaxies with broad H$\alpha$ emission, we find no dependence of variability properties with black hole mass. However, we caution that the variable AGN fraction is strongly dependent on baseline. For baselines less than two years, the variable fraction for the full sample is 0.25%, compared to 1.0% for baselines longer than two years. Finally, comparing Stripe 82 light curves (Baldassare et al. 2018) to PTF light curves, we find populations of changing-look AGN: 8 galaxies that are variable in Stripe 82, but quiescent in PTF, and 15 galaxies where the reverse is true. Our PTF work demonstrates the promise of long-term optical variability searches in low-mass galaxies for finding AGNs missed by other selection techniques., Comment: Submitted to Astrophysical Journal

The properties of low-mass galaxies hosting central black holes provide clues about the formation and evolution of the progenitors of supermassive black holes. In this letter, we present HSC-XD 52, a spectroscopically confirmed low-mass active galactic nucleus (AGN) at an intermediate redshift of $z\sim0.56$. We detect this object as a very luminous X-ray source coincident with a galaxy observed by the Hyper Suprime-Cam (HSC) as part of a broader search for low-mass AGN. We constrain its stellar mass through spectral energy distribution modeling to be LMC-like at $M_\star \approx 3 \times 10^9 M_\odot$, placing it in the dwarf regime. We estimate a central black hole mass of $M_\mathrm{BH} \sim 10^{6} M_\odot$. With an average X-ray luminosity of $L_X \approx 3.5 \times 10^{43}~\mathrm{erg}~\mathrm{s}^{-1}$, HSC-XD 52 is among the most luminous X-ray selected AGN in dwarf galaxies. The spectroscopic and photometric properties of HSC-XD 52 indicate that it is an intermediate redshift counterpart to local low-mass AGN., Comment: accepted to ApJL; 8 pages, 4 figures, 1 table

We present a sample of nearby dwarf galaxies with radio-selected accreting massive black holes (BHs), the majority of which are non-nuclear. We observed 111 galaxies using sensitive, high-resolution observations from the Karl G. Jansky Very Large Array (VLA) in its most extended A-configuration at X-band (~8-12 GHz), yielding a typical angular resolution of ~0.25" and rms noise of ~15 uJy. Our targets were selected by cross matching galaxies with stellar masses M_stellar < 3 x 10^9 M_sun and redshifts z<0.055 in the NASA-Sloan Atlas with the VLA Faint Images of the Radio Sky at Twenty centimeters (FIRST) Survey. With our new high-resolution VLA observations, we detect compact radio sources towards 39 galaxies and carefully evaluate possible origins for the radio emission including thermal HII regions, supernova remnants, younger radio supernovae, background interlopers, and AGNs in the target galaxies. We find that 13 dwarf galaxies almost certainly host active massive BHs despite the fact that only one object was previously identified as having optical signatures of an AGN. We also identify a candidate dual radio AGN in a more massive galaxy system. The majority of the radio-detected BHs are offset from the center of the host galaxies with some systems showing signs of interactions/mergers. Our results indicate that massive BHs need not always live in the nuclei of dwarf galaxies, confirming predictions from simulations. Moreover, searches attempting to constrain BH seed formation using observations of dwarf galaxies need to account for such a population of "wandering" BHs., Comment: Accepted for publication in the Astrophysical Journal; 18 pages; 16 figures

Supermassive black hole (SMBH) binaries with masses of ~10^8--10^9 Msun are expected to dominate the contribution to the as-yet undetected gravitational wave background (GWB) signal at the nanohertz frequencies accessible to Pulsar Timing Arrays (PTA). We currently lack firm empirical constraints on the amplitude of the GWB due to the dearth of confirmed SMBH binaries in the required mass range. Using HST/WFC3 images, we have discovered a z~0.2 quasar hosted in a merger remnant with two closely separated (0.13'' or ~430pc) continuum cores at the heart of the galaxy SDSSJ1010+1413. The two cores are spatially coincident with two powerful [OIII]-emitting point sources with quasar-like luminosities (L_AGN ~ 5x10^46 erg/s, suggesting the presence of a bound SMBH system, each with M_BH > 4x10^8 Msun. We place an upper limit on the merging timescale of the SMBH pair of 2.5 billion years, roughly the Universe lookback time at z~0.2. There is likely a population of quasar binaries similar to SDSSJ1010+1413 that contribute to a stochastic GWB that should be detected in the next several years. If the GWB is not detected this could indicate that SMBHs merge only over extremely long timescales, remaining as close separation binaries for many Hubble times, the so-called `final-parsec problem'., Comment: Accepted to ApJL (in press), 9 pages, 5 figures

