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Ho, Luis C.,Kim, Minjin IOP Publishing 2015 The Astrophysical journal Vol.809 No.2
<P>The masses of supermassive black holes (BHs) in broad-line active galactic nuclei (AGNs) can be measured through reverberation mapping, but this method currently cannot be applied to very large samples or to high-redshift AGNs. As a practical alternative, one can devise empirical scaling relations, based on the correlation between broad-line region size and AGN luminosity and the relation between BH mass and bulge stellar velocity dispersion, to estimate the virial masses of BHs from single-epoch spectroscopy. We present a revised calibration of the BH mass estimator for the commonly used H beta emission line. Our new calibration takes into account the recent determination of the virial coefficient for pseudo and classical bulges.</P>
Ho, Luis C.,Kim, Minjin,Terashima, Yuichi IOP Publishing 2012 ASTROPHYSICAL JOURNAL LETTERS - Vol.759 No.1
<P>Optical spectra and images taken with the Baade 6.5 m Magellan telescope confirm that 2XMM J123103.2+110648, a highly variable X-ray source with an unusually soft spectrum, is indeed associated with a type 2 (narrow-line) active nucleus at a redshift of z = 0.11871. The absence of broad H alpha or H beta emission in an otherwise X-ray unabsorbed source suggests that it intrinsically lacks a broad-line region. If, as in other active galaxies, the ionized gas and stars in J1231+1106 are in approximate virial equilibrium, and the M-BH-sigma(*) relation holds, the exceptionally small velocity dispersion of sigma = 33.5 km s (1) for [OIII] lambda 5007 implies that M-BH approximate to 10(5) M-circle dot, among the lowest ever detected. Such a low black hole mass is consistent with the general characteristics of the host, a small, low-luminosity, low-mass disk galaxy. We estimate the Eddington ratio of the black hole to be greater than or similar to 0.5, in good agreement with expectations based on the X-ray properties of the source.</P>
LOW-MASS ACTIVE GALACTIC NUCLEI WITH RAPID X-RAY VARIABILITY
Ho, Luis C.,Kim, Minjin American Astronomical Society 2016 The Astrophysical journal Vol.821 No.1
<P>We present a detailed study of the optical spectroscopic properties of 12 active galactic nuclei (AGNs) with candidate low-mass black holes (BHs) selected by Kamizasa et al. through rapid X-ray variability. The high-quality, echellette Magellan spectra reveal broad Ha emission in all the sources, allowing us to estimate robust virial BH masses and Eddington ratios for this unique sample. We confirm that the sample contains low-mass BHs accreting at high rates: the median M-BH = 1.2. x. 10(6) M-circle dot and median L-bol/L-Edd = 0.44. The sample follows the M-BH-sigma(*) relation, within the considerable scatter typical of pseudobulges, the probable hosts of these low-mass AGNs. Various lines of evidence suggest that ongoing star formation is prevalent in these systems. We propose a new strategy to estimate star formation rates in AGNs hosted by low-mass, low-metallicity galaxies, based on modification of an existing method using the strength of [O II] lambda 3727, [O III] lambda 5007, and X-rays.</P>
Stellar Photometric Structures of the Host Galaxies of Nearby Type 1 Active Galactic Nuclei
Kim, Minjin,Ho, Luis C.,Peng, Chien Y.,Barth, Aaron J.,Im, Myungshin Published by the University of Chicago Press for t 2017 The Astrophysical journal Supplement series Vol.232 No.2
<P>We present detailed image analysis of rest-frame optical images of 235 low-redshift (z less than or similar to 0.35) Type 1 active galactic nuclei (AGNs) observed with the Hubble Space Telescope. The high-resolution images enable us to perform rigorous two-dimensional image modeling to decouple the luminous central point source from the host galaxy, which, when warranted, is further decomposed into its principal structural components (bulge, bar, and disk). In many cases, care must be taken to account for structural complexities such as spiral arms, tidal features, and overlapping or interacting companion galaxies. We employ Fourier modes to characterize the degree of asymmetry of the light distribution of the stars as a quantitative measure of morphological distortion due to interactions or mergers. We examine the dependence of the physical parameters of the host galaxies on the properties of the AGNs, namely, radio-loudness and the width of the broad emission lines. In accordance with previous studies, narrow-line (H beta FWHM <= 2000 km s(-1)) Type 1 AGNs, in contrast to their broad-line (H beta FWHM > 2000 km s(-1)) counterparts, are preferentially hosted in later-type, lower-luminosity galaxies, which have a higher incidence of pseudo-bulges, are more frequently barred, and are less morphologically disturbed. This suggests that narrow-line Type 1 AGNs experienced a more quiescent evolutionary history driven primarily by internal secular evolution instead of external dynamical perturbations. The fraction of AGN hosts showing merger signatures is larger for more luminous sources. Radio-loud AGNs generally preferentially live in earlier-type (bulge-dominated), more massive hosts, although a minority of them appear to contain a significant disk component. We do not find convincing evidence for enhanced merger signatures in the radio-loud population.</P>