We present spectral energy distributions (SEDs) of 69 QSOs at z>5, covering a rest frame wavelength range of 0.1mu to ~80mu, and centered on new Spitzer and Herschel observations. The detection rate of the QSOs with Spitzer is very high (97% at lambd
a_rest ~< 4mu), but drops towards the Herschel bands with 30% detected in PACS (rest frame mid-infrared) and 15% additionally in the SPIRE (rest frame far-infrared; FIR). We perform multi-component SED fits for Herschel-detected objects and confirm that to match the observed SEDs, a clumpy torus model needs to be complemented by a hot (~1300K) component and, in cases with prominent FIR emission, also by a cold (~50K) component. In the FIR detected cases the luminosity of the cold component is on the order of 10^13 L_sun which is likely heated by star formation. From the SED fits we also determine that the AGN dust-to-accretion disk luminosity ratio declines with UV/optical luminosity. Emission from hot (~1300K) dust is common in our sample, showing that nuclear dust is ubiquitous in luminous QSOs out to redshift 6. However, about 15% of the objects appear under-luminous in the near infrared compared to their optical emission and seem to be deficient in (but not devoid of) hot dust. Within our full sample, the QSOs detected with Herschel are found at the high luminosity end in L_UV/opt and L_NIR and show low equivalent widths (EWs) in H_alpha and in Ly_alpha. In the distribution of H_alpha EWs, as determined from the Spitzer photometry, the high-redshift QSOs show little difference to low redshift AGN.
The detection of powerful near-infrared emission in high redshift (z>5) quasars demonstrates that very hot dust is present close to the active nucleus also in the very early universe. A number of high-redshift objects even show significant excess emi
ssion in the rest frame NIR over more local AGN spectral energy distribution (SED) templates. In order to test if this is a result of the very high luminosities and redshifts, we construct mean SEDs from the latest SDSS quasar catalogue in combination with MIR data from the WISE preliminary data release for several redshift and luminosity bins. Comparing these mean SEDs with a large sample of z>5 quasars we could not identify any significant trends of the NIR spectral slope with luminosity or redshift in the regime 2.5 < z < 6 and 10^45 < nuL_nu(1350AA) < 10^47 erg/s. In addition to the NIR regime, our combined Herschel and Spitzer photometry provides full infrared SED coverage of the same sample of z>5 quasars. These observations reveal strong FIR emission (L_FIR > 10^13 L_sun) in seven objects, possibly indicating star-formation rates of several thousand solar masses per year. The FIR excess emission has unusally high temperatures (T ~ 65 K) which is in contrast to the temperature typically expected from studies at lower redshift (T ~ 45 K). These objects are currently being investigated in more detail.
We present Herschel far-infrared (FIR) observations of two sub-mm bright quasars at high redshift: SDSS J1148+5251 (z=6.42) and BR 1202-0725 (z=4.69) obtained with the PACS instrument. Both objects are detected in the PACS photometric bands. The Hers
chel measurements provide additional data points that constrain the FIR spectral energy distributions (SEDs) of both sources, and they emphasise a broad range of dust temperatures in these objects. For lambda_rest ~< 20mu, the two SEDs are very similar to the average SEDs of quasars at low redshift. In the FIR, however, both quasars show excess emission compared to low-z QSO templates, most likely from cold dust powered by vigorous star formation in the QSO host galaxies. For SDSS J1148+5251 we detect another object at 160mu with a distance of ~10 arcseconds from the QSO. Although no physical connection between the quasar and this object can be shown with the available data, it could potentially confuse low-resolution measurements, thus resulting in an overestimate of the FIR luminosity of the z=6.42 quasar.
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 geometricall
y 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.