The origin and properties of black hole seeds that grow to produce the detected population of supermassive black holes are unconstrained at present. Despite the existence of several potentially feasible channels for the production of initial seeds in the high redshift universe, since even actively growing seeds are not directly observable at these epochs, discriminating between models remains challenging. Several new observables that encapsulate information about seeding have been proposed in recent years, and these offer exciting prospects for truly unraveling the nature of black hole seeds in the coming years. One of the key challenges for this task lies in the complexity of the problem, the required disentangling of the confounding effects of accretion physics and mergers, as mergers and accretion events over cosmic time stand to erase these initial conditions. Nevertheless, some unique signatures of seeding do survive and still exist in: local scaling relations between black holes and their galaxy hosts at low-masses; in high-redshift luminosity functions of accreting black holes; and in the total number and mass functions of gravitational wave coalescence events from merging binary black holes. One of the clearest discriminants for seed models are these high redshift gravitational wave detections of mergers from space detectable in the milliHertz range. These predicted event rates offer the most direct constraints on the properties of initial black hole seeds. Improving our theoretical understanding of black hole dynamics and accretion will also be pivotal in constraining seeding models in combination with the wide range of multi-messenger data., Comment: White paper submitted to the Astro2020 decadal survey

Massive 10^6-10^10 Msun black holes (BHs) are ubiquitous in local galactic nuclei. They were common by the time the Universe is several Gyr old, and many of them were in place within the first 1~Gyr after the Big Bang. Their quick assembly has been attributed to mechanisms such as the rapid collapse of gas into the nuclei of early protogalaxies, accretion and mergers of stellar-mass BHs accompanying structure formation at early times, and the runaway collapse of early, ultra-dense stellar clusters. The origin of the early massive BHs remains an intriguing and long-standing unsolved puzzle in astrophysics. Here we discuss strategies for discerning between BH seeding models using electromagnetic observations. We argue that the most direct answers will be obtained through detection of BHs with masses M<10^5 Msun at redshifts z>10, where we expect them to first form. Reaching out to these redshifts and down to these masses is crucial, because BHs are expected to lose the memory of their initial assembly by the time they grow well above 10^5 Msun and are incorporated into higher-mass galaxies. The best way to detect 10^4-10^5 Msun BHs at high redshifts is by a sensitive X-ray survey. Critical constraining power is augmented by establishing the properties and the environments of their host galaxies in deep optical/IR imaging surveys. Required OIR data can be obtained with the JWST and WFIRST missions. The required X-ray flux limits (down to 10^{-19} erg/s/cm^2) are accessible only with a next-generation X-ray observatory which has both high (sub-1") angular resolution and high throughput. A combination of deep X-ray and OIR surveys will be capable of probing several generic markers of the BH seed scenarios, and resolving the long-stanging puzzle of their origin. These electromagnetic observations are also highly synergistic with the information from LISA on high-z BH mergers., Comment: Science white paper submitted to the Astro2020 Decadal Survey

