Benjamin C. Bromley

Spectral Classification and Luminosity Function of Galaxies in the Las Campanas Redshift Survey

We construct a spectral classification scheme for the galaxies of the Las Campanas Redshift Survey (LCRS) based on a principal-component analysis of the measured galaxy spectra. We interpret the physical significance of our six spectral types and conclude that they are sensitive to morphological type and to the amount of active star formation. In this first analysis of the LCRS to include spectral classification, we estimate the general luminosity function, expressed as a weighted sum of the type-specific luminosity functions. In the R-band magnitude range of -23 < M ? -16.5 this function exhibits a broad shoulder centered on M ? -20 and an increasing faint-end slope that formally converges on ? ? -1.8 in the faint limit. The Schechter parameterization does not provide a good representation in this case, a fact that may partly explain the reported discrepancy between the luminosity functions of the LCRS and other redshift catalogs such as the Century Survey. The discrepancy may also arise from environmental effects such as the density-morphology relationship for which we see strong evidence in the LCRS galaxies. However, the Schechter parameterization is more effective for the luminosity functions of the individual spectral types. The data show a significant, progressive steepening of the faint-end slope, from ? ? +0.5 for early-type objects to ? ? -1.8 for the extreme late-type galaxies. The extreme late-type population has a sufficiently high space density that its contribution to the general luminosity function is expected to dominate at magnitudes fainter than M = -16. We conclude that an evaluation of type dependence is essential to any assessment of the general luminosity function.

Line Emission from an Accretion Disk around a Rotating Black Hole: Toward a Measurement of Frame Dragging

Line emission from an accretion disk and a corotating hot spot about a rotating black hole are considered for possible signatures of the frame-dragging effect. We explicitly compare integrated line profiles from a geometrically thin disk about a Schwarzschild and an extreme Kerr black hole, and show that the line profile differences are small if the inner radius of the disk is near or above the Schwarzschild stable-orbit limit of radius 6GM/c2. However, if the inner disk radius extends below this limit, as is possible in the extreme Kerr spacetime, then differences can become significant, especially if the disk emissivity is stronger near the inner regions. We demonstrate that the first three moments of a line profile define a three-dimensional space in which the presence of material at small radii becomes quantitatively evident in broad classes of disk models. In the context of the simple, thin disk paradigm, this moment-mapping scheme suggests formally that the iron line detected by the Advanced Satellite for Cosmology and Astrophysics mission from MCG -6-30-15 (Tanaka et al.) is ~3 times more likely to originate from a disk about a rotating black hole than from a Schwarzschild system. A statistically significant detection of black hole rotation in this way may be achieved after only modest improvements in the quality of data. We also consider light curves and frequency shifts in line emission as a function of time for corotating hot spots in extreme Kerr and Schwarzschild geometries. The frequency-shift profile is a valuable measure of orbital parameters and might possibly be used to detect frame dragging even at radii approaching 6GM/c2 if the inclination angle of the orbital plane is large. The light curve from a hot spot shows differences as well, although these too are pronounced only at large inclination angles.

The inner edge of the accretion disk around a supermassive black hole

Massive black holes are generally thought to exist at the centres of galaxies, but an unambiguous identification of a black hole has been impeded by a lack of evidence for the strong-field relativistic effects expected in the vicinity of such an object. Several years ago, a very broad iron emission line was discovered in the active galaxy MCG-6-30-15, indicative of emission from an accretion disk near the event horizon of a black hole. But this interpretation, based on the line profile, was somewhat model dependent (refs 2–5). Here we present an analysis of the iron-line emission from MCG-6-30-15 that is insensitive to the details (for example, diskthickness and emissivity) of the disk model used. We find that the inner edge of the disk material giving rise to the line is within 2.6 ± 0.3 times the Schwarzschild radius—the event horizon of a non-rotating black hole—at the 95% confidence level. Changes to the disk parameters can only decrease the inner radius of the emitting region, and so we can be confident that we are observing emission from gravitationally bound material in the strong-field region of a supermassive black hole. Moreover, we find that the black hole is rotating at a rate which is [gsims]23 ± 17% of the theoretical maximum, although this conclusion is model dependent.

Line Emission from an Accretion Disk Around a Black Hole: Effects of Disk Structure

The observed iron Kα fluorescence lines in Seyfert I galaxies provide strong evidence for an accretion disk near a supermassive black hole as a source of the line emission. These lines serve as powerful probes for examining the structure of inner regions of accretion disks. Previous studies of line emission have considered only geometrically thin disks, where the gas moves along geodesics in the equatorial plane of a black hole. Here we extend this work to consider the effects on line profiles from finite disk thickness, radial accretion flow, and turbulence. We adopt the Novikov-Thorne α-disk model and find that within this framework turbulent broadening is the dominant new effect. The most prominent change in the skewed, double-horned line profiles is a substantial reduction in the maximum flux at both red and blue peaks. The effect is most pronounced when the inclination angle is large and when the accretion rate is high. Thus, the effects discussed here may be important for future detailed modeling of high-quality observational data.

