501 research outputs found
Models relating the radio emission and ionised gas in Seyfert nuclei
Possible models are discussed in which the radio emitting components in Seyfert II nuclei can compress and accelerate the ambient nuclear medium to produce the characteristics of the narrow line region. A first order model, which considers only the expansion of the radio components, is briefly described. However, in many Seyfert nuclei it appears that the linear motion of the radio components is also important. This can result in shock heating of the ambient medium, and if the cooling time is long enough, can lead to a displacement between the radio component and the associated emission lines. This effect may be present in NGC 1068 and NGC 5929 and by considering ram pressure balance and the cooling length it is possible to estimate lobe velocities and ambient densities
A Spectropolarimetric Atlas of Seyfert 1 Galaxies
We present optical spectropolarimetry of the nuclei of 36 Seyfert 1 galaxies,
obtained with the William Herschel and the Anglo-Australian Telescopes from
1996 to 1999. In 20 of these, the optical emission from the active nucleus is
intrinsically polarized. We have measured a significant level of polarization
in a further 7 objects but these may be heavily contaminated by Galactic
interstellar polarization. The intrinsically polarized Seyfert 1s exhibit a
variety of characteristics, with the average polarization ranging from < 0.5 to
5 per cent and many showing variations in both the degree and position angle of
polarization across the broad H alpha emission line. We identify a small group
of Seyfert 1s that exhibit polarization properties similar to those of Seyfert
2 galaxies in which polarized broad-lines have been discovered. These objects
represent direct observational evidence that a Seyfert 2-like far-field polar
scattering region is also present in Seyfert 1s. Several other objects have
features that can be explained in terms of equatorial scattering of line
emission from a rotating disk. We propose that much of the diversity in the
polarization properties of Seyfert galaxies can be understood in terms of a
model involving both equatorial and polar scattering, the relative importance
of the two geometries as sources of polarized light being determined
principally by the inclination of the system axis to the line-of-sight.Comment: Accepted for publication in MNRAS (28 pages, 25 figures
HST and Spitzer point source detection and optical extinction in powerful narrow-line radio galaxies
We present the analysis of infrared HST and Spitzer data for a sample of 13
FRII radio galaxies at 0.03<z<0.11 that are classified as narrow-line radio
galaxies (NLRG). In the context of the unified schemes for active galactic
nuclei (AGN), our direct view of the AGN in NLRG is impeded by a parsec-scale
dusty torus structure. Our high resolution infrared observations provide new
information about the degree of extinction induced by the torus, and the
incidence of obscured AGN in NLRG.
We find that the point-like nucleus detection rate increases from 25 per cent
at 1.025m, to 80 per cent at 2.05m, and to 100 per cent at 8.0m.
This supports the idea that most NLRG host an obscured AGN in their centre. We
estimate the extinction from the obscuring structures using X-ray, near-IR and
mid-IR data. We find that the optical extinction derived from the 9.7m
silicate absorption feature is consistently lower than the extinction derived
using other techniques. This discrepancy challenges the assumption that all the
mid-infrared emission of NLRG is extinguished by a simple screen of dust at
larger radii. This disagreement can be explained in terms of either weakening
of the silicate absorption feature by (i) thermal mid-IR emission from the
narrow-line region, (ii) non-thermal emission from the base of the radio jets,
or (iii) by direct warm dust emission that leaks through a clumpy torus without
suffering major attenuation.Comment: 18 pages, 7 figures, 8 tables, accepted for publication in MNRA
Nuclear spirals as feeding channels to the Supermassive Black Hole: the case of the galaxy NGC 6951
We report the discovery of gas streaming motions along nuclear spiral arms
towards the LINER nucleus of the galaxy NGC 6951. The observations, obtained
using the GMOS integral field spectrograph on the Gemini North telescope,
yielded maps of the flux distributions and gas kinematics in the Halpha,
[NII]6584 and [SII]6717,31 emission lines of the inner 7x5 arcsec^2 of the
galaxy. This region includes a circumnuclear star-forming ring with radius
500pc, a nuclear spiral inside the ring and the LINER nucleus. The kinematics
of the ionized gas is dominated by rotation, but subtraction of a kinematic
model of a rotating exponential disk reveals deviations from circular rotation
within the nuclear ring which can be attributed to (1) streaming motions along
the nuclear spiral arms and (2) a bipolar outflow which seems to be associated
to a nuclear jet. On the basis of the observed streaming velocities and
geometry of the spiral arms we estimate a mass inflow rate of ionized gas of
3x10^(-4) Msun/yr, which is of the order of the accretion rate necessary to
power the LINER nucleus of NGC 6951. Similar streaming motions towards the
nucleus of another galaxy with LINER nucleus -- NGC 1097 -- have been reported
by our group in a previous paper. Taken together, these results support a
scenario in which nuclear spirals are channels through which matter is
transferred from galactic scales to the nuclear region to feed the supermassive
black hole.Comment: 25 pages, 6 eps figures, accepted for publication in Ap
Scenarios for the development of smart grids in the UK: synthesis report
âSmart gridâ is a catch-all term for the smart options that could transform the ways society produces, delivers and consumes energy, and potentially the way we conceive of these services. Delivering energy more intelligently will be fundamental to decarbonising the UK electricity system at least possible cost, while maintaining security and reliability of supply.
