Intermediate-mass Black Holes in Galactic Nuclei

We present the first homogeneous sample of intermediate-mass black hole candidates in active galactic nuclei. Starting with broad-line active nuclei from the Sloan Digital Sky Survey, we use the linewidth-luminosity-mass scaling relation to select a sample of 19 galaxies in the mass range M_BH ~ 8 x 10^4 - 10^6 solar masses. In contrast to the local active galaxy population, the host galaxies are ~1 mag fainter than M* and thus are probably late-type systems. The active nuclei are also faint, with M_g ~ -15 to -18 mag, while the bolometric luminosities are close to the Eddington limit. The spectral properties of the sample are compared to the related class of objects known as narrow-line Seyfert 1 galaxies. We discuss the importance of our sample as observational analogues of primordial black holes, contributors to the integrated signal for future gravitational wave experiments, and as a valuable tool in the calibration of the M-sigma relation.Comment: 4 pages, 4 figures. To appear in "The Interplay among Black Holes, Stars and ISM in Galactic Nuclei," Proc. IAU 222 (Gramado, Brazil), eds Th. Storchi Bergmann, L.C. Ho, H.R. Schmit

The Bright and the Dark Side of Malin 1

Malin 1 has long been considered a prototype giant, dark matter dominated Low Surface Brightness galaxy. Two recent studies, one based on a re-analysis of VLA HI observations and the other on an archival Hubble I-band image, throw a new light on this enigmatic galaxy and on its dark/luminous matter properties.Comment: 4 pages, 3 figures, to appear in the Proceedings of the 41st ESLAB Symposium "The Impact of HST on European Astronomy", 29 May to 1 June 2007, ESTEC, Noordwijk, N

TOPEX orbital radiation study

The space radiation environment of the TOPEX spacecraft is investigated. A single trajectory was considered. The external (surface incident) charged particle radiation, predicted for the satellite, is determined by orbital flux integration for the specified trajectory. The latest standard models of the environment are used in the calculations. The evaluation is performed for solar maximum conditions. The spacecraft exposure to cosmic rays of galactic origin is evaluated over its flight path through the magnetosphere in terms of geomagnetic shielding effects, both for surface incident heavy ions and for particles emerging behind different material thickness. Limited shielding and dose evaluations are performed for simple infinite slab and spherical geometries. Results, given in graphical and tabular form, are analyzed, explained, and discussed. Conclusions are presented and commented on

Dynamical Constraints on The Masses of the Nuclear Star Cluster and Black Hole in the Late-Type Spiral Galaxy NGC 3621

NGC 3621 is a late-type (Sd) spiral galaxy with an active nucleus, previously detected through mid-infrared [Ne V] line emission. Archival Hubble Space Telescope (HST) images reveal that the galaxy contains a bright and compact nuclear star cluster. We present a new high-resolution optical spectrum of this nuclear cluster, obtained with the ESI Spectrograph at the Keck Observatory. The nucleus has a Seyfert 2 emission-line spectrum at optical wavelengths, supporting the hypothesis that a black hole is present. The line-of-sight stellar velocity dispersion of the cluster is sigma=43+/-3 km/s, one of the largest dispersions measured for any nuclear cluster in a late-type spiral galaxy. Combining this measurement with structural parameters measured from archival HST images, we carry out dynamical modeling based on the Jeans equation for a spherical star cluster containing a central point mass. The maximum black hole mass consistent with the measured stellar velocity dispersion is 3*10^6 solar masses. If the black hole mass is small compared with the cluster's stellar mass, then the dynamical models imply a total stellar mass of ~1*10^7 solar masses, which is consistent with rough estimates of the stellar mass based on photometric measurements from HST images. From structural decomposition of 2MASS images, we find no clear evidence for a bulge in NGC 3621; the galaxy contains at most a very faint and inconspicuous pseudobulge component (M_K>-17.6 mag). NGC 3621 provides one of the best demonstrations that very late-type spirals can host both active nuclei and nuclear star clusters, and that low-mass black holes can occur in disk galaxies even in the absence of a substantial bulge.Comment: 13 pages, 10 figures. Accepted for publication in Ap

