Quasars and Dust Emission at Low and High Redshifts

Max Camenzind, ... & Marc Schartmann (MPIA)

Motivation: Quasars and their Global Spectra

The energy spectra of Quasars are dominated by three peaks:

The energy distribution of high-redshift quasars: the blue curve represents the observed spectrum of the bright PG Quasar 1634+706 with a redshift of 1.334 (Haas et al. 2000), the red dots the observed spectra of Quasars with redshifts about 4 (VLA, SCUBA and optical data). Distances are measured with a Hubble constant of 65 km/s/Mpc in a standard inflationary cosmological model. The blue and red dotted curves represent thermal emission from the coldest dust in these sources. We expect that the global spectra of these high-redshift Quasars are quite similar to that of the PG Quasar 1634+706, probably a factor two more luminous, indicating the presence of a central Black Hole mass of a few billion solar masses.

Project 1: Dusti Tori in Seyfert Galaxies
(with Marc Schartmann & Klaus Meisenheimer (MPIA))

IRAS and ISO have shown that Seyfert galaxies are strong emitters in the Infrared. According to our present understanding, this is emission from dust heated by the UV spectrum of the central quasar. Dust distribution in the cores of these galaxies is extremely compact, as shown by recent observations with the Keck telescopes in the wavelength region of 8 - 20 µm for the the Seyfert 1 galaxy NGC 7469 and the archetypical Seyfert 2 galaxy NGC 1068 (Soifer et al. 2003). Better spatial resolution will be achieved with interferometric techniques. This distribution of warm dust can be probed with the MIDI instrument using two VLT unit telescopes which provide an interferometric arm length upto 120 m, corresponding to a spatial resolution which is 15 times better than for a single unit telescope.


The left pannel shows the image of NGC 7469 with a logarithmic scale, while the right pannel shows the same image with a linear scale and cuts to enhance the starburst ring around it (Source: Keck adaptive optics). This image was obtained through the wide band H filter (1.65 microns). It is a mosaic of images obtained over two nights (July 23rd and 24th 1999); each individual frame was 5 second long and the image is the sum of approximately one hundred such frames. The central point source is the quasar, while the extended emission is from young stars in the starburst region.

This is an image of NGC 7469 at 12.5 µm obtained with the Keck telescope showing warm dust emission heated in the starburst ring. The central dust is heated by the quasar´s UV emission. 0.1 arcsecond corresponds to 30 parsecs. The central dust torus is very compact when observed at 12 µm, corresponding to a temperature of ~ 300 K. There should be a hole in the middle created by the sublimation of the dust in the strong UV radiation field. This is not resolved by the Keck telescope, i.e. the hole is less than 10 parsecs. On theoretical grounds, we expect a hole with a dimension of less than one parsec ! The Black Hole has a mass of about 10 Million solar masses, as derived from reverberation mapping (St. Andrews). This central gas and dust distribution is the reservoir from which the Black Hole gets fuelled. Gas located beyond ~ 50 parsecs is not relevant for feeding the monster.

NGC 1068 is the archetypical Seyfert 2 galaxy with its famous ionization cone extending in NE direction. A jet is emanating within the ionization cone. At a distance of 18 Mpc, one arcsecond corresponds to ~ 80 parsecs. The emission seen with VLBA occurs probably within the sublimation hole in the dust distribution and could be due to Bremsstrahlung from the accretion started towards the central Black Hole.

A disk of water molecules orbiting a supermassive black hole at the core of NGC 1068 which is ~15 Mpc away is "reverberating" in response to variations in the energy output from the galaxy's powerful "central engine" close to the black hole. A team of astronomers used the National Science Foundation's (NSF) Very Large Array (VLA) radio telescope in New Mexico and the 100-meter-diameter radio telescope of the Max Planck Institute for Radio Astronomy at Effelsberg, Germany, to observe the galaxy NGC 1068 in the constellation Cetus. In NGC 1068 lies a megamaser source which enables us to probe the structure and the velocity field of the innermost part of the torus. Greenhill et al. (1996) observed the water maser emission in NGC 1068 with the VLBA and obtained images with sub-milliarcsecond angular resolution. Their best fit rotation curve for the redshifted maser emission was sub-Keplerian; the line-of-sight velocity of the maser source decreased as for 0.57pc < r < 0.92pc.

