2,087 research outputs found
Revealing the large nuclear dust structures in NGC 1068 with MIDI/VLTI
To understand the relation between the small "obscuring torus" and dusty
structures at larger scales (5-10 pc) in NGC 1068, we use ESO's Mid-Infrared
Interferometer (MIDI) with the 1.8 m Auxiliary Telescopes to achieve the
necessary spatial resolution (~ 20-100 millarcsec). We use the chromatic phases
in the data to improve the spatial fidelity of the analysis. We present
interferometric data for NGC 1068 obtained in 2007 and 2012. We find no
evidence of source variability. Many (u,v) points show non-zero chromatic
phases indicating significant asymmetries. Gaussian model fitting of the
correlated fluxes and chromatic phases provides a 3-component best fit with
estimates of sizes, temperatures and positions of the components. A large,
warm, off-center component is required at a distance approximately 90 mas to
the north-west at a PA ~ -18 deg. The dust at 5-10 pc in the polar region
contributes 4 times more to the mid-infrared flux at 12 um than the dust
located at the center. This dust may represent the inner wall of a dusty cone.
If similar regions are heated by the direct radiation from the nucleus, then
they will contribute substantially to the classification of many Seyfert
galaxies as Type 2. Such a region is also consistent in other Seyfert galaxies
(the Circinus galaxy, NGC 3783 and NGC 424).Comment: 21 pages, 10 figures; Accepted for publication on A&
Time-resolved infrared emission from radiation-driven central obscuring structures in Active Galactic Nuclei
The central engines of Seyfert galaxies are thought to be enshrouded by
geometrically thick gas and dust structures. In this article, we derive
observable properties for a self-consistent model of such toroidal gas and dust
distributions, where the geometrical thickness is achieved and maintained with
the help of X-ray heating and radiation pressure due to the central engine.
Spectral energy distributions (SEDs) and images are obtained with the help of
dust continuum radiative transfer calculations with RADMC-3D. For the first
time, we are able to present time-resolved SEDs and images for a physical model
of the central obscurer. Temporal changes are mostly visible at shorter
wavelengths, close to the combined peak of the dust opacity as well as the
central source spectrum and are caused by variations in the column densities of
the generated outflow. Due to the three-component morphology of the
hydrodynamical models -- a thin disc with high density filaments, a surrounding
fluffy component (the obscurer) and a low density outflow along the rotation
axis -- we find dramatic differences depending on wavelength: whereas the
mid-infrared images are dominated by the elongated appearance of the outflow
cone, the long wavelength emission is mainly given by the cold and dense disc
component. Overall, we find good agreement with observed characteristics,
especially for those models, which show clear outflow cones in combination with
a geometrically thick distribution of gas and dust, as well as a geometrically
thin, but high column density disc in the equatorial plane.Comment: 16 pages, 12 figures, accepted for publication in MNRA
Radiative transfer modelling of parsec-scale dusty warped discs
Warped discs have been found on (sub-)parsec scale in some nearby Seyfert
nuclei, identified by their maser emission. Using dust radiative transfer
simulations we explore their observational signatures in the infrared in order
to find out whether they can partly replace the molecular torus. Strong
variations of the brightness distributions are found, depending on the
orientation of the warp with respect to the line of sight. Whereas images at
short wavelengths typically show a disc-like and a point source component, the
warp itself only becomes visible at far-infrared wavelengths. A similar variety
is visible in the shapes of the spectral energy distributions. Especially for
close to edge-on views, the models show silicate feature strengths ranging from
deep absorption to strong emission for variations of the lines of sight towards
the warp. To test the applicability of our model, we use the case of the
Circinus galaxy, where infrared interferometry has revealed a highly elongated
emission component matching a warped maser disc in orientation and size. Our
model is for the first time able to present a physical explanation for the
observed dust morphology as coming from the AGN heated dust. As opposed to
available torus models, a warped disc morphology produces a variety of silicate
feature shapes for grazing lines of sight, close to an edge-on view. This could
be an attractive alternative to a claimed change of the dust composition for
the case of the nearby Seyfert 2 galaxy NGC 1068, which harbours a warped maser
disc as well.Comment: accepted by MNRA
Embedded AGN and star formation in the central 80 pc of IC 3639
[Abridged] Methods: We use interferometric observations in the -band with
VLTI/MIDI to resolve the mid-IR nucleus of IC 3639. The origin of the nuclear
infrared emission is determined from: 1) the comparison of the correlated
fluxes from VLTI/MIDI with the fluxes measured at subarcsec resolution
(VLT/VISIR, VLT/ISAAC); 2) diagnostics based on IR fine-structure line ratios,
the IR continuum emission, IR bands produced by polycyclic aromatic
hydrocarbons (PAH) and silicates; and 3) the high-angular resolution spectral
energy distribution. Results: The unresolved flux of IC 3639 is at , measured with three different baselines in
VLTI (UT1-UT2, UT3-UT4, and UT2-UT3; -), making this the
faintest measurement so far achieved with mid-IR interferometry. The correlated
flux is a factor of - times fainter than the VLT/VISIR total flux
measurement. The observations suggest that most of the mid-IR emission has its
origin on spatial scales between and (-). A composite scenario where the star formation component dominates
over the AGN is favoured by the diagnostics based on ratios of IR
fine-structure emission lines, the shape of the IR continuum, and the PAH and
silicate bands. Conclusions: A composite AGN-starburst scenario is able to
explain both the mid-IR brightness distribution and the IR spectral properties
observed in the nucleus of IC 3639. The nuclear starburst would dominate the
mid-IR emission and the ionisation of low-excitation lines (e.g. [NeII]) with a net contribution of . The AGN accounts for the
remaining of the mid-IR flux, ascribed to the unresolved component
in the MIDI observations, and the ionisation of high-excitation lines (e.g.
