Transcript NIR interferometry of the Seyfert galaxy NGC 1068

NIR
interferometry of
the Seyfert galaxy
NGC 1068:
present
interferometric
NIR results and
future goals
G. Weigelt
T. Beckert
M. Wittkowski
Infrared interferometry and theoretical
studies of the environment of AGN:
→ structure of the dust environment,
→ evolution fuelling & growth of the BH
Activity in the centres of galaxies manifests itself in the form of both
•
non-stellar nuclear activity (AGN, i.e. BH, accretion disk, BLR, torus, NLR..),
•
and of circumnuclear star formation (starburst).
AGN activity is caused by accretion into a super-massive BH, whose evolution
is driven by this accretion.
We can investigate, by the combination of interferometric observations and
theoretical studies, how accretion processes at different length scales work
and how they relate to each other.
The length scales of interest range from :
•
•
•
approx. 100 pc – where starbursts may dominate the appearance –
to a few pc - where a dusty torus is the main component
and to the centre of the AGN with outflows and jets.
Physical torus models and radiative transfer modelling
Observations: Bispectrum speckle interferometry
of NGC 1068 with the SAO 6 m telescope
K-band speckle
interferogram of NGC
1068:
exposure time 200 ms,
FOV 1.8 x1.8 ’’
1.8’’
First resolution
of NGC 1068
Azimuthally averaged
visibility of NGC 1068
(top) and an
unresolved reference
star (bottom)
Wittkowski et al.
A&A 329, L45, 1998
1999: Weinberger et al.
confirmed this result (AJ
117, 2748)
FWHM Gauss fit
diameter 30 mas ~ 2 pc!
extension at PA ~ - 20o
Bispectrum speckle
interferometry of NGC 1068:
First diffraction-limited,
elongated K-band image
of NGC 1068
(Wittkowski et al. 1998, A&A 329, L45)
major axis
19 x 40 mas
NGC 1068:
visibility
functions
derived
from
4 different
data sets
minor axis
20 x 40 mas
24 x 40 mas, 600 frames,
2’’ K-band seeing
Weigelt et al.
2004
18 x 38 mas ~ 1.5 x 3 pc
error +/- 4 mas
11 000 frames,
1.2’’ seeing
K-band flux of the 18 x 39 mas object
~18 %
Photometric K-band results
Flux in ratio in 2.5’’ FOV:
Total flux: phot. calibrators
18 x 39 mas: 82% = 350 mJy
Extended components: 70 mJy
H-band visibility of NGC 1068
FWHM Gauss fit diameter: 18 x 45 mas (+/- 4 mas)
Real-time Bispectrum
Speckle Interferometry
Data recording and data processing: ~ 2 frames/s
SAO 6 m telescope, 2 arcsec K-band seeing, 76 mas resolution
Bispectrum speckle
interferometry:
First diffraction-limited
K‘- and H-band images
of the resolved 18 x 39
mas core of NGC 1068
The images show the
compact core with its
tail-shaped, northwestern extension at PA
of -16o as well as the
northern and southeastern extended
components.
Weigelt et al. 2004 (A&A 9/2004)
Diffraction-limited K’- and H-band images of NGC 1068
PA ~ -16o
Northern
400 mas
object
North-western
tail of the
compact
18 x 39 mas
object
Compact component: 18 x 39 mas ~ 1.3 x 2.8 pc
Northern component: 400 mas
Summary:
H- and K-band bispectrum speckle interferometry of
NGC 1068:
57 mas / 74 mas resolution images
● A diffraction-limited K'-band image with 74 mas resolution and
the first H-band image with 57 mas resolution were reconstructed
from speckle interferograms obtained with the SAO 6 m telescope.
● The resolved compact core has a north-western, tail-shaped
extension
Bispectrum speckle interferometry method: Weigelt, Optics Com. 21, 55,
1977; Lohmann et al. Appl. Opt. 22, 4028, 1983
Summary: Bispectrum speckle interferometry of NGC 1068:
1.3 x 2.8 pc core
►
The K'-band FWHM diameter of the resolved compact core is
18 x 39 mas or 1.3 x 2.8 pc (FWHM of a Gaussian fit; fit range
30-80% of the telescope cut-off frequency).
►
The diameter errors smaller than ± 4 mas.
► The
►
►
►
PA of the north-western extension is -16 ± 4o.
In the H band, the FWHM diameter of the compact core is
18 x 45 mas (± 4 mas), and the PA is -18 ± 4o.
The extended northern component (PA ~ 0o) has an elongated
structure with a length of ~ 400 mas or 29 pc.
