Literature Study for the Bachelor Research Project: The Dusty Torus of NGC1068 Bas Nefs Maarten Zwetsloot

Overview Active Galactic Nuclei Dusti Tori NGC1068 VLTI / MIDI Interferometry Our Research Project

Active Galactic Nuclei 1943 – Astronomer Carl Seyfert notices that certain nearby spiral galaxies have very bright almost starlike nuclei. First Active Nuclei to be recognized. Spectra of these galaxies show strong and often broad emission lines.

Active Galactic Nuclei Some other observations: AGN are very bright in a wide range of the EM spectrum: Luminosities can get up to 1010-1013 Lsun 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 Nuclei Building blocks Many models of AGN predict existence of a Central Engine (CE): Supermassive Black Hole (106 - 109 Msun) at the very centers of galaxies. Surrounding gas clouds fall in → flat accretion disk of hot gas. Release of gravitational energy → high energy ionizing radiation

Active Galactic Nuclei Building blocks (continued) From spectroscopy: Large regions of gas clouds surrounding the central engine ionised by radiation. BLR (Broad Line Regions): Strong and broad emission lines. Large range in velocities due to rotation and turbulent motions → Large Doppler broadening. NLR (Narrow Line Regions): Narrow lines. lower velocities of gas and lower electron densities.

Dusty Tori Unification Torus in Seyfert galaxies: Antonucci (1993): Zoo of AGN consists of only two types: Radio loud and radio quiet. Other types are due to orientation-dependent observational differences instead of intrinsic differences. Torus in Seyfert galaxies: Face-on view: BLR is well exposed so broad and narrow lines in spectrum (Seyfert I). Edge-on view: BLR obscured by torus (Seyfert II). In between: BLR only detected by scattered light.

Dusty Tori First indirect measurements First direct measurements Broad line features in polarized UV-spectrum of a Seyfert II galaxy (Antonucci and Miller, 1985) Indicates presence of hidden type I nucleus. Light from nucleus travels through torus hole and is scattered (and polarized) into our viewing direction. First direct measurements MIDI interferometric measurements of the core of NGC1068 (Jaffe et al., 2003). Fitting of a ‘2 Gaussian’ model: Inner hot component (>800 K) and outer warm component (320 K). Antonucci and Miller (1985) discovered broad line features in the polarized UV-spectrum of a Seyfert II galaxy, coming from 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. Jaffe 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).

Dusty Tori Spectrum Dust, heated by the central engine, emitting BB-radiation. Probably two different types of dust absorption. Fig. 2.— Spectral energy distribution of Cygnus A at 3–30 μm. The ordinate is F in W m−2, and the abscissa is wavelength in μm in the observed frame. The dashed line is the output spectrum of DUSTY with an outer-to-inner radius ratio of 200, a power-law index of 2.5 in the dust radial distribution, and an optical depth at 0.55 μm of 140 toward the central engine. Large filled circles: Data from the literature (Table 1). Small filled circles: Our LWS data. Any model output spectra that fit the observed data at >10 μm overpredict the fluxes at 4.8 and 7.3 μm. This could be caused by time variation.

NGC1068 Properties: Seyfert II spiral galaxy (Sb type) Brightest (9.6 mag) and closest (14.4 Mpc) Seyfert galaxy Size: 7.0’ x 5.9’

VLTI / MIDI VLTI: 4 VLT Unit Telescopes (D = 8.2 m) and 4 movable auxiliary telescopes (D = 1.8 m). Baselines up to 130 (UT) and 200 m (AT). MIDI: mid-infrared inter-ferometer installed at the VLTI, measures from 8 to 13 micron with 10 mas-resolution. Dust emits thermally in the infrared, MIDI measures with high resolution in mid-infrared → MIDI is the perfect instrument for detecting dust tori. MIDI is a mid-infrared interferometer that combines light coming from two telescopes of the VLT. The VLT has four 8.2 m unit telescopes (UT) for which baselines up to 130 m can be obtained, and several relocatable 1.8 m auxiliary telescopes (AT) with baselines up to 200 m. For 10 micron this means that MIDI can reach an angular resolution of 10 mas. Because of this high resolution and the fact that dust emits light in the infrared due to thermal radiation, the MIDI instrument is very suitable for detecting dust tori around AGN’s directly. In the initial scientific runs in June and November 2003, MIDI observed the core of NGC1068 and measured two UV points at 42 m and 78 m baselines. This was done with two UT’s of the VLT. For the first time ever MIDI made direct observations of a dust torus around an AGN.

Interferometry How does it work? Two telescopes detect incoming EM-waves at a given wavelength. These signals are added and form an interferometric pattern. This gives us the visibility-function. The visibility V(u,v) tells us something about the structure of the light distribution at the sky. Bigger baselines means information of smaller structures ( = 1.22  / D). UV coordinates: (u,v) = B(x,y) /  More visibility measurements by different baselines means a better sampling of the UV-plane. Fourier transform visibilities to obtain image

Interferometry Measurements: Total flux: The total flux received from the source. Correlated flux: Amplitude of the visibility times the total flux. Differential phase: Normally we would like to measure the phase of the visibility. But the data reduction process removes the linear part with the wavenumber of the phase. What’s left is called the differential phase.

Research Project Goal of this Bachelor Project Approach Checking the validity of a simple 2 component model with new interferometric observations from the VLTI. Introducing differential phase in torus modelling. Approach Low uv-sampling, can’t Fouriertransform to obtain image. Make a model instead and calculate flux and visibilities. Compare with real observations and fit parameters.

Research Project 2 Gaussian model Clumpy torus Warped disk 2003 MIDI observations resolve the putative torus for the first time. Initial modelling (Jaffe et al. 2004): Warm resolved (T=320 K) and small hot unresolved (T>800K) components with Gaussian brightness distribution and distinct silicate absorption profiles. Clumpy torus Warped disk

Research Project Our initial 3D model: 3D torus consisting of two dust components. Hot inner component: Temperature decreases with a power Phot of the radius from Thot to Twarm. BB-radiation is affected by Silicate absorption. Warm outer component: Temperature decreases with power Pwarm. BB-radiation is affected by Calcium absorption. Free parameters: Inclination, Thot, Twarm, Tcold, Rwarm, Phot, Pwarm, ααhot, ααcold

Research Project

Research Project When time permits we could try Different geometries, like a flared disk or a clumpy torus. Other dust properties by using different dust absorption types.