Low-Mass Seyfert 2 Galaxies in the Sloan Digital Sky Survey

(Abridged) We describe a sample of low-mass Seyfert 2 galaxies selected from the Sloan Digital Sky Survey, having a median absolute magnitude of M_g = -19.0 mag. These galaxies are Type 2 counterparts to the Seyfert 1 galaxies with intermediate-mass black holes identified by Greene & Ho (2004). Spectra obtained with the Echellette Spectrograph and Imager at the Keck Observatory are used to determine the central stellar velocity dispersions and to examine the emission-line properties. Overall, the stellar velocity dispersions are low (40-90 km/s), and we find 12 objects having sigma < 60 km/s, a range where very few Seyfert 2 galaxies were previously known. The sample follows the correlation between stellar velocity dispersion and FWHM([OIII]) seen in more massive Seyfert galaxies, indicating that the narrow-line FWHM values are largely determined by virial motion of gas in the central regions of the host galaxies. Using estimates of the black hole masses and AGN bolometric luminosities, we find that these galaxies are typically radiating at a high fraction of their Eddington rate, with a median L_bol/L_Edd = 0.4. We identify one galaxy, SDSS J110912.40+612346.7, as a Type 2 analog of the nearby dwarf Seyfert 1 galaxy NGC 4395, with a nearly identical narrow-line spectrum and a dwarf spiral host of only M_g = -16.8 mag. Forthcoming observations of this sample, including X-ray and mid-infrared spectroscopy, can provide new tests of the obscuring torus model for active galaxies at low luminosities.Comment: 34 pages, 16 figures. Some figures bitmapped to reduce size. To appear in A

Feedback In Luminous Obscured Quasars

We use spatially resolved long-slit spectroscopy from Magellan to investigate the extent, kinematics, and ionization structure in the narrow-line regions of 15 luminous, obscured quasars with z < 0.5. Increasing the dynamic range in luminosity by an order of magnitude, as well as improving the depth of existing observations by a similar factor, we revisit relations between narrow-line region size and the luminosity and linewidth of the narrow emission lines. We find a slope of 0.22 +/- 0.04 for the power-law relationship between size and luminosity, suggesting that the nebulae are limited by availability of gas to ionize at these luminosities. In fact, we find that the active galactic nucleus is effectively ionizing the interstellar medium over the full extent of the host galaxy. Broad (similar to 300-1000 km s(-1)) linewidths across the galaxies reveal that the gas is kinematically disturbed. Furthermore, the rotation curves and velocity dispersions of the ionized gas remain constant out to large distances, in striking contrast to normal and starburst galaxies. We argue that the gas in the entire host galaxy is significantly disturbed by the central active galactic nucleus. While only similar to 10(7)-10(8) M-circle dot worth of gas are directly observed to be leaving the host galaxies at or above their escape velocities, these estimates are likely lower limits because of the biases in both mass and outflow velocity measurements and may in fact be in accord with expectations of recent feedback models. Additionally, we report the discovery of two dual obscured quasars, one of which is blowing a large-scale (similar to 10 kpc) bubble of ionized gas into the intergalactic medium.NSF AST-0548198Astronom

Physical Properties of the Narrow-Line Region of Low-Mass Active Galaxies

We present spectroscopic observations of 27 active galactic nuclei (AGN) with some of the lowest black hole (BH) masses known. We use the high spectral resolution and small aperture of our Keck data, taken with the Echellette Spectrograph and Imager, to isolate the narrow-line regions (NLRs) of these low-mass BHs. We investigate their emission-line properties and compare them with those of AGN with higher-mass black holes. While we are unable to determine absolute metallicities, some of our objects plausibly represent examples of the low-metallicity AGN described by Groves et al. (2006), based on their [N II]/H_alpha ratios and their consistency with the Kewley & Ellison (2008) mass-metallicity relation. We find tentative evidence for steeper far-UV spectral slopes in lower-mass systems. Overall, NLR emission lines in these low-mass AGN exhibit trends similar to those seen in AGN with higher-mass BHs, such as increasing blueshifts and broadening with increasing ionization potential. Additionally, we see evidence of an intermediate line region whose intensity correlates with L/L_Edd, as seen in higher-mass AGN. We highlight the interesting trend that, at least in these low-mass AGN, the [O III] equivalent width (EW) is highest in symmetric NLR lines with no blue wing. This trend of increasing [O III] EW with line symmetry could be explained by a high covering factor of lower ionization gas in the NLR. In general, low-mass AGN preserve many well-known trends in the structure of the NLR, while exhibiting steeper ionizing continuum slopes and somewhat lower gas-phase metallicities.Comment: 46 pages, 14 figures, 7 table