Estimating Ω from Galaxy Redshifts: Linear Flow Distortions and Nonlinear Clustering

We propose a method to determine the cosmic mass density Ω from redshift-space distortions induced by large-scale flows in the presence of nonlinear clustering. Nonlinear structures in redshift space, such as fingers of God, can contaminate distortions from linear flows on scales as large as several times the small-scale pairwise velocity dispersion σv. Following Peacock & Dodds, we work in the Fourier domain and propose a model to describe the anisotropy in the redshift-space power spectrum; tests with high-resolution numerical data demonstrate that the model is robust for both mass and biased galaxy halos on translinear scales and above. On the basis of this model, we propose an estimator of the linear growth parameter β = Ω0.6/b, where b measures bias, derived from sampling functions that are tuned to eliminate distortions from nonlinear clustering. The measure is tested on the numerical data and found to recover the true value of β to within ~10%. An analysis of IRAS 1.2 Jy galaxies yields β=0.8+0.4-0.3 at a scale of 1000 km s-1, which is close to optimal given the shot noise and finite size of the survey. This measurement is consistent with dynamical estimates of β derived from both real-space and redshift-space information. The importance of the method presented here is that nonlinear clustering effects are removed to enable linear correlation anisotropy measurements on scales approaching the translinear regime. We discuss implications for analyses of forthcoming optical redshift surveys in which the dispersion is more than a factor of 2 greater than in the IRAS data.

Spectral line signatures of relativistic disks

We consider emission line formation in a thin accretion disk around a black hole, taking into account the differential flow of material in the disk. If the disk is optically thick in the line, local velocity gradients can cause the integrated intensity to have azimuthal dependence in the emitter frame. We examine this effect with simple parameterized models based on Sobolev theory to highlight the overall character of the changes in the observed line profile. The shape of the profile, which can serve as a diagnostic of the disk geometry and the spin of the black hole, may be significantly altered by the velocity-gradient effect. Specifically, we find that the effect causes a decrease of flux in the blue Doppler peak, which in turn would lead to an underestimate of the inner disk radius. If the inner radius were used as a signature of black hole rotation, as when the disk is not emissive within the marginally stable circular orbit, then the inferred rotation would be overestimated in cases where the emissi...

Line emission from an accretion disk around black hole: effects of the disk structure

The observed iron Kα fluorescence lines in Seyfert galaxies provide strong evidence for an accretion disk near a supermassive black hole as a source of the line emission. Previous studies of line emission have considered only geometrically thin disks, where the gas moves along geodesics in the equatorial plane of a black hole. Here we extend this work to include effects on line profiles from finite disk thickness, radial accretion flow and turbulence. We adopt the Novikov-Thorne solution, and find that within this framework, turbulent broadening is the most significant effect. The most prominent changes in the skewed, double-horned line profiles is a substantial reduction in the maximum flux at both red and blue peaks. We show that at the present level of signal-to-noise in X-ray spectra, proper treatment of the actual structure of the accretion disk can change estimates of the inclination angle of the disk. Thus these effects will be important for future detailed modeling of high quality observational data.The observed iron Kα fluorescence lines in Seyfert galaxies provide strong evidence for an accretion disk near a supermassive black hole as a source of the line emission. Previous studies of line emission have considered only geometrically thin disks, where the gas moves along geodesics in the equatorial plane of a black hole. Here we extend this work to include effects on line profiles from finite disk thickness, radial accretion flow and turbulence. We adopt the Novikov-Thorne solution, and find that within this framework, turbulent broadening is the most significant effect. The most prominent changes in the skewed, double-horned line profiles is a substantial reduction in the maximum flux at both red and blue peaks. We show that at the present level of signal-to-noise in X-ray spectra, proper treatment of the actual structure of the accretion disk can change estimates of the inclination angle of the disk. Thus these effects will be important for future detailed modeling of high quality observational data.

Bounds on the inner radius of emission around supermassive black holes

Observations of iron K-alpha fluorescence lines in Seyfert galaxies provide strong evidence for an accretion disk around a supermassive black hole as the source of the line emission. Here we consider a diagnostic of a disk line profile, based on the minimum and maximum frequency of emission, to robustly constrain the inner radius of the disk. This diagnostic is applied to spectra from Seyfert I galaxies (Iwasawa et al. 1996 [1]; Nandra et al. 1997 [2]), and a composite Seyfert II spectrum (Turner et al. 1997 [3]). We are able to place firm bounds on the inner radius of emission from the bright nucleus in MCG–6–30–15; we find that the disk extends to within 6.2 Rg of the central black hole, independent of inclination angle (Rg is GM/c2 for a black hole of mass M). With inclination angle constraints—we find 29±5° from frequency extrema of the [1] data—the inner radius bounded by 5.2±0.6 Rg. The frequency extrema of composite Seyfert I and II spectra both show inner radii below about 20 Rg and are consistent...

Estimation of relativistic accretion disk parameters from iron line emission

The observed iron Kα fluorescence lines in Seyfert I galaxies provide strong evidence for an accretion disk near a supermassive black hole as a source of the line emission. Here we present an analysis of the geometrical and kinematic properties of the disk based on the extreme frequency shifts of a line profile as determined by the measurable flux in both the red and blue wings. The edges of the line are insensitive to the distribution of the X-ray flux over the disk, and hence provide a robust alternative to profile fitting of disk parameters. Our approach yields new, strong bounds on the inclination angle of the disk and the location of the emitting region. We apply our method to interpret the observational data from a few Seyfert I nuclei.