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Register a new userThe effect of stellar feedback on the formation and evolution of gas and dust tori in AGN
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Recently, the existence of geometrically thick dust structures in Active Galactic Nuclei (AGN) has been directly proven with the help of mid-infrared interferometry. The observations are consistent with a two-component model made up of a geometrically thin and warm central disk, surrounded by a colder, fluffy torus component. In an exploratory study, we investigate one possible physical mechanism, which could produce such a structure, namely the effect of stellar feedback from a young nuclear star cluster on the interstellar medium in centres of AGN. The model is realised with the help of the hydrodynamics code TRAMP. We follow the evolution of the interstellar medium by taking discrete mass loss and energy ejection due to stellar processes, as well as optically thin radiative cooling into account. In a post-processing step, we calculate observable quantities (spectral energy distributions and images) with the help of the radiative transfer code MC3D. The interplay between injection of mass, supernova explosions and radiative cooling leads to a two-component structure made up of a cold geometrically thin, but optically thick and very turbulent disk residing in the vicinity of the angular momentum barrier, surrounded by a filamentary structure. The latter consists of cold long radial filaments flowing towards the disk and a hot tenuous medium in between, which shows both inwards and outwards directed motions. This modelling is able to reproduce the range of observed neutral hydrogen column densities of a sample of Seyfert galaxies as well as the relation between them and the strength of the silicate 10 micron spectral feature. Despite being quite crude, our mean Seyfert galaxy model is even able to describe the SEDs of two intermediate type Seyfert galaxies observed with the Spitzer Space Telescope.
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We present new, spatially resolved [CI]1-0, [CI]2-1, CO(7-6), and dust continuum observations of 4C 41.17 at $z=3.8$ obtained with the IRAM NOEMA interferometer. This is one of the best-studied radio galaxies in this epoch and is arguably the best candidate of jet-triggered star formation at high redshift currently known in the literature. 4C 41.17 shows a narrow ridge of dust continuum extending over 15 kpc near the radio jet axis. Line emission is found within the galaxy in the region with signatures of positive feedback. Using the [CI]1-0 line as a molecular gas tracer, and multifrequency observations of the far-infrared dust heated by star formation, we find a total gas mass of $7.6times 10^{10}$ M$_{odot}$, which is somewhat greater than that previously found from CO(4-3). The gas mass surface density of $10^3$ M$_{odot}$ yr$^{-1}$ pc$^{-2}$ and the star formation rate surface density of 10 M$_{odot}$ yr$^{-1}$ kpc$^{-2}$ were derived over the 12 kpc$times$8 kpc area, where signatures of positive feedback have previously been found. These densities are comparable to those in other populations of massive, dusty star-forming galaxies in this redshift range, suggesting that the jet does not currently enhance the efficiency with which stars form from the gas. This is consistent with expectations from simulations, whereby radio jets may facilitate the onset of star formation in galaxies without boosting its efficiency over longer timescales, in particular after the jet has broken out of the interstellar medium, as is the case in 4C 41.17.
We present preliminary results from high resolution (~ 0.05) adaptive optics observations of Cygnus A. The images show a bi-conic structure strongly suggestive of an obscuring torus around a quasar nucleus. A bright (K=18.5) point source is found near the expected position of the nucleus. We interpret this source as the hot inner rim of the torus seen through the opening of the torus. Using high angular resolution K-band spectroscopy, we measure the ratio of molecular to recombination hydrogen lines as a function of distance to the center of the putative torus. These measurements place constraints on the properties of the torus and indicate a projected diameter of ~600 pc.
Interferometric measurements in the mid-infrared have shown that the sizes of the warm dust distributions in active galactic nuclei are consistent with their scaling with the square root of their luminosity. We carry out a more detailed analysis of this size-luminosity relation to investigate which of the general properties of the dusty tori in active galactic nuclei can be derived from this relation. We improve the accuracy of the size-luminosity relation by adding a few additional size measurements from more recent interferometric observations and compare the measured sizes to those derived from hydrodynamical and radiative transfer models of AGN tori. We find that a Gaussian approximation yields a reasonable estimate of the size of the brightness distribution, as long as the visibilities are within 0.2 {leg} V {leg} 0.9. The size estimates derived from the models are consistent with those determined from the measurements. However, the models predict a significant offset between the sizes derived for face-on and edge-on tori. This offset is not observed in the current data, probably because of the large uncertainties and low statistics of the present interferometric measurements. Furthermore, we find a ratio of the mid- to near-infrared sizes of approximately 30, whereas the first probes the body of the torus and the second is an estimate of the inner rim. The size-luminosity relation of AGN tori using Gaussian size estimates is a very simple and effective tool to investigate the internal structure and geometry of AGN tori and obtain constraints on the differences between type 1 and type 2 AGN. However, to fully exploit the possibilities of investigating the nuclear distributions of gas and dust in AGN using this size-luminosity relation, more accurate interferometric measurements of a larger sample of AGN are needed.
When, and how, did the first galaxies and supermassive black holes (SMBH) form, and how did they reionization the Universe? First galaxy formation and cosmic reionization are among the last frontiers in studies of cosmic structure formation. We delineate the detailed astrophysical probes of early galaxy and SMBH formation afforded by observations at centimeter through submillimeter wavelengths. These observations include studies of the molecular gas (= the fuel for star formation in galaxies), atomic fine structure lines (= the dominant ISM gas coolant), thermal dust continuum emission (= an ideal star formation rate estimator), and radio continuum emission from star formation and relativistic jets. High resolution spectroscopic imaging can be used to study galaxy dynamics and star formation on sub-kpc scales. These cm and mm observations are the necessary compliment to near-IR observations, which probe the stars and ionized gas, and X-ray observations, which reveal the AGN. Together, a suite of revolutionary observatories planned for the next decade from centimeter to X-ray wavelengths will provide the requisite panchromatic view of the complex processes involved in the formation of the first generation of galaxies and SMBHs, and cosmic reionization.
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