Pulsar timing arrays (PTAs) are on the verge of detecting low-frequency gravitational waves (GWs) from supermassive black hole binaries (SMBHBs). With continued observations of a large sample of millisecond pulsars, PTAs will reach this major milestone within the next decade. Already, SMBHB candidates are being identified by electromagnetic surveys in ever-increasing numbers; upcoming surveys will enhance our ability to detect and verify candidates, and will be instrumental in identifying the host galaxies of GW sources. Multi-messenger (GW and electromagnetic) observations of SMBHBs will revolutionize our understanding of the co-evolution of SMBHs with their host galaxies, the dynamical interactions between binaries and their galactic environments, and the fundamental physics of accretion. Multi-messenger observations can also make SMBHBs 'standard sirens' for cosmological distance measurements out to $z\simeq0.5$. LIGO has already ushered in breakthrough insights in our knowledge of black holes. The multi-messenger detection of SMBHBs with PTAs will be a breakthrough in the years $2020-2030$ and beyond, and prepare us for LISA to help complete our views of black hole demographics and evolution at higher redshifts., Comment: Submitted to the Astro2020 decadal review. This is one of five core white papers written by members of the NANOGrav Collaboration, see also: J.Cordes et al., S.R.Taylor et al., X.Siemens et al., and E.Fonseca et al

This document illustrates the feasibility of a few per cent level measurement of the local black hole occupation fraction in low mass galaxies through wide-field, high angular resolution X-ray imaging observations of local volume galaxies. The occupation fraction, particularly at the low end of the galaxy luminosity function, is a key benchmark for any model which aims to reproduce the formation and growth of super-massive black holes and their host galaxies. Our proposed measurement will complement orthogonal efforts that are planned in X-rays at high red-shifts, as well as in the local Universe with ground-based facilities., Comment: Astro2020 Decadal Survey White Paper

LISA will open the mHz band of gravitational waves (GWs) to the astronomy community. The strong gravity which powers the variety of GW sources in this band is also crucial in a number of important astrophysical processes at the current frontiers of astronomy. These range from the beginning of structure formation in the early universe, through the origin and cosmic evolution of massive black holes in concert with their galactic environments, to the evolution of stellar remnant binaries in the Milky Way and in nearby galaxies. These processes and their associated populations also drive current and future observations across the electromagnetic (EM) spectrum. We review opportunities for science breakthroughs, involving either direct coincident EM+GW observations, or indirect multimessenger studies. We argue that for the US community to fully capitalize on the opportunities from the LISA mission, the US efforts should be accompanied by a coordinated and sustained program of multi-disciplinary science investment, following the GW data through to its impact on broad areas of astrophysics. Support for LISA-related multimessenger observers and theorists should be sized appropriately for a flagship observatory and may be coordinated through a dedicated mHz GW research center., Comment: Submitted to the Astro2020 Decadal Survey call for science white papers

Quasars may have played a key role in limiting the stellar mass of massive galaxies. Identifying those quasars in the process of removing star formation fuel from their hosts is an exciting ongoing challenge in extragalactic astronomy. In this paper we present X-ray observations of eleven extremely red quasars (ERQs) with $L_{\rm bol}\sim 10^{47}$ erg s$^{-1}$ at $z=1.5-3.2$ with evidence for high-velocity ($v > 1000$ km s$^{-1}$) [OIII]$\lambda$5007\AA\ outflows. X-rays allow us to directly probe circumnuclear obscuration and to measure the instantaneous accretion luminosity. We detect ten out of eleven extremely red quasars available in targeted and archival data. Using a combination of X-ray spectral fitting and hardness ratios, we find that all of the ERQs show signs of absorption in the X-rays with inferred column densities of $N_{\rm H}\approx 10^{23}$ cm$^{-2}$, including four Compton-thick candidates ($N_{\rm H} > 10^{24}$ cm$^{-2}$). We stack the X-ray emission of the seven weakly detected sources, measuring an average column density of $N_{\rm H}\sim 8\times 10^{23}$ cm$^{-2}$. The absorption-corrected (intrinsic) $2-10$ keV X-ray luminosity of the stack is $2.7\times 10^{45}$ erg s$^{-1}$, consistent with X-ray luminosities of type 1 quasars of the same infrared luminosity. Thus, we find that ERQs are a highly obscured, borderline Compton-thick population, and based on optical and infrared data we suggest that these objects are partially hidden by their own equatorial outflows. However, unlike some quasars with known outflows, ERQs do not appear to be intrinsically underluminous in X-rays for their bolometric luminosity. Our observations indicate that low X-rays are not necessary to enable some types of radiatively driven winds., Comment: 16 pages, 6 figures, 2 tables. Re-submitted to ApJ following referee's comments

