84 research outputs found
Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy
To test the dust torus model for active galactic nuclei directly, we study
the extent and morphology of the nuclear dust distribution in the Circinus
galaxy using high resolution interferometric observations in the mid-infrared
with the MIDI instrument at the Very Large Telescope Interferometer. We find
that the dust distribution in the nucleus of Circinus can be explained by two
components, a dense and warm disk-like component of 0.4 pc size and a slightly
cooler, geometrically thick torus component with a size of 2.0 pc. The disk
component is oriented perpendicular to the ionisation cone and outflow and
seems to show the silicate feature at 10 micron in emission. It coincides with
a nuclear maser disk in orientation and size. From the energy needed to heat
the dust, we infer a luminosity of the accretion disk corresponding to 20% of
the Eddington luminosity of the nuclear black hole. We find that the
interferometric data are inconsistent with a simple, smooth and axisymmetric
dust emission. The irregular behaviour of the visibilities and the shallow
decrease of the dust temperature with radius provide strong evidence for a
clumpy or filamentary dust structure. We see no evidence for dust reprocessing,
as the silicate absorption profile is consistent with that of standard galactic
dust. We argue that the collimation of the ionising radiation must originate in
the geometrically thick torus component. Our findings confirm the presence of a
geometrically thick, torus-like dust distribution in the nucleus of Circinus,
as required in unified schemes of Seyfert galaxies. Several aspects of our data
require that this torus is irregular, or "clumpy".Comment: 20 pages, 16 figures, accepted for publication by A&
The complexity of parsec-scaled dusty tori in AGN
Warm gas and dust surround the innermost regions of active galactic nuclei
(AGN). They provide the material for accretion onto the super-massive black
hole and they are held responsible for the orientation-dependent obscuration of
the central engine. The AGN-heated dust distributions turn out to be very
compact with sizes on scales of about a parsec in the mid-infrared. Only
infrared interferometry currently provides the necessary angular resolution to
directly study the physical properties of this dust. Size estimates for the
dust distributions derived from interferometric observations can be used to
construct a size--luminosity relation for the dust distributions. The large
scatter about this relation suggests significant differences between the dust
tori in the individual galaxies, even for nuclei of the same class of objects
and with similar luminosities. This questions the simple picture of the same
dusty doughnut in all AGN. The Circinus galaxy is the closest Seyfert 2 galaxy.
Because its mid-infrared emission is well resolved interferometrically, it is a
prime target for detailed studies of its nuclear dust distribution. An
extensive new interferometric data set was obtained for this galaxy. It shows
that the dust emission comes from a very dense, disk-like structure which is
surrounded by a geometrically thick, similarly warm dust distribution as well
as significant amounts of warm dust within the ionisation cone.Comment: 8 pages, 3 figures, to appear in the proceedings of the conference
"The central kiloparsec in Galactic Nuclei: Astronomy at High Angular
Resolution 2011", open access Journal of Physics: Conference Series (JPCS),
published by IOP Publishin
Parsec-scale dust distributions in Seyfert galaxies - Results of the MIDI AGN snapshot survey
The emission of warm dust dominates the mid-infrared spectra of active
galactic nuclei (AGN). Only interferometric observations provide the necessary
angular resolution to resolve the nuclear dust and to study its distribution
and properties. The investigation of dust in AGN cores is hence one of the main
science goals for the MID-infrared Interferometric instrument MIDI at the VLTI.
As the first step, the feasibility of AGN observations was verified and the
most promising sources for detailed studies were identified. This was carried
out in a "snapshot survey" with MIDI using Guaranteed Time Observations. In the
survey, observations were attempted for 13 of the brightest AGN in the
mid-infrared which are visible from Paranal. The results of the three
brightest, best studied sources have been published in separate papers. Here we
present the interferometric observations for the remaining 10, fainter AGN. For
8 of these, interferometric measurements could be carried out. Size estimates
or limits on the spatial extent of the AGN-heated dust were derived from the
interferometric data of 7 AGN. These indicate that the dust distributions are
compact, with sizes on the order of a few parsec. The derived sizes roughly
scale with the square root of the luminosity in the mid-infrared, s ~ sqrt(L),
with no clear distinction between type 1 and type 2 objects. This is in
agreement with a model of nearly optically thick dust structures heated to T ~
300 K. For three sources, the 10 micron feature due to silicates is tentatively
detected either in emission or in absorption. Based on the results for all AGN
studied with MIDI so far, we conclude that in the mid-infrared the differences
between individual galactic nuclei are greater than the generic differences
between type 1 and type 2 objects.Comment: 18 pages, 8 figures, updated to version published in A&A 502, 67-8
A Dipole Vortex Model of Obscuring Tori in Active Galaxy Nuclei
The torus concept as an essential structural component of active galactic
nuclei (AGN) is generally accepted. Here, the situation is discussed when the
torus "twisting" by the radiation or wind transforms it into a dipole toroidal
vortex which in turn can be a source of matter replenishing the accretion disk.