The strongest evidence that the masers are responding to variations in the output of the central engine came from watching variations in the brightness of masers on opposite sides of the water molecule disk. The masers on both sides of the molecular disk, some 5 light-years across, brightened within about two weeks of each other. If this were caused by something within that molecular disk itself, it would take about 10,000 years to affect both sides of the disk, because of the orbital times involved. However, both sides of the disk are the same distance from the central engine, so they can both respond to the central engine simultaneously.

The dust emission from funnels:

The dust emission seen at 12.5 µm in NGC 1068 is extremely compact (Keck observation). But even here, the sublimation hole in the core is not resolved. The extended dust emission is probably due to extremely steep funnel walls and only the right funnel edge is visible. Some emission from the counter-walls is also seen.

This comparison between the HST image and the Keck 12.5 µm image shows that the central funnel is probably very compact, extending in vertical direction only to ~ 50 parsecs. The left funnel wall is nicely visible in the [OIII] image, but the optical emission fills in the entire funnel, as indicated by the dashed lines. This comparison also suggests that dust coexist with hotter phases, the torus is obviously very cloudy. It could also coexist with X-ray gas.

The torus has a complicated temperature distribution, where sublimation temperatures are achieved near the funnel edges. This provides a nice global spectrum. The fact that the energy distribution is flat downto 4 µm indicates that sublimation temeprature is achieved and that the sublimation hole is less than one parsec !

Spectra:

The core (b) and the counter-wall (a) show the 10 µm silicate absorption feature. The dust emission from the counter-wall is strongly absorbed below 10 µm, suggesting that the total optical depth at 10 µm strongly exceeds unity.

Project 2: Gas and Dust Accumulation in the Bulge of Ellipticals

Massive Black Holes are hosted by very massive bulges . The cores of massive ellipticals form at very high redshifts in the centers of young clusters by merging processes. After about one Gyear, the initial starburst deceases away and stellar evolution leads to heavy mass injection into the core. At high redshifts, gas and dust are accumulated in the bulge of the spheroidal component of the stellar distribution and form a toroidal structure on the parsec-scale extending to the core-radius of the stellar bulge. This injection decreases in the local Universe so that bulges are merely filled up by a thin hot gas ( see M87) , dust will be dissolved by the high temperature.

In giant ellipticals, the stellar density profiles are very flat in the core region and decay rapidly outside the core radius of about one kiloparsec (red distribution, Nuker profile). Within this core gas and dust form a highly turbulent medium confined by a toroidal structure, which decays into small turbulent cells. Emission from the accretion disk (white core) is scattered along the funnel of the torus into the polar direction. This geometry is typical for a Type I Quasar at high redshift. In Type II Quasars, the inclination towards the polar axis is higher and no emission from the central accretion disk can be detected. Dust emission from the torus is however still detectable in the FIR (ISO, SIRTF and SCUBA). Hard X-rays can also be detected, since they still penetrate the dusty torus.

Project 3: Quasars, Dust and Jet Propagation

When jets are launched from the vicinity of Black Holes inside of less than one parsec, they have to propagate through this hot gas. Inside some characteristic distance, they move freely and expand due to internal dissipation. At the scale of some 10 kiloparsecs, they have expanded to diameters in the range of one kiloparsec, their beam density is typically fallen below the cluster gas density, depending somewhat on the mass-loss rate in the jets. The most powerful jets loose masses of the order of a fraction of a solar mass per year, micro-jets such as the one in M 87 less than 0.1 percent of a solar mass per year. The plasma in these jets is quite exotic. Due to their propagation at nearly the speed of light, the entire plasma is heated to temperatures in the range of a few 100 billion degrees Kelvin. This means that the electrons are relativistic with minimal Lorentz factors in the range of 10 - 100. Electrons can easily be accelerated to even higher energies by various processes in the plasma.

Jets in Quasars are launched along the polar funnel.

The thermal cluster gas is heated by the bow shock to temperature of 100 Mio Kelvin, provided the head of the jet propagates with a speed of a few thousand kilometers through the cluster gas. In thinner clusters, the cluster gas is heated to even higher temperatures, since the bow shocks can propagate with some fraction of the speed of light. This gas cools by the emission of Bremsstrahlung photons.