110 research outputs found
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
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
The Planetary Nebula A39: An Observational Benchmark for Numerical Modeling of Photoionized Plasmas
Galactic nebulae are the main probes for the chemical evolution of the interstellar medium. Yet, recent observations have shown that chemical abundances determined from recombination and collisionally excited emission lines can differ by as much as an order of magnitude in some planetary nebulae (PNs). Many PNs have complex geometries and morphological evidence for interactions from stellar winds, and it is not clear to what extent winds, inhomogeneities, or shocked gas affect the observed spectrum. There currently is no full explanation for this discrepancy, which brings into question whether we understand the physical state of these low-density plasmas at all. This paper presents new spectroscopy from the KPNO Mayall 4 m telescope and imagery from the WIYN 3.5 m telescope of A39, a large PN with an exceptionally simple geometry. It appears to be a limb-brightened spherical shell, the simplest possible nebula. There is little evidence for external interactions, so this is the case in which photoionization simulations should be in near-perfect agreement with observation. We combine optical and UV spectroscopy to form a composite spectrum and compare this with photoionization models. No problems were encountered in reproducing the observed spectrum, although even this simple object has two distinct emission-line regions and exhibits several anomalies. A39 was too faint to detect the crucial heavy-element recombination lines in our data set, so it was not possible to compare collisional and recombination abundances. We predict the spectrum over a broad range of bandpasses to facilitate future observations that may probe deeper than our instrumentation allowed
Measuring Column Densities in Quasar Outflows: VLT Observations of QSO 2359-1241
We present high resolution spectroscopic VLT observations of the outflow seen
in QSO 2359-1241. These data contain absorption troughs from five resonance Fe
II lines with a resolution of ~7 km/s and signal-to-noise ratio per resolution
element of order 100. We use this unprecedented high quality data set to
investigate the physical distribution of the material in front of the source,
and by that determine the column densities of the absorbed troughs. We find
that the apparent optical depth model gives a very poor fit to the data and
greatly underestimates the column density measurements. Power-law distributions
and partial covering models give much better fits with some advantage to
power-law models, while both models yield similar column density estimates. The
better fit of the power-law model solves a long standing problem plaguing the
partial covering model when applied to large distance scale outflow: How to
obtain a velocity dependent covering factor for an outflow situated at
distances thousands of time greater than the size of the AGN emission source.
This problem does not affect power-law models. Therefore, based on the better
fit and plausibility of the physical model, we conclude that in QSO 2359-1241,
the outflow covers the full extent of the emission source but in a
non-homogeneous way.Comment: 27 pages, 6 figures, to appear on ApJ Jul 10. The full (online)
version of figure 2 can be obtained here:
http://www.phys.vt.edu/~arav/f2_online_version.p
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