Ground-based studies of emission-line variability: Recent results for NGC 5548 and future plans

Ever since emission-line variability was first detected in AGNs, it has been clearly understood that light-travel time effects in the broadline region (BLR) afford a tool for studying the structure of these spatially unresolved regions. However, only within the last few years has the observational problem become well defined in terms of the sampling rates and quality of data necessary to address the problem correctly. The amount of data required to extract structural information about the BLR from the continuum and emission-line light curves is so considerable that the most promising approach seemed to be to combine the observational efforts. The goal was to work together to produce a large, high-quality database which would then be released to individual investigators for more complete analysis. The cornerstone of this effort, was a joint NASA/ESA/SERC program to monitor the spectrum of NGC 5548 every four days with IUE from December 1988 through August 1988. A concurrent ground-based program was organized in an effort to enhance the scientific return on the project by extending the wavelength coverage and providing higher spectral resolution and signal to noise ratios than would be possible with IUE. It also turned out that the temporal resolution of the ground-based program was somewhat better than the temporal resolution of the IUE program, and the temporal baseline is longer and continues to grow. The database and initial results are presented by Peterson et al., (1991). The important results of the ground-based program are summarized, some additional applications of the existing data are described, and a few comments are made on what can be done in the future to expand on this work

Toward Precision Measurement of Central Black Hole Masses

We review briefly direct and indirect methods of measuring the masses of black holes in galactic nuclei, and then focus attention on supermassive black holes in active nuclei, with special attention to results from reverberation mapping and their limitations. We find that the intrinsic scatter in the relationship between the AGN luminosity and the broad-line region size is very small, ~0.11 dex, comparable to the uncertainties in the better reverberation measurements. We also find that the relationship between reverberation-based black hole masses and host-galaxy bulge luminosities also seems to have surprisingly little intrinsic scatter, ~0.17 dex. We note, however, that there are still potential systematics that could affect the overall mass calibration at the level of a factor of a few.Comment: 10 pages, 2 figures. To be published in the Proceedings of IAU Symposium 267 "Co-Evolution of Central Black Holes and Galaxies

Determining Central Black Hole Masses in Distant Active Galaxies and Quasars. II. Improved Optical and UV Scaling Relationships

We present four improved empirical relationships useful for estimating the central black hole mass in nearby AGNs and distant luminous quasars alike using either optical or UV single-epoch spectroscopy. These mass-scaling relationships between line widths and luminosity are based on recently improved empirical relationships between the broad-line region size and luminosities in various energy bands and are calibrated to the improved mass measurements of nearby AGNs based on emission-line reverberation mapping. The mass-scaling relationship based on the Hbeta line luminosity allows mass estimates for low-redshift sources with strong contamination of the optical continuum luminosity by stellar or non-thermal emission, while that based on the C IV lambda 1549 line dispersion allows mass estimates in cases where only the line dispersion (as opposed to the FWHM) can be reliably determined. We estimate that the absolute uncertainties in masses given by these mass-scaling relationships are typically around a factor of 4. We include in an Appendix mass estimates for all the Bright Quasar Survey (PG) quasars for which direct reverberation-based mass measurements are not available.Comment: 48 pages including 12 figures and 7 tables. Accepted by Ap

Black Hole Masses from Reverberation Measurements

We have reanalyzed in a consistent way existing reverberation data for 35 AGNs for the purpose of refining the black hole masses derived from these data. We find that the precision (or random component of the error) of reverberation-based black hole mass measurements is typically around 30%, comparable to the precision attained in measurement of black hole masses in quiescent galaxies by gas or stellar dynamical methods. As discussed in this volume by Onken et al., we have established an absolute calibration for AGN reverberation-based masses by assuming that AGNs and quiescent galaxies follow an identical relationship between black hole mass and host-galaxy bulge velocity dispersion. The scatter around this relationship implies that the typical systematic uncertainties in reverberation-based black hole masses are smaller than a factor of three. We present a preliminary version of a mass-luminosity relationship that is much better defined than any previous attempt. Scatter about the mass-luminosity relationship for these AGNs appears to be real and could be correlated with either Eddington ratio or source inclination.Comment: To appear in "The Interplay Among Black Holes, Stars, and ISM in Galactic Nuclei," IAU Coll. 222, eds. Storchi-Bergmann, Ho, and Schmitt. 6 pages, 2 figures. Higher resolution versions of the figures are available at http://www.astronomy.ohio-state.edu/~peterson/Astroph

