Quasar Tomography: Unification of Echo Mapping and Photoionisation Models

Reverberation mapping uses time-delayed variations in photoionised emission lines to map the geometry and kinematics of emission-line gas in active galactic nuclei. In previous work, the light travel time delay tau=R(1+cos(theta))/c and Doppler shift v give a 2-d map Psi(tau,v) for each emission line. Here we combine the velocity-delay information with photoionisation physics in a maximum entropy fit to the full reverberating spectrum F_lam(lam,t) to recover a 5-d map of the differential covering fraction f(R,theta,n,N,v), with n and N the density and column density of the gas clouds. We test the method for a variety of geometries (shells, rings, disks, clouds, jets) by recovering a 3-d map f(R,theta,n) from reverberations in 7 uv emission lines. The best test recovers a hollow shell geometry, defining R to 0.15 dex, n to 0.3 dex, and ionisation parameter U ~ 1/(n R^2) to 0.1 dex. The results are sensitive to the adopted distance and luminosity, suggesting that these parameters may be measurable as well.Comment: Accepted 4 Sep 2002 for publication in MNRA

The Ultraviolet-Optical Albedo of Broad Emission Line Clouds

We explore the effective UV-optical albedos of a variety of types of broad emission line clouds, as well as their possible effects on the observed spectra of active galactic nuclei (AGNs). An important albedo source in moderately ionized ionization-bounded clouds is that which is due to neutral hydrogen: Rayleigh scattering of continuum photons off the extreme damping wings of Lyα. The photons resulting from this scattering mechanism may contribute significantly to the Lyα emission line, especially in the very broad wings. In addition, line photons emitted near 1200 Å (e.g., N V λ1240) that stream toward the neutral portion of the cloud may be reflected off this Rayleigh scattering mirror, so that they preferentially escape from the illuminated face. Inclusion of this effect can alter predicted emission-line strengths and profiles. In more highly ionized ionization-bounded clouds, Thomson scattering dominates the UV-optical albedo, but this albedo is lessened by the hydrogen gas opacity; these clouds are most reflective on the long-wavelength side of the hydrogen recombination edges. This feature may then alter the shapes of the spectral regions near the recombination edges, e.g., the Balmer jump. We illustrate the effects of gas density and line broadening on the effective albedo. We also discuss the reflection effects of the accretion disk and the dusty torus.\u27\u27 The accretion disk is an effective reflector of UV-optical photons, whether by electron or Rayleigh scattering, and it is possible that we observe a significant fraction of this light from the AGNs in reflection. This effect can alter the emission-line profiles and even destroy emission at the Lyman jump emitted by broad-line clouds. Finally, we discuss the possibility that continuum reflection from broad-line clouds is at least in part responsible for the polarized broad absorption line troughs

Iron Abundance Diagnostics in High-Redshift QSOs

The abundance of alpha-process elements such as magnesium and carbon relative to iron measured from the broad emission lines of QSOs can serve as a diagnostic of the star formation and chemical enrichment histories of their host galaxies. We investigate the relationship between Fe/Mg and Fe/C abundance ratios and the resulting Fe II / Mg II 2800A and Fe II / 1900A-blend flux ratios, both of which have been measured in QSOs out to redshifts of approximately six. Using a galactic chemical evolution model based on a starburst in a giant elliptical galaxy, we find that these flux ratios are good tracers of the chemical enrichment of the nuclei. However, the values of these ratios measured in objects at redshifts of approximately six suggest that iron enrichment has occurred more rapidly in these objects than predicted by the assumed elliptical starburst model, under currently favored cosmologies.Comment: 2 pages, to appear in proceedings of IAU Symposium No. 222, The Interplay Among Black Holes, Stars and ISM in Galacti Nucle

On the Column Density of AGN Outflows: the Case of NGC 5548

We re-analyze the HST high resolution spectroscopic data of the intrinsic absorber in NGC 5548 and find that the C IV absorption column density is at least four times larger than previously determined. This increase arises from accounting for the kinematical nature of the absorber and from our conclusion that the outflow does not cover the narrow emission line region in this object. The improved column density determination begins to bridge the gap between the high column densities measured in the X-ray and the low ones previously inferred from the UV lines. Combined with our findings for outflows in high luminosity quasars these results suggest that traditional techniques for measuring column densities: equivalent width, curve-of-growth and Gaussian modeling, are of limited value when applied to absorption associated with AGN outflows.Comment: Published ApJ version (566, 699), including a new figure with FUSE data and a useful algebraic expression for the optical depth solutio

Locally Optimally-emitting Clouds and the Narrow Emission Lines in Seyfert Galaxies

The narrow emission line spectra of active galactic nuclei are not accurately described by simple photoionization models of single clouds. Recent Hubble Space Telescope images of Seyfert 2 galaxies show that these objects are rich with ionization cones, knots, filaments, and strands of ionized gas. Here we extend to the narrow line region the ``locally optimally emitting cloud'' (LOC) model, in which the observed spectra are predominantly determined by powerful selection effects. We present a large grid of photoionization models covering a wide range of physical conditions and show the optimal conditions for producing many of the strongest emission lines. We show that the integrated narrow line spectrum can be predicted by an integration of an ensemble of clouds, and we present these results in the form of diagnostic line ratio diagrams making comparisons with observations. We also predict key diagnostic line ratios as a function of distance from the ionizing source, and compare these to observations. The predicted radial dependence of the [O III]/[O II] ratio may be matched to the observed one in NGC4151, if the narrow line clouds see a more intense continuum than we see. The LOC scenario when coupled with a simple Keplerian gravitational velocity field will quite naturally predict the observed line width versus critical density relationship. The influence of dust within the ionized portion of the clouds is discussed and we show that the more neutral gas is likely to be dusty, although a high ionization dust-free region is most likely present too. This argues for a variety of NLR cloud origins.Comment: 29 pages plus 16 figures, accepted for publication in Ap

Quasars as Cosmological Probes: The Ionizing Continuum, Gas Metallicity, and the \u3cem\u3eW\u3csup\u3eλ\u3c/sup\u3e-L\u3c/em\u3e Relation

Using a realistic model for line emission from the broad emission line regions of quasars, we are able to reproduce the previously observed correlations of emission-line ratios with the shape of the spectral energy distribution (SED). In agreement with previous studies, we find that the primary driving force behind the Baldwin effect (Wλ ~ Lβ, β \u3c 0) is a global change in the SED with quasar luminosity, in that more luminous quasars must have characteristically softer ionizing continua. This is completely consistent with observations that show (1) a correlation between Luv, αox, and αuvx (2) correlations of SED shape-sensitive line ratios with αox, αuvx, and Luv; and (3) correlations between line equivalent widths and αox, αuvx, and Luv. However, to explain the complete lack of a correlation in the Wλ(N V)-Luv diagram, we propose that the more luminous quasars have characteristically larger gas metallicities (Z). As a secondary element, nitrogen\u27s rapidly increasing abundance with increasing Z compensates for the losses in Wλ(N V) emitted by gas illuminated by softer continua in higher luminosity quasars. A characteristic relationship between Z and L has an impact on the Wλ-Luv relations for other lines as well. For a fixed SED, an increasing gas metallicity reduces the Wλ of the stronger metal lines (the gas cools), as well as that of Lyα and especially He II (because of the increasing metal opacity), while the weaker lines (e.g., C III] λ1909) generally respond positively. The interplay between the effects of a changing SED and Z with L results in the observed luminosity-dependent spectral variations. All of the resulting dependences on Luv are within the range of the observed slopes