Smarter energy delivery is expected to allow the integration of more low carbon technologies and to be much more cost effective than traditional methods, as well as contributing to economic growth by opening up new business and innovation opportunities. Innovating new options for energy system management could lead to cost savings of up to ÂŁ10bn, even if low carbon technologies do not emerge. This saving will be much higher if UK renewable energy targets are achieved.
Building on extensive expert feedback and input, this report describes four smart grid scenarios which consider how the UKâs electricity system might develop to 2050. The scenarios outline how political decisions, as well as those made in regulation, finance, technology, consumer and social behaviour, market design or response, might affect the decisions of other actors and limit or allow the availability of future options. The project aims to explore the degree of uncertainty around the current direction of the electricity system and the complex interactions of a whole host of factors that may lead to any one of a wide range of outcomes. Our addition to this discussion will help decision makers to understand the implications of possible actions and better plan for the future, whilst recognising that it may take any one of a number of forms
Jet driven motions in the Narrow Line region of NGC1068
We have obtained HST FOC f/48 long-slit spectroscopy of the inner 4" of the
Narrow Line Region of NGC 1068 between 3500-5400A with a spectral resolution of
1.78A/pixel. At a spatial scale of 0.0287" per pixel these data provide an
order of magnitude improvement in resolution over previous ground based spectra
and allow us to trace the interaction between the radio jet and the gas in the
NLR. Our results show that, within +/-0.5" of the radio-jet the emission lines
are kinematically disturbed and split into two components whose velocity
separation is 1500 km/sec. The filaments associated with the radio lobe also
show a redshifted kinematic disturbance of the order of 300 km/sec which
probably is a consequence of the expansion of the radio plasma.
Furthermore, the material enveloping the radio-jet is in a much higher
ionization state than that of the surrounding NLR gas. The highest excitation
is coincident with the jet axis where emission in the coronal line of [FeVII]
3769A is detected and the HeII 4686A is strong but where [OII] 3727A is
depressed. This large localized increase in ionization on the jet axis is
accompanied by the presence of an excess continuum. Because the electron
density is substantially larger in the jet compared to the surrounding NLR,
these results can only be explained if there is a more intense ionizing
continuum associated with the jet. This can be accomplished in a variety of
ways which include an intrinsically anisotropic nuclear radiation field, a
reduced gas covering factor or the presence of a local ionization source.
The morphology, kinematics and, possibly, the ionization structure of the NLR
in the vicinity of the jet of NGC 1068 are a direct consequence of the
interaction with the radio outflow.Comment: 11 pages, 5 figures, ApJ Letters in pres
Hubble Space Telescope Observations of NGC 6240: a Case Study of an Ultra-Luminous Infrared Galaxy with Obscured Activity
We present results from an HST study of the morphology and kinematics of NGC
6240. This merging galaxy with a double nucleus is one of the nearest and
best-studied ultraluminous infrared galaxies. HST resolves both nuclei into
seperate components. The distance between the northern and southern
optical/near-infrared components is greater than that observed in radio and
X-ray studies, arguing that even in K-band we may not be seeing all the way
through the dust to the true nuclei. The ionized gas does not display rotation
around either of the nuclei, or equilibrium motion in general. There is a
strong velocity gradient between the nuclei, similar to what is seen in CO
data. There is no such gradient in our stellar kinematics. The velocity
dispersion of the gas is larger than expected for a cold disk. We also map and
model the emission-line velocity field at an off-nuclear position where a steep
velocity gradient was previously detected in ground-based data. Overall, the
data indicate that line-of-sight projection effects, dust absorption,
non-equilibrium merger dynamics, and the possible influence of a wind may be
playing an important role in the observed kinematics. Chandra observations of
hard X-rays have shown that both of the nuclei contain an Active Galactic
Nucleus (AGN). The HST data show no clear sign of the two AGNs: neither
continuum nor narrow-band imaging shows evidence for unresolved components in
the nuclei, and there are no increased emission line widths or rapid rotation
near the nuclei. This underscores the importance of X-ray data for identifying
AGNs in highly dust-enshrouded environments.Comment: LaTeX, 32 pages, 9 figures, 2 tables, accepted for publication in The
Astronomical Journal (Jan 2004). Paper with high-resolution (non-compressed)
color figures in gzipped postscript format available at
http://www.stsci.edu/~marel/psgzdir/ngc6240v11.ps.g
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