The effects of galaxy interactions on star formation

This dissertation explores the effects of galaxy interactions on star formation through three separate projects. In the first two projects, we examine enhanced star formation by studying the star cluster populations of the interacting galaxies Arp 284 (NGC 7714/5) and Arp 261, using data from the Hubble Space Telescope along with ancillary data from the Spitzer Space Telescope and Galaxy Evolution Explorer to obtain broader wavelength coverage. Combined with Starburst99 evolutionary synthesis models, we estimate the ages and masses of the clusters. The mass and luminosity distributions are found to be in good agreement with other systems from the literature. The clusters in Arp 284 are predominantly young, with ages less than 20 Myr, though observational limits make the significance of this result uncertain. Older clusters, though not numerous, have nearly the same spatial distribution within the imaged portion of NGC 7714 as young clusters. The cluster population in the bridge connecting the galaxies appears to be older, though the data in this part of the system are too limited to draw firm conclusions. The ages of the giant H II regions in NGC 7714 are generally older than those of their constituent clusters, possibly indicating that the young clusters we detect are surrounded by their dispersed predecessors. We call this the jewels in the crown effect. The age distribution of the Arp 261 cluster population is more difficult to interpret because the metallicity of the galaxies is currently unknown, making the ages highly uncertain. Despite these uncertainties, it is clear that the majority of the clusters have ages ~20 Myr or less. We also find more evidence of the jewels in the crown effect in this system. The cluster age distributions in the features of this system have significant implications for its dynamical history. Radio data from the NVSS already indicates that the Taffy-like collision scenario suggested by the optical morphology may not be correct. Analysis of optical spectra, which have already been obtained, will allow us to determine the metallicity of the galaxies and improve our estimates of the cluster ages. In the final project, we examine the suppression of star formation in the bridge between the Taffy galaxies using strong, resolved emission from warm H2. Relative to the continuum and faint PAH emission, the H2 emission in the system is the strongest in the bridge, where the purely rotational lines of H2 dominate the mid-infrared spectrum in a way very reminiscent of the group-wide shock in the strongly interacting group Stephan\u27s Quintet. We use excitation diagrams to characterize the warm molecular gas, finding an average surface mass of ~5 Msol pc-2 and typical excitation temperatures of 150-175~K. H2 emission is also seen in the galaxy disks, although there the emission is more consistent with that seen in normal star forming galaxies. We investigate several possible heating mechanisms for the bridge gas, but favor the conversion of mechanical energy from the head-on collision via turbulence and shocks as the main heating source. Since the cooling time for the warm H2 is short, shocks must be permeating the bridge region in order to continue heating the molecular hydrogen

Keplerian Motion of Broad-Line Region Gas as Evidence for Supermassive Black Holes in Active Galactic Nuclei

Emission-line variability data on NGC 5548 argue strongly for the existence of a mass of order 7 x 10^7 solar masses within the inner few light days of the nucleus in the Seyfert 1 galaxy NGC 5548. The time-delayed response of the emission lines to continuum variations is used to infer the size of the line-emitting region, and these determinations are combined with measurements of the Doppler widths of the variable line components to estimate a virial mass. The data for several different emission lines spanning an order of magnitude in distance from the central source show the expected V proportional to r^{-1/2} correlation and are consistent with a single value for the mass.Comment: 9 pages, 2 Figures. accepted by ApJ Letter