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

Selection of Forage and Avoidance of Predation Risk by Partially Migratory Mule Deer

Migration by ungulates has traditionally been thought of as a strategy that increases access to forage quality or reduces exposure to risk of predation, but the benefits of migration may be waning globally. In partially migratory populations, the persistence of both migrant and resident strategies is an intriguing ecological phenomenon, because migrants and residents often face contrasting fitness consequences. Partial migration is common in mule deer (Odocoileus hemionus), a species that has experienced widespread declines across the western United States during recent decades. Mule deer seldom switch between migratory strategies throughout their lifetime, which may make them less resilient to environmental change than more behaviorally plastic ungulate species. To indicate the mechanisms maintaining partial migration, we investigated how predation risk, forage quality, and habitat selection in relation to these factors varied between migrant and resident mule deer. First, we developed resource selection functions (RSFs) for wolves and mountain lions to estimate predation risk. Then, we modeled forage quality throughout mule deer summer ranges. We then compared forage quality (kcal/m2) and predation risk in migrant and resident summer ranges of 3 partially migratory populations across Western Montana. We found no substantial differences in forage quality between migrant and resident summer ranges, and predation risk did not differ predictably between the 2 groups. We used RSFs to assess how home range (2nd order) and within-home range (3rd order) selection varied between migrants and residents. At the 2nd order, neither migrants or residents selected forage or avoided wolf predation risk, but both groups avoided mountain lion predation risk. At the 3rd order, both migrants and residents selected for forage and avoided wolf and mountain lion predation risk. Given their exposure to similar forage and risk conditions between groups, and similar habitat selection patterns, our results suggest that the benefits of a migrant strategy did not outweigh those of a resident strategy during our study. Within mule deer populations, partial migration may be maintained due to changes in the relative benefits of migration over time. Mule deer behavior was consistent across different ecosystem types and migratory strategies, suggesting a general mechanism for summer habitat selection may exist for mule deer in forested environments of the Northern Rockies

Evidence for Supermassive Black Holes in Active Galactic Nuclei from Emission-Line Reverberation

Emission-line variability data for Seyfert 1 galaxies provide strong evidence for the existence of supermassive black holes in the nuclei of these galaxies, and that the line-emitting gas is moving in the gravitational potential of that black hole. The time-delayed response of the emission lines to continuum variations is used to infer the size of the line-emitting region, which is then combined with measurements of the Doppler widths of the variable line components to estimate a virial mass. In the case of the best-studied galaxy, NGC 5548, various emission lines spanning an order of magnitude in distance from the central source show the expected velocity proportional to inverse square root of the distance correlation between distance and line width, and are thus consistent with a single value for the mass. Two other Seyfert galaxies, NGC 7469 and 3C 390.3, show a similar relationship. We compute the ratio of luminosity to mass for these three objects and the narrow-line Seyfert 1 galaxy NGC 4051 and find that that the gravitational force on the line-emitting gas is much stronger than radiation pressure. These results strongly support the paradigm of gravitationally bound broad emission-line region clouds.Comment: 10 pages, 2 figures, Accepted for publication in Astrophysical Journal Letter

Long term variability of the Broad Emission Line profiles in AGN

Results of a long-term monitoring ($\gtrsim 10$ years) of the broad line and continuum fluxes of three Active Galactic Nuclei (AGN), 3C 390.3, NGC 4151, and NGC 5548, are presented. We analyze the H$\alpha$ and H$\beta$ profile variations during the monitoring period and study different details (as bumps, absorption bands) which can indicate structural changes in the Broad Line Region (BLR). The BLR dimensions are estimated using the time lags between the continuum and the broad lines flux variations. We find that in the case of 3C 390.3 and NGC 5548 a disk geometry can explain both the broad line profiles and their flux variations, while the BLR of NGC 4151 seems more complex and is probably composed of two or three kinematically different regions.Comment: 10 pages, 9 figures, New Astronomy Reviews (Proceeding of 7th SCSLSA), in pres

On the size of the Fe II emitting region in the AGN Akn 120

We present a reverberation analysis of the strong, variable optical Fe II emission bands in the spectrum of Akn 120, a low-redshift AGN which is one of the best candidates for such a study. On time scales of several years the Fe II line strengths follow the variations in the continuum strength. However, we are unable to measure a clear reverberation lag time for these Fe II lines on any time scale. This is due to the very broad and flat-topped nature of the Fe II cross correlation functions, as compared to the H-beta response which is much more sharply localized in time. Although there is some suggestion in the light curve of a 300-day response time, our statistical analysis does not pick up such a feature. We conclude that the optical Fe II emission does not come from a photoionization-powered region similar in size to the H-beta emitting region, but we cannot say for sure where it does come from. Our results are generally consistent either with emission from a photoionized region several times larger than the H-beta zone, or with emission from gas heated by some other means, perhaps responding only indirectly to the continuum variations.Comment: Accepted for publication in the Ap

