Efstathiou, Andreas

We present ISOSWS, ISOPHOT_S and 8-13 μm observations of Centaurus A which show prominent PAH and silicate features. These and other data are used to construct a model for the infrared continuum. We find that, in a small nuclear aperture (∼4 arcsec, ∼60 pc), the spectral energy distribution is characteristic of emission from a starburst and dusty AGN torus; in larger apertures, additional components of cirrus and starburst emission are required. The model components are based on the radiative transfer models of Efstathiou et al. which include multiple scattering and the radiative effects of a dust-embedded source with a distribution of grain species and sizes. The torus component is modelled in terms of a tapered dusty disc centrally illuminated by a quasar-like source. The cirrus and starburst components are, respectively, modelled in terms of diffuse dust illuminated by the interstellar medium and an ensemble of optically thick molecular clouds centrally illuminated by hot stars. These latter components additionally include emission from small graphite particles and PAHs. Based on our overall model, the torus diameter is estimated to be 3.6pc and the best inclination angle of the torus is 45°. We present independent observational evidence for this structure. This result has implications for the detectability of tori in low-power AGN and for the use of the IRAS 60/25-μm flux ratio as an indicator of the torus inclination.

While the torus plays an obvious role in blocking direct light, and thus increasing the observed polarization from scattering, its role in directly producing polarized flux is less clear. Contributions to the infrared polarization from aligned grains within the torus are discussed.

We present a model for the nuclear infrared (IR) continuum spectrum of the Seyfert galaxy NGC 1068. The torus emission is modelled in terms of the tapered disc models of Efstathiou & Rowan-Robinson, which give a good fit to the global infrared properties of active galactic nuclei. The models include the effects of a distribution of grain species and sizes and multiple scattering from dust. Our analysis is constrained by the inclination of the torus predicted by optical spectropolarimetry. We assume in particular that our line of sight is inclined to the axis of symmetry by about 35°, and that the half-opening angle of the cone is 30°. We find that the torus emission alone cannot account for the whole of the IR continuum spectrum. While this is in agreement with recent mid-IR imaging observations, which show that up to 60 per cent of the flux is not originating from the torus, our model suggests that the difference between the observed and predicted torus emission is actually much greater at near-IR wavelengths. We attribute this excess IR emission to a component of optically thin dust [Av=0.1-0.5 mag) located in the ionization cone between the BLR and the NLR. This dust must be distributed as r-2 in order to produce the required spectrum. Flatter density distributions peak at longer density distributions peak at longer wavelengths and also produce a strong emission feature at 10 µm, contrary to observations. Even with an r-2 distribution, the grain mixture in the cone needs to be modified in order to suppress further the silicate emission feature. We suggest that this may be due to either destruction of silicate grains by shocks or the clumping of NLR dust. In addition, our model requires that the flux radiated by the central source towards the cone is at least a factor of 6 higher than that directed towards the bulk of the torus, which is naturally explained if the central source is an accretion disc. This conclusion depends mainly on the assumed inclination and opening angle of the torus, but is rather insensitive to other geometrical parameters.

We present positions and fluxes of point sources found in the Infrared Space Observatory (ISO) images of the Hubble Deep Field (HDF) at 6.7 and 15 μm. We have constructed algorithmically selected 'complete' flux-limited samples of 19 sources in the 15-μm image, and seven sources in the 6.7-μm image. The typical flux limit at 15 μm is ∼0.2 mJy and at 6.7 μm is ∼0.04 mJy. We have selected 'supplementary' samples of three sources at 15 μm and 20 sources at 6.7 μm by eye. We discuss the completeness and reliability of the connected pixel source detection algorithm used, by comparing the intrinsic and estimated properties of simulated data, and also by estimating the noise properties of the real data. The most pessimistic estimate of the number of spurious sources in the 'complete' samples is one at 15 μm and two at 6.7 μm, and in the 'supplementary' lists is one at 15 μm and five at 6.7 μm.

