189 research outputs found
Are the Narrow Line Regions in Active Galaxies Dusty and Radiation Pressure Dominated?
The remarkable similarity between emission spectra of narrow line regions
(NLR) in Seyfert Galaxies has long presented a mystery. In photoionization
models, this similarity implies that the ionization parameter is nearly always
the same, about U ~ 0.01. Here we present dusty, radiation-pressure dominated
photoionization models that can provide natural physical insight into this
problem. In these models, dust and the radiation pressure acting on it provide
the controlling factor in moderating the density, excitation and surface
brightness of photoionized NLR structures. Additionally, photoelectric heating
by the dust is important in determining the temperature structure of the
models. These models can also explain the coexistence of the low-,
intermediate- and coronal ionization zones within a single self-consistent
physical structure. The radiation pressure acting on dust may also be capable
of driving the fast (~3000 km/s) outflows such as are seen in the HST
observations of NGC 1068.Comment: 23 pages, 8 figures, Accepted by Ap
Spatially extended absorption around the z=2.63 radio galaxy MRC 2025-218: outflow or infall?
We present an investigation into the absorber in front of the z=2.63 radio
galaxy MRC 2025-218, using integral field spectroscopy obtained at the Very
Large Telescope, and long slit spectroscopy obtained at the Keck II telescope.
The properties of MRC 2025-218 are particularly conducive to study the nature
of the absorbing gas, i.e., this galaxy shows bright and spatially extended
Ly-alpha emission, along with bright continuum emission from the active
nucleus. Ly-alpha absorption is detected across ~40x30 kpc^2, has a covering
factor of ~1, and shows remarkably little variation in its properties across
its entire spatial extent. This absorber is kinematically detached from the
extended emission line region (EELR). Its properties suggest that the absorber
is outside of the EELR. We derive lower limits to the HI, HII and H column
densities for this absorber of 3x10^16, 7x10^17 and 2x10^18 cm^-2,
respectively. Moreover, the relatively bright emission from the active nucleus
has allowed us to measure a number of metal absorption lines: CI, CII, CIV, NV,
OI, SiII, SiIV, AlII and AlIII. The column density ratios are most naturally
explained using photoionization by a hard continuum, with an ionization
parameter U~0.0005-0.005. Shocks or photoionization by young stars cannot
reproduce satisfactorily the measured column ratios. Using the ratio between
the SiII* and SiII column densities, we derive a lower limit of >10 cm^-3 for
the electron density of the absorber. The data do not allow useful constraints
to be placed on the metallicity of the absorber. We consider two possibilities
for the nature of this absorber: the cosmological infall of gas, and an outflow
driven by supernovae or the radio-jets.Comment: Accepted for publication in MNRA
Ionized Absorbers in AGN: the Role of Collisional Ionization and Time-Evolving Photoionization
In this paper we explore collisional ionization and time-evolving
photoionization in the, X-ray discovered, ionized absorbers in Seyfert
galaxies. These absorbers show temporal changes inconsistent with simple
equilibrium models. We develop a simple code to follow the temporal evolution
of non-equilibrium photoionized gas. As a result several effects appear that
are easily observable; and which, in fact, may explain otherwise paradoxical
behavior.
Specifically we find that: 1) In many important astrophysical conditions pure
collisional and photoionization equilibria can be distinguished with moderate
spectral resolution observations, due to a strong absorption structure between
1 and 3 keV. 2) In time-evolving non-equilibrium photoionization models the
response of the ionization state of the gas to sudden changes of the ionizing
continuum is smoothed and delayed at low gas densities, even when the
luminosity increases. 3) If the changes of the ionizing luminosity are not
instantaneous, and the electron density is low enough (the limit depends on the
average ionization state of the gas), the ionization state of the gas can
continue to increase while the source luminosity decreases, so a maximum in the
ionization state of a given element may occur during a minimum of the ionizing
intensity (the opposite of the prediction of equilibrium models). 4) Different
ions of different elements reach their equilibrium configuration on different
time-scales.
These properties are similar to those seen in several ionized absorbers in
AGN, properties which had hitherto been puzzling. We applied these models to a
high S/N ROSAT PSPC observation of the Seyfert 1 galaxy NGC 4051.Comment: 36 pages, 10 figures, accepted for publication on Apj, in pres
Gas and Dust Emission from the Nuclear Region of the Circinus Galaxy
Simultaneous modeling of the line and continuum emission from the nuclear
region of the Circinus galaxy is presented. Composite models which include the
combined effect of shocks and photoionization from the active center and from
the circumnuclear star forming region are considered. The effects of dust
reradiation, bremsstrahlung from the gas and synchrotron radiation are treated
consistently. The proposed model accounts for two important observational
features. First, the high obscuration of Circinus central source is produced by
high velocity and dense clouds with characteristic high dust-to-gas ratios.