[NeV] and [OIV]).Comment: Accepted for publication in A&
The dusty torus in the Circinus galaxy: a dense disk and the torus funnel
(Abridged) With infrared interferometry it is possible to resolve the nuclear
dust distributions that are commonly associated with the dusty torus in active
galactic nuclei (AGN). The Circinus galaxy hosts the closest Seyfert 2 nucleus
and previous interferometric observations have shown that its nuclear dust
emission is well resolved.
To better constrain the dust morphology in this active nucleus, extensive new
observations were carried out with MIDI at the Very Large Telescope
Interferometer.
The emission is distributed in two distinct components: a disk-like emission
component with a size of ~ 0.2 1.1 pc and an extended component with a
size of ~ 0.8 1.9 pc. The disk-like component is elongated along PA ~
46{\deg} and oriented perpendicular to the ionisation cone and outflow. The
extended component is elongated along PA ~ 107{\deg}, roughly perpendicular to
the disk component and thus in polar direction. It is interpreted as emission
from the inner funnel of an extended dust distribution and shows a strong
increase in the extinction towards the south-east. We find no evidence of an
increase in the temperature of the dust towards the centre. From this we infer
that most of the near-infrared emission probably comes from parsec scales as
well. We further argue that the disk component alone is not sufficient to
provide the necessary obscuration and collimation of the ionising radiation and
outflow. The material responsible for this must instead be located on scales of
~ 1 pc, surrounding the disk.
The clear separation of the dust emission into a disk-like emitter and a
polar elongated source will require an adaptation of our current understanding
of the dust emission in AGN. The lack of any evidence of an increase in the
dust temperature towards the centre poses a challenge for the picture of a
centrally heated dust distribution.Comment: 30 pages, 12 figures; A&A in pres
Dust emission from a parsec-scale structure in the Seyfert 1 nucleus of NGC 4151
We report mid-IR interferometric measurements with \sim 10 mas resolution,
which resolve the warm (T = 285 +25 / -50 K) thermal emission at the center of
NGC 4151. Using pairs of VLT 8.2 m telescopes with MIDI and by comparing the
data to a Gaussian model, we determined the diameter of the dust emission
region, albeit only along one position angle, to be 2.0 +/- 0.4 pc (FWHM). This
is the first size and temperature estimate for the nuclear warm dust
distribution in a Seyfert 1 galaxy. The parameters found are comparable to
those in Seyfert 2 galaxies, thus providing direct support for the unified
model. Using simple analytic temperature distributions, we find that the
mid-infrared emission is probably not the smooth continuation of the hot
nuclear source that is marginally resolved with K band interferometry. We also
detected weak excess emission around 10.5 micron in our shorter baseline
observation, possibly indicating that silicate emission is extended to the
parsec scale.Comment: 5 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
NGC 1068: No change in the mid-IR torus structure despite X-ray variability
Context. Recent NuSTAR observations revealed a somewhat unexpected increase
in the X-ray flux of the nucleus of NGC 1068. We expect the infrared emission
of the dusty torus to react on the intrinsic changes of the accretion disk.
Aims. We aim to investigate the origin of the X-ray variation by
investigating the response of the mid-infrared environment.
Methods. We obtained single-aperture and interferometric mid-infrared
measurements and directly compared the measurements observed before and
immediately after the X-ray variations. The average correlated and
single-aperture fluxes as well as the differential phases were directly
compared to detect a possible change in the structure of the nuclear emission
on scales of 2 pc.
Results. The flux densities and differential phases of the observations
before and during the X-ray variation show no significant change over a period
of ten years. Possible minor variations in the infrared emission are
8 %.
Conclusions. Our results suggest that the mid-infrared environment of NGC
1068 has remained unchanged for a decade. The recent transient change in the
X-rays did not cause a significant variation in the infrared emission. This
independent study supports previous conclusions that stated that the X-ray
variation detected by NuSTAR observations is due to X-ray emission piercing
through a patchy section of the dusty region.Comment: 6 pages, 5 figures, 3 tables. Accepted for publication on A&
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