Thus emerging instability which can be responsible for quasar radiation flares
accompanied by matter outbursts is also discussed. The "Matreshka" scheme for
an obscuring vortex torus structure capable of explaining the AGN variability
and evolution is proposed. The model parameters estimated numerically for the
luminosity close to the Eddington limit agree well with the observations.Comment: 17 pages, 11 figures, version of this paper is published in Astronomy
Report
The dusty heart of nearby active galaxies. I. High-spatial resolution mid-IR spectro-photometry of Seyfert galaxies
We present 8-13 micron imaging and spectroscopy of 9 type 1 and 10 type 2 AGN
obtained with the VLT/VISIR instrument at spatial resolution <100 pc. The
emission from the host galaxy sources is resolved out in most cases. The
silicate absorption features are moderately deep and emission features are
shallow. We compare the mid-IR luminosities to AGN luminosity tracers and found
that the mid-IR radiation is emitted quite isotropically. In two cases, IC5063
and MCG-3-34-64, we find evidence for extended dust emission in the narrow-line
region. We confirm the correlation between observed silicate feature strength
and Hydrogen column density recently found in Spitzer data. In a further step,
our 3D clumpy torus model has been used to interpret the data. We show that the
strength of the silicate feature and the mid-IR spectral index can be used to
get reasonable constraints on the dust distribution in the torus. The mid-IR
spectral index, alpha, is almost exclusively determined by the radial dust
distribution power-law index, a, and the silicate feature depth is mostly
depending on the average number of clouds, N0, along an equatorial
line-of-sight and the torus inclination. A comparison of model predictions to
our type 1 and type 2 AGN reveals typical average parameters a=-1.0+/-0.5 and
N0=5-8, which means that the radial dust distribution is rather shallow. As a
proof-of-concept of this method, we compared the model parameters derived from
alpha and the silicate feature to more detailed studies of IR SEDs and
interferometry and found that the constraints on a and N0 are consistent.
Finally, we might have found evidence that the radial structure of the torus
changes from low to high AGN luminosities towards steeper dust distributions,
and we discuss implications for the IR size-luminosity relation. (abridged)Comment: 22 pages, 13 figues, 6 tables; Accepted for publication in A&A; Note
that this is the second submitted paper from the series, but we changed paper
order. This one will be referred to as paper I, the previously submitted
arXiv:0909.4539 will become paper I
The multiphase gas structure and kinematics in the circumnuclear region of NGC 5728
We report on our combined analysis of HST, VLT/MUSE, VLT/SINFONI, and ALMA observations of the local Seyfert 2 galaxy, NGC 5728 to investigate in detail the feeding and feedback of the active galactic nucleus (AGN). The data sets simultaneously probe the morphology, excitation, and kinematics of the stars, ionized gas, and molecular gas over a large range of spatial scales (10 pc to 10 kpc). NGC 5728 contains a large stellar bar that is driving gas along prominent dust lanes to the inner 1 kpc where the gas settles into a circumnuclear ring. The ring is strongly star forming and contains a substantial population of young stars as indicated by the lowered stellar velocity dispersion and gas excitation consistent with H II regions. We model the kinematics of the ring using the velocity field of the CO (2–1) emission and stars and find it is consistent with a rotating disc. The outer regions of the disc, where the dust lanes meet the ring, show signatures of inflow at a rate of 1 M⊙ yr−1. Inside the ring, we observe three molecular gas components corresponding to the circular rotation of the outer ring, a warped disc, and the nuclear stellar bar. The AGN is driving an ionized gas outflow that reaches a radius of 250 pc with a mass outflow rate of 0.08 M⊙ yr−1 consistent with its luminosity and scaling relations from previous studies. While we observe distinct holes in CO emission which could be signs of molecular gas removal, we find that largely the AGN is not disrupting the structure of the circumnuclear region
Near-infrared proper motions and spectroscopy of infrared excess sources at the Galactic Center
There are a number of faint compact infrared excess sources in the central
stellar cluster of the Milky Way. Their nature and origin is unclear. In
addition to several isolated objects of this kind we find a small but dense
cluster of co-moving sources (IRS13N) about 3" west of SgrA* just 0.5" north of
the bright IRS13E cluster of WR and O-type stars. Based on their color and
brightness, there are two main possibilities: (1) they may be dust embedded
stars older than few Myr, or (2) extremely young, dusty stars with ages less
than 1Myr. We present fist H- and Ks-band identifications or proper motions of
the IRS13N members, the high velocity dusty S-cluster object (DSO), and other
infrared excess sources in the central field. We also present results of NIR H-
and Ks-band ESO-SINFONI integral field spectroscopy of ISR13N. We show that
within the uncertainties, the proper motions of the IRS13N sources in Ks- and
L'-band are identical. This indicates that the bright L'-band IRS13N sources