The K'- and H-band fluxes of the compact core are 350 ± 90 mJy, or
K‘ = 8.2, and 70 ± 20 mJy, or H = 10.4, respectively.
Comparison with HST,
Merlin, and VLBA
images
Heat image: HST 502 nm
MERLIN 5 GHz,
Muxlow et al. 1996
100 mas
VLBA 8.4 GHz radio continuum;
Gallimore et al. 1997: ~ 5 x 20 mas
Ionization cone
C
200 mas
S1
MERLIN 5 GHz contour map
(Gallimore et al.1996) superposed
on our K'-band image.
The center of the radio component
S1 coincides with the center of the
K'-band peak. The P.A. of –16o of the
18 x 39 mas K'-band core is very
similar to that of the western wall (PA
-15o) of the ionization cone. The
northern extended 400 mas structure
(red) aligns with the direction of the
inner radio jet (S1 - C). The northern
borderline of the northern component
is almost perpendicular to the radio
jet direction between C and NE.
Interpretation
The PA of -16 ± 4o of the compact 18 x 39 mas core is very similar to that of
the western wall (PA = -15o) of the ionization cone.
This suggests that the H- and K'-band emission from the 18 x 39 mas =
1.3 x 2.8 pc core is both thermal emission & scattered light from
● dust near the western wall of a low-density, conical outflow cavity or from
● the innermost region of a pc-scale dusty torus heated by the central source.
The dust sublimation radius of NGC 1068 is approximately 0.2 – 1 pc.
The northern extended 400 mas structure lies near the western wall of the
ionization cone and coincides with the inner radio jet (PA = 11o). The large
distance from the core suggests that the K'-band emission of the northern
extended component is scattered light from the western cavity region and the
radio jet region.
Interpretation:
Flux and structure predicted by various torus models
For a quantitative analysis of the K'-band flux emerging from the optically thick
torus, accurate radiative transfer modeling is necessary. A number of models have
been published for NGC 1068 (Pier & Krolik1992, 1993, Granato & Danese1994,
Efstathiou & Rowan-Robinson 1995, Efstathiou et al. 1995, Granato et al. 1997,
Manske et al. 1998, Nenkova et al. 2002, Galliano et al. 2003). All these models try
to fit the SED of Rieke & Low (1975) with a K-band flux of 0.3 Jy.
Most authors introduced in their extended torus an additional dust component
located within the ionization cone to reach the required K band flux.
Torus models which can explain the observed infrared SED by emission solely
from the torus (i.e., without dust within the ionization cone) were presented by
Granato & Danese (1994).
A simulated 2.2 μm image reported by Granato et al. 1997 shows an elongated
shape along the direction with low absorption. The model images are equal or
somewhat larger than the dust sublimation radius). As discussed above, our images
of the compact core also have a size which is similar to the dust sublimation radius.
First VINCI-VLT interferometry of NGC 1068:
resolution of a new 0 - 5 mas structure (46 m baseline, K band):
3 mas clumps in the torus? Accretion disk?
30 mas
3 mas
6 m telescope / speckle
or 0.1 mas ?
VINCI VLTI, 46 m
Wittkowski, Kervella, Arsenault, Paresce, Beckert, Weigelt 2004, A&A 418, L39
First MIDI-VLT interferometry of NGC 1068
Resolution of two
components:
42 m baseline
- Central hot component:
warm
hot comp.
72 m baseline
Dust temperature > 800 K
Size: 0.7 x <1 pc (0.7 pc parallel jet)
Hot inner torus wall?
- Large warm component:
T = 320 K
Size 2.1 x 3.4 pc
Jaffe et al. 2004, Nature 429, May 6
Science goals of
next generation interferometric facilities?
•
Testing the unification scheme and torus models (clumpy tori)
•
Physical properties of outflows and jets
•
AGN – starburst connection
•
Nuclear disks, their evolution, and black hole fuelling & growth
•
Connection between massive, large-scale gas disks with starbursts, dusty tori,
and AGN activity
(BLR: talk by Alessandro Marconi)
Testing the unification scheme and torus models
Based on the pioneering work of Miller and Antonucci (Miller et al. 1983, Antonucci
et al. 1985), the major difference between type 1 and type 2 AGN is attributed to
aspect-angle-dependent obscuration effects by a dusty torus.
For more than a decade, clumpy models for the circumnuclear torus have been
under discussion (Krolik & Begelman 1988). However this idea was not included
in the first radiative transfer calculations (Pier & Krolik 1992, 1993; Granato &
Danese 1994; Efstathiou & Rowan-Robinson 1995) of dust emission from these tori.