Collisions and interactions between gas-rich galaxies are thought to be pivotal stages in their formation and evolution, causing the rapid production of new stars, and possibly serving as a mechanism for fueling supermassive black holes (BH). Harnessing the exquisite spatial resolution (~0.5 arcsec) afforded by the first ~170 deg^2 of the Hyper Suprime-Cam (HSC) Survey, we present our new constraints on the importance of galaxy-galaxy major mergers (1:4) in growing BHs throughout the last ~8 Gyrs. Utilizing mid-infrared observations in the WISE All-Sky survey, we robustly select active galactic nuclei (AGN) and mass-matched control galaxy samples, totaling ~140,000 spectroscopically confirmed systems at i<22 mag. We identify galaxy interaction signatures using a novel machine-learning random forest decision tree technique allowing us to select statistically significant samples of major-mergers, minor-mergers/irregular-systems, and non-interacting galaxies. We use these samples to show that galaxies undergoing mergers are a factor ~2-7 more likely to contain luminous obscured AGN than non-interacting galaxies, and this is independent of both stellar mass and redshift to z < 0.9. Furthermore, based on our comparison of AGN fractions in mass-matched samples, we determine that the most luminous AGN population (L_AGN > 10^45 erg/s) systematically reside in merging systems over non-interacting galaxies. Our findings show that galaxy-galaxy interactions do, on average, trigger luminous AGN activity substantially more often than in secularly evolving non-interacting galaxies, and we further suggest that the BH growth rate may be closely tied to the dynamical time of the merger system., Comment: 26 pages, 12 figures, replaced with accepted PASJ HSC-SSP special issue version

Galaxy-scale bars are expected to provide an effective means for driving material towards the central region in spiral galaxies, and possibly feeding supermassive black holes (BHs). Here we present a statistically-complete study of the effect of bars on average BH accretion. From a well-selected sample of 50,794 spiral galaxies (with M* ~ 0.2-30 x 10^10 Msun) extracted from the Sloan Digital Sky Survey Galaxy Zoo 2 project, we separate those sources considered to contain galaxy-scale bars from those that do not. Using archival data taken by the Chandra X-ray Observatory, we identify X-ray luminous (L_X >~ 10^41 erg/s) active galactic nuclei (AGN) and perform an X-ray stacking analysis on the remaining X-ray undetected sources. Through X-ray stacking, we derive a time-averaged look at accretion for galaxies at fixed stellar mass and star formation rate, finding that the average nuclear accretion rates of galaxies with bar structures are fully consistent with those lacking bars (Mdot_acc ~ 3 x 10^-5 Msun/yr). Hence, we robustly conclude that large-scale bars have little or no effect on the average growth of BHs in nearby (z < 0.15) galaxies over gigayear timescales., Comment: 10 pages, 5 figures, accepted to ApJ

We study the time lags between the continuum emission of quasars at different wavelengths, based on more than four years of multi-band ($g$, $r$, $i$, $z$) light-curves in the Pan-STARRS Medium Deep Fields. As photons from different bands emerge from different radial ranges in the accretion disk, the lags constrain the sizes of the accretion disks. We select 240 quasars with redshifts $z \approx 1$ or $z \approx 0.3$ that are relatively emission line free. The light curves are sampled from day to month timescales, which makes it possible to detect lags on the scale of the light crossing time of the accretion disks. With the code JAVELIN, we detect typical lags of several days in the rest frame between the $g$ band and the $riz$ bands. The detected lags are $\sim 2-3$ times larger than the light crossing time estimated from the standard thin disk model, consistent with the recently measured lag in NGC5548 and micro-lensing measurements of quasars. The lags in our sample are found to increase with increasing luminosity. Furthermore, the increase in lags going from $g-r$ to $g-i$ and then to $g-z$ is slower than predicted in the thin disk model, particularly for high luminosity quasars. The radial temperature profile in the disk must be different from what is assumed. We also find evidence that the lags decrease with increasing line ratios between ultraviolet FeII lines and MgII, which may point to changes in the accretion disk structure at higher metallicity., Comment: 18 pages, 17 Figures, 3 Tables, submitted to ApJ