A modified version of the code of Efstathiou and Rowan-Robinson (1990), that solves accurately the axially symmetric radiative transfer problem in dust clouds, is used to model the infrared emission from dust in Active Galactic Nuclei. The method takes into account a distribution of grain species and sizes and includes treatment of multiple scattering from grains. Arguments are presented supporting the idea that tapered discs (discs whose height increases with distance from the central source but tapers off to a constant height in their outer part) with steep density gradients are the most successful in satisfying the observational constraints.

The ISOPHOT Serendipity Survey utilizes the slew time between ISO's pointed observations with strip scanning measurements of the sky in the far-IR at 170 micrometers. The slews contain information about two fundamentally different types of objects, namely unresolved galactic and extragalactic far-IR sources as well as extended regions of galactic cirrus emission. Since the structure of the obtained data is almost unique, the development of dedicated software to extract astrophysically interesting parameters for the crossed sources is mandatory. Data analysis is currently in its early stages and concentrates on the detection of point sources. First results from an investigation of a high galactic latitude field near the North Galactic Pole indicate that the detection completeness with respect to previously known IRAS sources will be almost 100 percent for sources with f(subscript 100micrometers > 2 Jy, dropping below approximately equals 50 percent for f(subscript 100micrometers < 1.5 Jy. Nevertheless, even faint sources down to a level of f(subscript 170micrometers approximately equals 1 Jy can be detected. Since the majority of the detected point sources are galaxies, the Serendipity Survey will result in a large database of approximately equals 2000 galaxies.

We present a method for calculating the infrared polarization due to absorption and emission by aligned grains distributed in a disc or a torus. The temperature distribution in the torus is calculated with the axially symmetric radiative transfer code of Efstathiou & Rowan-Robinson. The polarization patterns predicted for a torus that is believed to be typical of those found in active galactic nuclei (AGN) show a characteristic flip in position angle, as the polarization changes from dichroic absorption to dichroic emission. The wavelength at which the flip occurs is very sensitive to the optical thickness of the torus but not to the fraction of aligned grains, their elongation or any other parameter we have explored. We also find that the predicted polarization is not sensitive to whether grain alignment extends into the hot (inner) part of the disc

We discuss the identification of sources detected by the Infrared Space Observatory (ISO) at 6.7 and 15 μm in the Hubble Deep Field (HDF) region. We conservatively associate ISO sources with objects in existing optical and near-infrared HDF catalogues using the likelihood ratio method, confirming these results (and, in one case, clarifying them) with independent visual searches. We find 15 ISO sources to be reliably associated with bright [I814 (AB) < 23] galaxies in the HDF, and one with an I814(AB) = 19.9 star, while a further 11 are associated with objects in the Hubble Flanking Fields (10 galaxies and one star). Amongst optically bright HDF galaxies, ISO tends to detect luminous, star-forming galaxies at fairly high redshift and with disturbed morphologies, in preference to nearby ellipticals.

We present a generalization of the method of Efstathiou & Rowan-Robinson for the solution of the radiative transfer problem in dust clouds that takes into account a distribution of grain species and sizes. The method includes treatment of multiple scattering from grains and can be applied to axisymmetric density distributions. In this paper, however, we restrict our attention to the special case of spherical symmetry. We find that, in the parameter space we have explored, composite grains with 'average' absorption and scattering efficiencies approximate very well the emission from the grain mixture of Rowan-Robinson. We also re-examine the sample of compact H n regions of Crawford & Rowan-Robinson, and find that the combined effect of the multigrain calculations and a new model for interstellar grains by Rowan-Robinson is to improve considerably the agreement of models that assume high-optical-depth (Av ≈ 20-200) homogeneous clouds around hot stars with the IRAS data. For four objects we are able to compare our models with submillimetre data (350-1300 (xm). For W3(OH) the fit is good, but for the other three objects the models predict too much flux at long wavelengths. Possible reasons for this are discussed.

We present optical/infrared broad-band filter polarimetry and optical spectropolarimetry of the powerful FR II galaxy 3C 321. The latter observations reveal a scattered broad component to the Hα emission line, detectable in polarized flux. This implies that 3C 321 is actually a quasar the continuum and broad-line region of which are obscured from direct view, possibly by the geometrically thick torus invoked in the unified theory of active galaxies.