Only recently, Nenkova et al. (2002) developed a scheme which uses the
clumpiness of the torus for an approximative and statistical treatment of the
radiative transfer problem with optically thick clouds.
dust sublimation radii
K-band brightness distribution of our radiative transfer calculations based on
the method of Nenkova et al. (2002) (Beckert et al. 2004).
The inclination is 20o from the midplane (contour range of 213).
Physical properties of tori and radiative transfer modelling
Vollmer et al. (2004) and Beckert & Duschl (2004) and other authors reported
stationary models of obscuring dusty tori around AGN. Such (more realistic) models
are required as input for radiative transfer modelling.
● The key ingredient of the model is the clumpiness of the torus in which dusty
molecular clouds move with a high velocity dispersion.
● Within the model, the radial accretion flow is seen as a consequence of cloudcloud collisions, which can redistribute angular momentum.
Accretion in the combined gravitational potential of a central black hole and stellar
cluster transfers gravitational binding energy to random kinetic energy
and maintains the thickness of the torus.
Theoretical
studies of tori
Cut through the probability distribution of finding a dusty cloud in the torus.
The distribution is derived from the model of Beckert & Duschl (2004).
The spatial scale is in units of the dust sublimation radius. The mean
number of clouds along a line of sight to the centre drops below 1 for angles
larger that 40o from the midplane.
dust sublimation radii
K-band brightness distribution of a radiative transfer model based on the
method of Nenkova et al. (2002) (Beckert et al. 2004).
The inclination is 20o from the midplane (contour range of 213).
Interferometric/theoretical studies of the
AGN – starburst connection
The effects of star formation on the gas dynamics and the star formation rate can
most likely influence the channelling of matter to the centre (e.g., Vollmer & Beckert
2003).
● However, it is difficult to establish a direct causal connection between
starburst and AGN activity (Knapen 2004);
since the steering of turbulence and subsequent transport of gas to the centre
happens on a viscous transport time scale, which introduces a yet unknown time lag
between starburst and AGN activity. Therefore, any causal connection between a
starburst and the AGN is difficult to test.
● We need studies of the gaseous and dusty AGN environment during the accretion
of mass by the central black hole (in the continuum and in emission lines).
Nuclear disks, their evolution, and their relation to starbursts
It seems that black holes in the local universe have predominantly
increased their mass by accretion (Marconi et al. 2004). The existence of
black holes with masses >109 solar masses in the quasars with the
highest known redshifts (i.e., young objects) indicates that the viscous time scales for
very massive disks have to be remarkably short, much shorter than predicted by
standard disk models (e.g., Shlosman, Frank, & Begelman 1989).
● The original interpretation that this is indicative of non-axisymmetric processes has,
however, become questionable by the finding that the bar fraction in AGN disks is
not considerably higher than in normal galaxies (e.g., Martini et al. 2003).
● Alternative? It has been shown that self-gravity of disks may also enhance the radial mass
transport (Duschl, Strittmatter & Biermann 2000; Duschl 2004). An interesting
feature of these self-gravitating disks is that the transport of angular momentum is
more efficient and that material from the outer disk regions will accrete more
efficiently than that at smaller radii. Self-gravity in massive gaseous disks will lead to
gravitational instabilities and star formation.
Interferometry and theory: the connection between
massive gas disks, starbursts, dusty tori, and AGN
activity?
● The properties of the circumnuclear dust distribution (e.g., torus, outflow
cavities, etc.) derived from observations and modelling can be related to the
luminosity and morphology of circumnuclear star forming regions. This will provide
constraints for a scenario which connects the time evolution of gas from large
scales to the activity in the centre.
● The observable properties of the circumnuclear star formation regions will
provide constraints for the dynamical models of massive gas disks discussed
above. In particular, the location of the inner edge of a starburst ring will allow us to
determine where self-gravity becomes important in the disk and thus to get
information about the mass distribution in the disk.
AMBER
Observation of 51 Hya:
Commissioning May 2004
Medium spectral resolution R=1200
K‘ band (2.0-2.3 μm)
Two 8.2 m UTs equipped with MACAO AO
Next generation facilities
Size and shape of tori:
100 m baseline
four 8 m telescopes
imaging (visibilities + closure phases)
Detailed structure of tori (clumpiness, outflow cavities):
1 km baseline
> 10 telescopes
imaging
BLR:
size of the nearest BLRs (~ 0.1 mas):
100 m baseline if differential visibilities and phases are measured
detailed structure: 1 to 10 km baseline