We present follow-up X-ray observations of the candidate massive black hole (BH) in the nucleus of the low-mass, compact starburst galaxy Henize 2-10. Using new high-resolution observations from the Chandra X-ray Observatory totaling 200 ks in duration, as well as archival Chandra observations from 2001, we demonstrate the presence of a previously unidentified X-ray point source that is spatially coincident with the known nuclear radio source in Henize 2-10 (i.e., the massive BH). We show that the hard X-ray emission previously identified in the 2001 observation is dominated by a source that is distinct from the nucleus, with the properties expected for a high-mass X-ray binary. The X-ray luminosity of the nuclear source suggests the massive BH is radiating significantly below its Eddington limit (~10^-6 L_Edd), and the soft spectrum resembles other weakly accreting massive BHs including Sagittarius A*. Analysis of the X-ray light curve of the nucleus reveals the tentative detection of a ~9-hour periodicity, although additional observations are required to confirm this result. Our study highlights the need for sensitive high-resolution X-ray observations to probe low-level accretion, which is the dominant mode of BH activity throughout the Universe., Comment: Accepted for publication in ApJ Letters. 7 pages, 4 figures

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 ApJ

We use time-domain optical spectroscopy to distinguish between broad emission lines powered by accreting black holes (BHs) or stellar processes (i.e., supernovae) for 16 galaxies identified as AGN candidates by Reines \etal (2013). Our study is primarily focused on those objects with narrow emission-line ratios dominated by star formation. Based on follow-up spectra taken with the Magellan Echellette Spectrograph (MagE), the Dual Imaging Spectrograph, and the Ohio State Multi-Object Spectrograph, we find that the broad H$\alpha$ emission has faded or was ambiguous for all of the star-forming objects (14/16) over baselines ranging from 5 to 14 years. For the two objects in our follow-up sample with narrow-line AGN signatures (RGG 9 and RGG 119), we find persistent broad H$\alpha$ emission consistent with an AGN origin. Additionally, we use our MagE observations to measure stellar velocity dispersions for 15 objects in the Reines et al. (2013) sample, all with narrow-line ratios indicating the presence of an AGN. Stellar masses range from $\sim5\times10^{8}$ to $3\times10^{9}$~\msun, and we measure $\sigma_{\ast}$ ranging from $28-71~{\rm km~s^{-1}}$. These $\sigma_{\ast}$ correspond to some of the lowest-mass galaxies with optical signatures of AGN activity. We show that RGG 119, the one object which has both a measured $\sigma_{\ast}$ and persistent broad H$\alpha$ emission, falls near the extrapolation of the $\rm M_{BH}-\sigma_{\star}$ relation to the low-mass end., Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 12 figures

We present new Chandra X-ray observations of seven low-mass black holes (~1e6 Msun) accreting at low Eddington ratios between -2.0

Studies of the physical properties of local elliptical galaxies (e.g., gas temperatures, halo masses, stellar kinematics) are shedding new light on galaxy formation. Here we present the hot X-ray gas properties of 33 early-type systems within the MASSIVE galaxy survey sample that have archival Chandra X-ray observations. Through careful X-ray spectral modeling, we derive X-ray luminosities (L_X) and plasma temperatures (T_gas) for the diffuse gas components in these galaxies. We combine the MASSIVE sample with 41 galaxies from the ATLAS^3D survey to investigate the X-ray and optical properties of a statistically significant sample of nearby early-type galaxies across a wide-range of environments. We deduce that all early-type galaxies (independent of galaxy mass and rotational support) follow a universal scaling law such that L_X~T_gas^4.5. When X-ray measurements are performed consistently in apertures set by the galaxy stellar content, the wide-scale environment does not contribute to the intrinsic scatter (~0.5dex) within the scaling relation. We further demonstrate that the scatter in L_X around both K-band luminosity (L_K) and the galaxy stellar velocity dispersion is primarily driven by T_gas, with no clear trends with halo mass, radio power, or angular momentum of the stars. It is not trivial to tie the origin of the gas directly to either the stellar mass or the galaxy potential. Indeed, our data require a steeper relation between L_X, L_K, and sigma_e than predicted by standard mass-loss models. Finally, we find a statistically significant correlation between sigma_e and T_gas, suggesting that T_gas is set by the galaxy potential inside the optical effective radius. We conclude that within the inner-most 10-30kpc region, early-types maintain pressure-supported hot gas, with a minimum T_gas set by the virial temperature, but the majority show evidence for some additional heating., Comment: 23 pages, 10 figures. Accepted ApJ version

