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The Dusty Torus of NGC1068Literature Study for the Bachelor Research Project:
Bas Nefs
Maarten Zwetsloot
Overview
• Active Galactic Nuclei• Dusti Tori• NGC1068• MIDI Measurements with the VLTI• Our Research Project
Active Galactic Nuclei• 1943 – Astronomer Carl Seyfert no-
tices that certain nearby spiral gala-xies have very bright almost starlike nuclei.
• First Active Nuclei to be recognized.
• Spectra of these galaxies show strong and often broad emis- sion lines.
• •
Active Galactic Nuclei Some other observations:
AGN are very bright in a wide range of the EM spectrum: luminocities can get up to 1010-1013 L
sun
and higher. They come in a wide variety of types with slightly
different properties – the 'AGN zoo' These nuclei are variable in time, periods from hours
up to months → objects are really small, in the pc size scale.
Nonstellar energy source(s)?
Active Galactic NucleiBuilding blocks
Many models of AGN predict existance of a Central Engine:supermassive Black Hole (106 -
109 Msun
) at the very centres of
galaxies.Surrounding gas clouds fall in
→flat accretion disk of hot gasRelease of gravitational energy→ high energy ionizing
radiation
Active Galactic Nuclei
• Building blocks (continued)
From spectroscophy:
Large regions of gas clouds surrounding the centralengine ionised by radiation.BLR (Broad Line Regions) -- strong and broademission lines -- large Doppler broadening→ large range in velocities due to rotation and turbulent
motionsNLR (Narrow Line Regions) -- narrow lines -- lower
velocities of gas and lower electron densities.
The building blocks of AGN
Active Galactic Nuclei
Active Galactic Nuclei
The unified model (Antonucci 1993): no more zoology
Properties and evolution of different kinds of AGN depend solely on orientation (different viewing angles) and black hole mass.
Jets
Dusty torus
Ionisation cones -- torus collimates the radiation from the central engine in coneshaped regions.
Active Galactic NucleiThe Zoo of AGN
*Seyfert Galaxies
At small distances and at low end of luminocity range (e.g. NGC1068 at 14.4 Mpc)
Active Galactic NucleiSeyfert Galaxies
2 'flavours' depending on the viewing angle:
Seyfert I Seyfert II
The torus is viewed face-on: BLR is well exposed so broad and narrow lines in spectrum.
Edge-on view on torus: BLR obscured by torus
In between:
Dusty Tori
• What are they?Dusty tori are large structures of dust (~100 pc) around the central engines of AGN. They were ‘invented’ by astronomers for the unification of the Seyfert I and Seyfert II galaxies.
These dust structures should have some sort of doughnut shape. When viewed face-on, the broad line region is clearly visible (type I) while this is not the case when viewed edge-on (type II).
Dusty Tori
• First indirect measurementsAntonucci and Miller (1985) discovered broad line features in the polarized UV-spectrum of a Seyfert II galaxy. This indicates this galaxy contains a hidden type I nucleus. The light from the nucleus leaves the dusty torus through the hole and is scattered (and polarized) into our viewing direction. [pic]
• First direct measurementsJaffe et al. (2003) used the newly installed Mid-infrared instrument (MIDI) at the VLTI to obtain measurements of the core of NGC1068 with mas-resolution. By fitting parameters of a ‘2 Gaussian’ model they found an inner hot component (>800 K) and a warm component (350 K). [pic]
Dusty Tori
What does a torus explain?*Hidden Broad Line Regions*Ionization cones*Ratio of type2/type1 AGNs ~ 4:1
•SpectrumDust, heated by the central engine, emitting BB-radiation. [pics]
Dusty Tori
• Problems– By looking at the ratio of Seyfert I and Seyfert II galaxies
we can estimate a mean height for the dusty tori. The height we find is to large for the dust structures to be stable.
NGC1068
VLTI / MIDI
2 Gaussian model
Research Project
What do we hope to achieve in this BP?
*Checking the validity of a simple 2 component model with new interferometric observations from VLTI*Introducing differential phase in torus modelling*If given enough time: extending the modelling to more complex torus geometries, like a warped disk or clum- py torus*Changing dust properties
• Coverage of the (u,v)-plane needs to be increased -> more accurate modelling for different projected baselines