Their large velocities, up to 1500 km\s, place them very close to the active
center. Second, the derived size of the line emitting region is well in
agreement with the observed limits for the coronal and narrow line region of
Circinus.Comment: 36 pages, LaTex (including 4 Tables and 9 figures), removed from
Abstract To appear in "The Astrophysical Journal
Spatial Resolution of High-Velocity Filaments in the Narrow-Line Region of NGC 1068: Associated Absorbers Caught in Emission?
Using the HST STIS spectrograph we have obtained a grid of [O III] and H-beta
emission-line spectra at 0"05x0"19 and 60 km/s (FWHM) resolution that covers
much of the NLR of NGC 1068. We find emitting knots that have blueshifted
radial velocities up to 3200 km/s relative to galaxy systemic, are 70-150 pc NE
of the nucleus and up to 40 pc from the radio jet, emit several percent of the
NLR line flux but no significant continuum, span velocity extents of up to 1250
km/s but a small fraction of the sky seen from the nucleus, coincide with a
region of enhanced IR coronal-line emission, and have ionized masses 200
Msun/ne4 (ne4=10^4 cm^{-3}). We argue that the blueshifted knots are ablata
from disintegrating molecular clouds that are being photoionized by the AGN,
and are being accelerated readiatively by the AGN or mechanically by the radio
jet. In their kinematic properties, the knots resemble the associated absorbers
seen projected on the UV continua of some AGN. Between 2"5-4"5 from the
nucleus, emission is redshifted relative to systemic, a pattern that we
interpret as gas in the galaxy disk being pushed away from us by the NE radio
lobe.Comment: 13 pages LaTeX w/ convenient embedded EPS figs, scheduled for ApJ
April 1/0
Infrared spectroscopy of NGC 1068: Probing the obscured ionizing AGN continuum
The ISO-SWS 2.5-45 um infrared spectroscopic observations of the nucleus of
the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a
compilation of UV to IR narrow emission line data to determine the spectral
energy distribution (SED) of the obscured extreme-UV continuum that
photoionizes the narrow line emitting gas in the active galactic nucleus. We
search a large grid of gas cloud models and SEDs for the combination that best
reproduces the observed line fluxes and NLR geometry. Our best fit model
reproduces the observed line fluxes to better than a factor of 2 on average and
is in general agreement with the observed NLR geometry. It has two gas
components that are consistent with a clumpy distribution of dense outflowing
gas in the center and a more extended distribution of less dense and more
clumpy gas farther out that has no net outflow. The best fit SED has a deep
trough at ~4 Ryd, which is consistent with an intrinsic Big Blue Bump that is
partially absorbed by ~6x10^19 cm^-2 of neutral hydrogen interior to the NLR.Comment: 15 pp, 4 figures, ApJ accepte
The Extended Line Region of 3C 299
We present results of HST observations of the radio galaxy 3C 299. The
broad-band F702W (R) and F555W (V) images (WFPC2/PC) show an elliptical galaxy,
with a comet-like structure extending to the NE in the radio jet direction. The
[OIII]5007 emission line map, shows a bi-conical structure centered on
the nucleus, that overlaps the structure found in the broad-band filters. The
radio core coincides with the center of the bi-conical structure and the radio
axes are aligned with the direction of the cones. These data show clear
evidence of a strong interaction between the radio jet and the NE morphology of
the galaxy. We show evidence that this NE region is an ENLR; the line-ratio
diagnostics show that models involving gas shocked by the radio-jet plus
ionization from a precursor HII region, produced itself by the ionizing photons
of the postshocked gas on the preshocked gas provide a good match to the
observations. We investigate the spatial behavior of the ionizing parameter
, by determining the [OIII]/[OII] line ratio which is sensitive to the
change of the ionization parameter, and trace its behavior over the ENLR along
the radio jet direction. We find that [OIII]/[OII] does not follow a simple
dilution model, but rather that it is approximately constant over a large range
of distance from the nucleus thus requiring a local source of ionization which
seems to be compatible with the shock models driven by the radio jet.Comment: 17 pages, 9 Postscript figures, ApJ accepted, uses aaspp.st
- …