Tori of Active Galactic Nuclei are made up of a mixture of hot and cold gas, as well as dust. In order to protect the dust grains from destruction by the hot gas as well as by the energetic radiation of the accretion disk, the dust is often assumed t
o be distributed in clouds. In our new 3D model of AGN dust tori, the torus is modelled as a wedge-shaped disk in which dusty clouds are randomly distributed, by taking the dust density distribution of the corresponding continuous model into account. We especially concentrate on the differences between clumpy and continuous models in terms of the temperature distributions, the surface brightness distributions and interferometric visibilities, as well as spectral energy distributions. To this end, we employ radiative transfer calculations with the help of the 3D Monte Carlo code MC3D. In a second step, interferometric visibilities are calculated from the simulated surface brightness distributions, which can be directly compared to observations with the MIDI instrument. The radial temperature distributions of clumpy models possess significantly enhanced scatter compared to the continuous cases. Even at large distances, clouds can be heated directly by the central accretion disk. The existence of the silicate 10 micron-feature in absorption or in emission depends sensitively on the distribution, the size and optical depth of clouds in the innermost part of the torus, due to shadowing effects of clouds there. This explains failure and success of previous modelling efforts of clumpy tori. After adapting the parameters of our clumpy standard model to the circumstances of the Seyfert 2 Circinus galaxy, it can qualitatively explain recent mid-infrared interferometric observations performed with MIDI, as well as high resolution spectral data.
We report the results of interferometric observations of Centaurus A with the MID-infrared Interferometer (MIDI) at ESOs VLTI telescope array. The interferometric measurements are spectrally resolved (R = 30) in the wavelength range 8 to 13 micron. T
heir spatial resolution reaches 15 mas at the shortest wavelengths. Supplementary observations were obtained in the near-infrared with the adaptive optics instrument NACO, and at mm wavelengths with SEST and JCMT. We find that he mid-infrared emission from the core of Centaurus A is dominated by an unresolved point source (<10 mas). Observations with baselines orientated perpendicular to the radio jet reveal an extended component which can be interpreted as a geometrically thin, dusty disk, the axis of which is aligned with the radio jet. Its diameter is about 0.6 pc. We argue, that the unresolved emission is dominated by a synchrotron source. Its overall spectrum is characterized by a u^{-0.36} power-law which cuts off exponentially towards high frequencies at 8x10^{13} Hz and becomes optically thick at u < 45 GHz. Based on a Synchrotron Self Compton (SSC) interpretation for the gamma-ray emission, we find a magnetic field strength of 26 microTesla and a maximum energy of relativistic electrons of gamma_c = E_c/m_e c^2 = 8500. Near gamma_c, the acceleration time scale is 4 days, in good agreement with the fastest X-ray variations. Our SSC model argues for an upper limit for the bulk Lorentz factor < 2.5, at variance with the concept of a mis-directed BL Lac object. We estimate a thermal core luminosity of 1.3x10^{34} W, intermediate between the values for highly efficiently accreting AGN (e.g. Seyfert galaxies) and those of typical FR I radio galaxies.
To test the dust torus model for active galactic nuclei directly, we study the extent and morphology of the nuclear dust distribution in the Circinus galaxy using high resolution interferometric observations in the mid-infrared with the MIDI instrume
nt at the Very Large Telescope Interferometer. We find that the dust distribution in the nucleus of Circinus can be explained by two components, a dense and warm disk-like component of 0.4 pc size and a slightly cooler, geometrically thick torus component with a size of 2.0 pc. The disk component is oriented perpendicular to the ionisation cone and outflow and seems to show the silicate feature at 10 micron in emission. It coincides with a nuclear maser disk in orientation and size. From the energy needed to heat the dust, we infer a luminosity of the accretion disk corresponding to 20% of the Eddington luminosity of the nuclear black hole. We find that the interferometric data are inconsistent with a simple, smooth and axisymmetric dust emission. The irregular behaviour of the visibilities and the shallow decrease of the dust temperature with radius provide strong evidence for a clumpy or filamentary dust structure. We see no evidence for dust reprocessing, as the silicate absorption profile is consistent with that of standard galactic dust. We argue that the collimation of the ionising radiation must originate in the geometrically thick torus component. Our findings confirm the presence of a geometrically thick, torus-like dust distribution in the nucleus of Circinus, as required in unified schemes of Seyfert galaxies. Several aspects of our data require that this torus is irregular, or clumpy.