Binary active galactic nuclei (AGNs) provide clues to how gas-rich mergers trigger and fuel AGNs and how supermassive black hole (SMBH) pairs evolve in a gas-rich environment. While significant effort has been invested in their identification, the detailed properties of binary AGNs and their host galaxies are still poorly constrained. In a companion paper, we examined the nature of ionizing sources in the double nuclei of four kpc-scale binary AGNs with redshifts between 0.1~0.2. Here, we present their host galaxy morphology based on F336W (U-band) and F105W (Y-band) images taken by the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope. Our targets have double-peaked narrow emission lines and were confirmed to host binary AGNs with follow up observations. We find that kpc-scale binary AGNs occur in galaxy mergers with diverse morphological types. There are three major mergers with intermediate morphologies and a minor merger with a dominant disk component. We estimate the masses of the SMBHs from their host bulge stellar masses and obtain Eddington ratios for each AGN. Compared with a representative control sample drawn at the same redshift and stellar mass, the AGN luminosities and Eddington ratios of our binary AGNs are similar to those of single AGNs. The U-Y color maps indicate that clumpy star forming regions could significantly affect the X-ray detection of binary AGNs, e.g., the hardness ratio. Considering the weak X-ray emission in AGNs triggered in merger systems, we suggest that samples of X-ray selected AGNs may be biased against gas-rich mergers., Comment: 12 pages, 4 figures, accepted for publication in ApJ

Capitalizing on the all-sky coverage of {\it WISE}, and the 35\% and 50\% sky coverage from SDSS and Pan-STARRS, respectively, we explore the efficacy of $m_{R}$ (optical) - $m_{3.4 \mu m}$ (mid-infrared), hereafter $R-W1$, as a color diagnostic to identify obscured supermassive black hole accretion in wide-area X-ray surveys. We use the $\sim$16.5 deg$^2$ Stripe 82 X-ray survey data as a test-bed to compare $R-W1$ with $R-K$, an oft-used obscured AGN selection criterion, and examine where different classes of objects lie in this parameter space. Most stars follow a well-defined path in $R-K$ vs. $R-W1$ space. We demonstrate that optically normal galaxies hosting X-ray AGN at redshifts $0.54$ color-cut, while they typically are not selected as AGN based on their $W1-W2$ colors. Additionally, different observed X-ray luminosity bins favor different regions in $R-W1$ parameter space: moderate luminosity AGN ($10^{43}$ erg s$^{-1} < L_{\rm 0.5-10 keV} < 10^{44}$ erg s$^{-1}$) tend to have red colors while the highest luminosity AGN ($L_{\rm 0.5-10 keV} > 10^{45}$ erg s$^{-1}$) have bluer colors; higher spectroscopic completeness of the Stripe 82X sample is needed to determine whether this is a selection effect or an intrinsic property. Finally, we parameterize X-ray obscuration of Stripe 82X AGN by calculating their hardness ratios (HRs) and find no clear trends between HR and optical reddening. Our results will help inform best-effort practices in following-up obscured AGN candidates in current and future wide-area, shallow X-ray surveys, including the all-sky {\it eROSITA} mission., Comment: accepted for publication to ApJ; 17 pages (emulateapj style), 21 figures, 3 tables

