72 research outputs found
A complete census of silicate features in the mid-infrared spectra of active galaxies
We present a comprehensive study of the silicate features at 9.7 and 18
micron of a sample of almost 800 active galactic nuclei (AGN) with available
spectra from the Spitzer InfraRed Spectrograph (IRS). We measure the strength
of the silicate feature at 9.7 micron, S9.7, before and after subtracting the
host galaxy emission from the IRS spectra. The numbers of type 1 and 2 AGN with
the feature in emission increase by 20 and 50%, respectively, once the host
galaxy is removed, while 35% of objects with the feature originally in
absorption exhibit it in even deeper absorption. The peak of S9.7, lambda_peak,
has a bimodal distribution when the feature is in emission, with about 65% of
the cases showing lambda_peak > 10.2 micron. Silicates can appear in emission
in objects with mid-infrared (MIR) luminosity spanning over six orders of
magnitude. The derived distributions of the strength of the silicate features
at 9.7 and 18 micron provide a solid test bed for modeling the dust
distribution in AGN. Clumpiness is needed in order to produce absorption
features in unobscured AGN and can also cause the silicates to be in absorption
at 9.7 micron and in emission at 18 micron in type 1 sources. We find the
`cosmic' silicates of Ossenkopf et al. to be more consistent with the
observations than Draine's `astronomical' silicates. Finally, we discuss the
possibility of a foreground absorber to explain the deep silicate absorption
features in the MIR spectra of some type 2 AGN.Comment: 10 pages, 11 figures. Accepted for publication in Ap
Scale Length of Disk Galaxies
As a part of a Euro-VO research initiative, we have undertaken a programme
aimed at studying the scale length of 54909 Sa-Sd spiral galaxies from the SDSS
DR6 catalogue. We have retrieved u,g,r,i,z-band images for all galaxies in
order to derive the light profiles. We also calculate asymmetry parameters to
select non-disturbed disks for which we will derive exponential disk scale
lengths. As images in different bands probe different optical depths and
stellar populations, it is likely that a derived scale length value should
depend on waveband, and our goal is to use the scale length variations with
band pass, inclination, galaxy type, redshift, and surface brightness, in order
to better understand the nature of spiral galaxies.Comment: Invited talk at the workshop "Multiwavelegth Astronomy and Virtual
Observatory" at ESA/ESAC in december 200
Disentangling accretion disk and dust emissions in the infrared spectrum of type 1 AGN
We use a semi-empirical model to reproduce the 0.1-10um spectral energy
distribution (SED) of a sample of 85 luminous quasars. In the model, the
continuum emission from the accretion disk as well as the nebular lines are
represented by a single empirical template (disk), where differences in the
optical spectral index are reproduced by varying the amount of extinction. The
near- and mid-infrared emission of the AGN-heated dust is modelled as the
combination of two black-bodies (dust). The model fitting shows that the disk
and dust components are remarkably uniform among individual quasars, with
differences in the observed SED largely accounted for by varying levels of
obscuration in the disk as well as differences in the relative luminosity of
the disk and dust components. By combining the disk-subtracted SEDs of the 85
quasars, we generate a template for the 1-10um emission of the AGN-heated dust.
Additionally, we use a sample of local Seyfert 1 galaxies with full
spectroscopic coverage in the 0.37um to 39um range to demonstrate a method for
stitching together spectral segments obtained with different PSF and extraction
apertures. We show that the disk and dust templates obtained from luminous
quasars also reproduce the optical-to-mid-infrared spectra of local Seyfert 1s
when the contribution from the host galaxy is properly subtracted.Comment: 8 pages, 4 figures; accepted for publication in Frontiers in
Astronomy and Space Sciences, Research Topic "Quasars at all cosmic epochs";
proceedings of the conference "Quasars at all cosmic epochs", held in Padova,
April 2-7, 201
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