67 research outputs found
A new model for the infrared emission of IRAS F10214+4724
We present a new model for the infrared emission of the high redshift
hyperluminous infrared galaxy IRAS F10214+4724 which takes into account recent
photometric data from Spitzer and Herschel that sample the peak of its spectral
energy distribution. We first demonstrate that the combination of the AGN
tapered disc and starburst models of Efstathiou and coworkers, while able to
give an excellent fit to the average spectrum of type 2 AGN measured by
Spitzer, fails to match the spectral energy distribution of IRAS F10214+4724.
This is mainly due to the fact that the nuSnu distribution of the galaxy falls
very steeply with increasing frequency (a characteristic of heavy absorption by
dust) but shows a silicate feature in emission. We propose a model that assumes
two components of emission: clouds that are associated with the narrow-line
region and a highly obscured starburst. The emission from the clouds must
suffer significantly stronger gravitational lensing compared to the emission
from the torus to explain the observed spectral energy distribution.Comment: 4 pages, 1 figure, to be published in 'The Spectral Energy
Distribution of Galaxies' J. Tuffs \& C.C.Popescu, ed
Self-consistent 2-phase AGN torus models: SED library for observers
We assume that dust near active galactic nuclei (AGN) is distributed in a
torus-like geometry, which may be described by a clumpy medium or a homogeneous
disk or as a combination of the two (i.e. a 2-phase medium). The dust particles
considered are fluffy and have higher submillimeter emissivities than grains in
the diffuse ISM. The dust-photon interaction is treated in a fully
self-consistent three dimensional radiative transfer code. We provide an AGN
library of spectral energy distributions (SEDs). Its purpose is to quickly
obtain estimates of the basic parameters of the AGN, such as the intrinsic
luminosity of the central source, the viewing angle, the inner radius, the
volume filling factor and optical depth of the clouds, and the optical depth of
the disk midplane, and to predict the flux at yet unobserved wavelengths. The
procedure is simple and consists of finding an element in the library that
matches the observations. We discuss the general properties of the models and
in particular the 10mic. silicate band. The AGN library accounts well for the
observed scatter of the feature strengths and wavelengths of the peak emission.
AGN extinction curves are discussed and we find that there is no direct
one-to-one link between the observed extinction and the wavelength dependence
of the dust cross sections. We show that objects of the library cover the
observed range of mid IR colors of known AGN. The validity of the approach is
demonstrated by matching the SEDs of a number of representative objects: Four
Seyferts and two quasars for which we present new Herschel photometry, two
radio galaxies, and one hyperluminous infrared galaxy. Strikingly, for the five
luminous objects we find pure AGN models fit the SED without a need to
postulate starburst activity.Comment: A&A accepted by referee, AGN library available at
http://www.eso.org/~rsiebenm/agn_models/index.htm
Optical depth in polarised Monte Carlo radiative transfer
Context: The Monte Carlo method is the most widely used method to solve radiative transfer problems in astronomy, especially in a fully general 3D geometry. A crucial concept in any Monte Carlo radiative transfer code is the random generation of the next interaction location. In polarised Monte Carlo radiative transfer with aligned non-spherical grains, the nature of dichroism complicates the concept of optical depth.
Aims: We investigate, in detail, the relation between optical depth and the optical properties and density of the attenuating medium in polarised Monte Carlo radiative transfer codes that take dichroic extinction into account.
Methods: Based on solutions for the radiative transfer equation, we discuss the optical depth scale in polarised radiative transfer with spheroidal grains. We compare the dichroic optical depth to the extinction and total optical depth scale.
Results: In a dichroic medium, the optical depth is not equal to the usual extinction optical depth, nor to the total optical depth. For representative values of the optical properties of dust grains, the dichroic optical depth can differ from the extinction or total optical depth by several tens of percent. A closed expression for the dichroic optical depth cannot be given, but it can be derived efficiently through an algorithm that is based on the analytical result corresponding to elongated grains with a uniform grain alignment.
Conclusions: Optical depth is more complex in dichroic media than in systems without dichroic attenuation, and this complexity needs to be considered when generating random free path lengths in Monte Carlo radiative transfer simulations. There is no benefit in using approximations instead of the dichroic optical depth
- …