The uncertain origin of the recently-discovered `changing-looking' quasar phenomenon -- in which a luminous quasar dims significantly to a quiescent state in repeat spectroscopy over ~10 year timescales -- may present unexpected challenges to our understanding of quasar accretion. To better understand this phenomenon, we take a first step to building a sample of changing-look quasars with a systematic but simple archival search for these objects in the Sloan Digital Sky Survey Data Release 12. By leveraging the >10 year baselines for objects with repeat spectroscopy, we uncover two new changing-look quasars, and a third discovered previously. Decomposition of the multi-epoch spectra and analysis of the broad emission lines suggest that the quasar accretion disk emission dims due to rapidly decreasing accretion rates (by factors of >2.5), while disfavoring changes in intrinsic dust extinction for the two objects where these analyses are possible. Broad emission line energetics also support intrinsic dimming of quasar emission as the origin for this phenomenon rather than transient tidal disruption events or supernovae. Although our search criteria included quasars at all redshifts and transitions from either quasar-like to galaxy-like states or the reverse, all of the clear cases of changing-look quasars discovered were at relatively low-redshift (z ~ 0.2 - 0.3) and only exhibit quasar-like to galaxy-like transitions., Comment: 15 pages, 8 figures. Updated to accepted version

We present an analysis of the X-ray spectrum and long-term variability of the nearby dwarf starburst galaxy Henize 2-10. Recent observations suggest that this galaxy hosts an actively accreting black hole with mass ~10^6 M_sun. The presence of an AGN in a low-mass starburst galaxy marks a new environment for active galactic nuclei (AGNs), with implications for the processes by which "seed" black holes may form in the early Universe. In this paper, we analyze four epochs of X-ray observations of Henize 2-10, to characterize the long-term behavior of its hard nuclear emission. We analyze observations with Chandra from 2001 and XMM-Newton from 2004 and 2011, as well as an earlier, less sensitive observation with ASCA from 1997. Based on detailed analysis of the source and background, we find that the hard (2-10 keV) flux of the putative AGN has decreased by approximately an order of magnitude between the 2001 Chandra observation and exposures with XMM-Newton in 2004 and 2011. The observed variability confirms that the emission is due to a single source. It is unlikely that the variable flux is due to a supernova or ultraluminous X-ray source, based on the observed long-term behavior of the X-ray and radio emission, while the observed X-ray variability is consistent with the behavior of well-studied AGNs., Comment: 7 pages, 4 figures, 2 tables; accepted for publication in ApJ

We present a sample of hard X-ray selected candidate black holes (BHs) in 19 dwarf galaxies. BH candidates are identified by cross-matching a parent sample of ~44,000 local dwarf galaxies (M_stellar < 3 x 10^9 Msun, z<0.055) with the Chandra Source Catalog, and subsequently analyzing the original X-ray data products for matched sources. Of the 19 dwarf galaxies in our sample, 8 have X-ray detections reported here for the first time. We find a total of 43 point-like hard X-ray sources with individual luminosities L(2-10 keV) ~ 10^37 - 10^40 erg/s. Hard X-ray luminosities in this range can be attained by stellar-mass X-ray binaries (XRBs), and by massive BHs accreting at low Eddington ratio. We place an upper limit of 53% (10/19) on the fraction of galaxies in our sample hosting a detectable hard X-ray source consistent with the optical nucleus, although the galaxy center is poorly defined in many of our objects. We also find that 42% (8/19) of the galaxies in our sample exhibit statistically significant enhanced hard X-ray emission relative to the expected galaxy-wide contribution from low-mass and high-mass XRBs, based on the L(2-10 keV)-M_stellar-SFR relation defined by more massive and luminous systems. For the majority of these X-ray enhanced dwarf galaxies, the excess emission is consistent with (but not necessarily due to) a nuclear X-ray source. Follow-up observations are necessary to distinguish between stellar-mass XRBs and active galactic nuclei powered by more massive BHs. In any case, our results support the notion that X-ray emitting BHs in low-mass dwarf galaxies may have had an appreciable impact on reionization in the early Universe., Comment: Accepted for publication in the Astrophysical Journal