are indeed dust enshrouded stars rather than core-less dust clouds. The proper
motions show that the IRS13N sources are not strongly gravitationally bound to
each other implying that they have been formed recently. We also present a
first H- and Ks-band identification as well as proper motions and HKsL'-colors
of a fast moving DSO which was recently found in the cluster of high speed
S-stars that surround the super-massive black hole Sagittarius A* (SgrA*). Most
of the compact L'-band excess emission sources have a compact H- or Ks-band
counterpart and therefore are likely stars with dust shells or disks. Our new
results and orbital analysis from our previous work favor the hypothesis that
the infrared excess IRS13N members and other dusty sources close to SgrA* are
very young dusty stars and that star formation at the GC is a continuously
ongoing process.Comment: 20 pages, 18 figures, 4 tables plus appendix with 16 figures and 3
tables accepted by A&
The Galactic Center Black Hole Laboratory
The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*)
shows flare emission from the millimeter to the X-ray domain. A detailed
analysis of the infrared light curves allows us to address the accretion
phenomenon in a statistical way. The analysis shows that the near-infrared
flare amplitudes are dominated by a single state power law, with the low states
in SgrA* limited by confusion through the unresolved stellar background. There
are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO
is one of them. Its nature is unclear. It may be comparable to similar stellar
dusty sources in the region or may consist predominantly of gas and dust. In
this case a particularly enhanced accretion activity onto SgrA* may be expected
in the near future. Here the interpretation of recent data and ongoing
observations are discussed.Comment: 30 pages - 7 figures - accepted for publication by Springer's
"Fundamental Theories of Physics" series; summarizing GC contributions of 2
conferences: 'Equations of Motion in Relativistic Gravity' at the
Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the
COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov.
19 - 22, 2013
The Role of Host Galaxy for the Environmental Dependence of Active Nuclei in Local Galaxies
We discuss the environment of local hard X-ray selected active galaxies, with reference to two independent group catalogues. We find that the fraction of these AGN in S0 host galaxies decreases strongly as a function of galaxy group size (halo mass) – which contrasts with the increasing fraction of galaxies of S0 type in denser environments. However, there is no evidence for an environmental dependence of AGN in spiral galaxies. Because most AGN are found in spiral galaxies, this dilutes the signature of environmental dependence for the population as a whole. We argue that the differing results for AGN in disc-dominated and bulge-dominated galaxies are related to the source of the gas fuelling the AGN, and so may also impact the luminosity function, duty cycle and obscuration. We find that there is a significant difference in the luminosity function for AGN in spiral and S0 galaxies, and tentative evidence for some difference in the fraction of obscured AGN
Modelling the spectral energy distribution of galaxies. V. The dust and PAH emission SEDs of disk galaxies
We present a self-consistent model of the spectral energy distributions
(SEDs) of spiral galaxies from the ultraviolet (UV) to the mid-infrared
(MIR)/far-infrared (FIR)/submillimeter (submm) based on a full radiative
transfer calculation of the propagation of starlight in galaxy disks. This
model predicts not only the total integrated energy absorbed in the UV/optical
and re-emitted in the infrared/submm, but also the colours of the dust emission
based on an explicit calculation of the strength and colour of the UV/optical
radiation fields heating the dust, and incorporating a full calculation of the
stochastic heating of small dust grains and PAH molecules.
The geometry of the translucent components of the model is empirically
constrained using the results from the radiation transfer analysis of Xilouris
et al. on spirals in the middle range of the Hubble sequence, while the
geometry of the optically thick components is constrained from physical
considerations with a posteriori checks of the model predictions with
observational data.
These geometrical constraints enable the dust emission to be predicted in
terms of a minimum set of free parameters: the central face-on dust opacity in
the B-band tau^f_B, a clumpiness factor F for the star-forming regions, the
star-formation rate SFR, the normalised luminosity of the old stellar
population old and the bulge-to-disk ratio B/D. We show that these parameters
are almost orthogonal in their predicted effect on the colours of the dust/PAH
emission.
The results of the calculations are made available in the form of a large
library of simulated dust emission SEDs spanning the whole parameter space of
our model, together with the corresponding library of dust attenuation
calculated using the same model. (see full abstract in the paper)Comment: 39 pages; accepted for publication in Astronomy & Astrophysics; For a
higher resolution version of Fig.1 and Fig.20 see
http://www.star.uclan.ac.uk/~ccp/index.shtm
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