Size-Mass-luminosity relations in AGN and the role of the accretion disc

We address the question of the relations between the black hole's mass, the accretion rate, the bolometric luminosity, the optical luminosity and the size of the Broad Line Region (BLR) in Active Galactic Nuclei, using recent observational data obtained from monitoring campaigns. We show that a standard accretion disc cannot account for the observed optical luminosity, unless it radiates at super-Eddington rates. This implies the existence of another, dominant emission mechanism in the optical range, or a non standard disc (non stationary, ADAF and/or strong outflows). Narrow Line Seyfert 1 galaxies (NLS1s) are most extreme in this context: they have larger bolometric to Eddington luminosity ratios than Broad Line Seyfert 1 (BLS1s), and most likely a larger "non disc" component in the optical range. From realistic simulations of self-gravitating $\alpha$-discs, we have systematically localized the gravitationally unstable disc and shown that, given uncertainties on both the model and observations, it coincides quite well with the size of the BLR. We therefore suggest that the gravitationally unstable disc is the source which releases BLR clouds in the medium. However the influence of the ionization parameter is also required to explain the correlation found between the size of the BLR and the luminosity. In this picture the size of the BLR in NLS1s (relative to the black hole size) is larger (and the emission line width smaller) than in BLS1s simply because their Eddington ratio is larger.Comment: 9 pages, 8 figures, accepted in A &

On reverberation and cross-correlation estimates of the size of the broad-line region in active galactic nuclei

It is known that the dependence of the emission-line luminosity of a typical cloud in the active galactic nuclei broad-line regions (BLRs) upon the incident flux of ionizing continuum can be nonlinear. We study how this nonlinearity can be taken into account in estimating the size of the BLR by means of the "reverberation" methods. We show that the BLR size estimates obtained by cross-correlation of emission-line and continuum light curves can be much (up to an order of magnitude) less than the values obtained by reverberation modelling. This is demonstrated by means of numerical cross-correlation and reverberation experiments with model continuum flares and emission-line transfer functions and by means of practical reverberation modelling of the observed optical spectral variability of NGC 4151. The time behaviour of NGC 4151 in the H_alpha and H_beta lines is modelled on the basis of the observational data by Kaspi et al. (1996, ApJ, 470, 336) and the theoretical BLR model by Shevchenko (1984, Sov. Astron. Lett., 10, 377; 1985, Sov. Astron. Lett., 11, 35). The values of the BLR parameters are estimated that allow to judge on the size and physical characteristics of the BLR. The small size of the BLR, as determined by the cross-correlation method from the data of Kaspi et al. (1996, ApJ, 470, 336), is shown to be an artifact of this method. So, the hypothesis that the BLR size varies in time is not necessitated by the observational data.Comment: 26 pages, including 11 figure

A Look at What Is (and Isn't) Known About Quasar Broad Line Regions and How Narrow-Line Seyfert 1 Galaxies Fit In

The evidence is reviewed that the Broad Line Region (BLR) probably has two distinct components located at about the same distance from the central black hole. One component, BLR II, is optically-thick, low-ionization emission at least some of which arises from a disc and the other, BLR I, is probably optically-thin emission from a more spherically symmetric halo or atmosphere. The high Fe II/H-beta ratios seen in Narrow-Line Seyfert 1 galaxies (NLS1s) are not due to strong Fe II emission, as is commonly thought, but to unusually weak Balmer emission, probably caused by higher densities. NLS1s probably differ from non-NLS1s because of the higher density of gas near the black hole. This produces a higher accretion rate, a denser BLR, and a view of the central regions that is more face-on.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho

Are quasars accreting at super-Eddington rates?

In a previous paper, Collin & Hur\'e (2001), using a sample of Active Galactic Nuclei (AGN) where the mass has been determined by reverberation studies (Kaspi et al. 2000), have shown that if the optical luminosity is emitted by a steady accretion disc, about half of the objects are accreting close to or higher than the Eddington rate. We conclude here that this result is unavoidable, unless the masses are strongly underestimated by reverberation studies, which does not seem to be the case. There are three issues to the problem: 1. Accretion proceeds at Eddington or super-Eddington rates through thick discs. Several consequences follow: an anti-correlation between the line widths of the lines and the Eddington ratios, and a decrease of the Eddington ratio with an increasing black hole mass. Extrapolated to all quasars, these results imply that the amount of mass locked in massive black holes should be larger than presently thought. 2. The optical luminosity is not produced directly by the gravitational release of energy, and super-Eddington rates are not required. The optical luminosity has to be emitted by a dense and thick medium located at large distances from the center (10$^3$ to $10^4$ gravitational radii). It can be due to reprocessing of the X-ray photons from the central source in a geometrically thin warped disc, or in dense "blobs" forming a geometrically thick system, which can be a part of the accretion flow or the basis of an outflow. 3. Accretion discs are completely "non standard". Presently neither the predictions of models nor the observed spectral distributions are sufficient to help choosing between these solutions.Comment: 16 pages, 11 figures, accepted in A&