Quasar feedback is suspected to play a key role in the evolution of massive galaxies, by removing or reheating gas in quasar host galaxies and thus limiting the amount of star formation. In this paper we continue our investigation of quasar-driven winds on galaxy-wide scales. We conduct Gemini Integral Field Unit spectroscopy of a sample of luminous unobscured (type 1) quasars, to determine the morphology and kinematics of ionized gas around these objects, predominantly via observations of the [O III]5007 emission line. We find that ionized gas nebulae extend out to ~13 kpc from the quasar, that they are smooth and round, and that their kinematics are inconsistent with gas in dynamical equilibrium with the host galaxy. The observed morphological and kinematic properties are strikingly similar to those of ionized gas around obscured (type 2) quasars with matched [O III] luminosity, with marginal evidence that nebulae around unobscured quasars are slightly more compact. Therefore in samples of obscured and unobscured quasars carefully matched in [O III] luminosity we find support for the standard geometry-based unification model of active galactic nuclei, in that the intrinsic properties of quasars, of their hosts and of their ionized gas appear to be very similar. Given the apparent ubiquity of extended ionized regions, we are forced to conclude that either the quasar is at least partially illuminating pre-existing gas or that both samples of quasars are seen during advanced stages of quasar feedback. In the latter case, we may be biased by our [O III]-based selection against quasars in the early "blow-out" phase, for example due to dust obscuration., Comment: 17 pages, 10 figures, 2 tables. Published in MNRAS, 2014

We present a sample of 151 dwarf galaxies (10^8.5 < M_stellar < 10^9.5 Msun) that exhibit optical spectroscopic signatures of accreting massive black holes (BHs), increasing the number of known active galaxies in this stellar mass range by more than an order of magnitude. Utilizing data from the Sloan Digital Sky Survey Data Release 8 and stellar masses from the NASA-Sloan Atlas, we have systematically searched for active BHs in ~25,000 emission-line galaxies with stellar masses comparable to the Magellanic Clouds and redshifts z<0.055. Using the narrow-line [OIII]/H-beta versus [NII]/H-alpha diagnostic diagram, we find photoionization signatures of BH accretion in 136 galaxies, a small fraction of which also exhibit broad H-alpha emission. For these broad-line AGN candidates, we estimate BH masses using standard virial techniques and find a range of 10^5 < M_BH < 10^6 Msun and a median of M_BH ~ 2 x 10^5 Msun. We also detect broad H-alpha in 15 galaxies that have narrow-line ratios consistent with star-forming galaxies. Follow-up observations are required to determine if these are true type 1 AGN or if the broad H-alpha is from stellar processes. The median absolute magnitude of the host galaxies in our active sample is Mg = -18.1 mag, which is ~1-2 magnitudes fainter than previous samples of AGN hosts with low-mass BHs. This work constrains the smallest galaxies that can form a massive BH, with implications for BH feedback in low-mass galaxies and the origin of the first supermassive BH seeds., Comment: 26 pages, 15 figures, 6 tables. Accepted for publication in The Astrophysical Journal

We present a systematic X-ray study, the third in a series, of 49 active galactic nuclei with intermediate-mass black holes (IMBH; ~10^5-10^6 M_sun) using Chandra observations. We detect 42 out of 49 targets with a 0.5-2 keV X-ray luminosity 10^41-10^43 erg/s. We perform spectral fitting for the 10 objects with enough counts (>200), and they are all well fit by a simple power-law model modified by Galactic absorption, with no sign of significant intrinsic absorption. While we cannot fit the X-ray spectral slope directly for the rest of the sample, we estimate it from the hardness ratio and find a range of photon indices consistent with those seen in more luminous and massive objects. The X-ray-to-optical spectral slope (alphaox) of our IMBH sample is systematically flatter than in active galaxies with more massive black holes, consistent with the well-known correlation between alphaox and UV luminosity. Thanks to the wide dynamic range of our sample, we find evidence that alphaox increases with decreasing M_BH as expected from accretion disk models, where the UV emission systematically decreases as M_BH decreases and the disk temperature increases. We also find a long tail toward low alphaox values. While some of these sources may be obscured, given the high L_bol/L_Eddington values in the sample, we argue that some may be intrinsically X-ray-weak, perhaps owing to a rare state that radiates very little coronal emission., Comment: 13 pages (double columns), 2 tables, 9 figures, ApJ accepted