43 research outputs found
OBSCURATION IN ACTIVE GALACTIC NUCLEI
All classes of Active Galactic Nuclei (AGN) are fundamentally powered by accretion of gas onto a supermassive black hole. The process converts the potential energy of the infalling matter to X-ray and ultraviolet (UV) radiation, releasing up to several 1012 solar luminosities.
Observations show that the accreting central engines in AGN are surrounded by dusty matter. The dust occupies a torus around the AGN which is comprised of discrete clumps. If the AGN radiation is propagating through the torus on its way to an observer, it will be heavily re-processed by the dust, i.e. converted from UV to infrared (IR) wavelengths. Much of the information about the input radiation is lost in this conversion process while an imprint of the dusty torus is left in the released IR photons.
Our group was the first to formulate a consistent treatment of radiative transfer in a clumpy medium an important improvement over simpler models with smooth dust distributions previously used by researchers. Our code CLUMPY computes spectral energy distributions (SED) for any set of model parameters values. Fitting these models to observed AGN SEDs allows us to determine important quantities, such as the torus size, the spatial distribution of clumps, the torus covering factor, or the intrinsic AGN luminosity. Detailed modeling also permits us to study the complex behavior of certain spectral features.
IR radiative transfer introduces degeneracies to the solution space: different parameter values can yield similar SEDs. The geometry of the torus further exacerbates the problem. Knowing the amount of parameter degeneracy present in our models is important for quantifying the confidence in data fits. When matching the models to observed SEDs we must employ modern statistical methods. In my research I use Bayesian statistics to determine the likely ranges of parameter values. I have developed all tools required for fitting observed SEDs with our large model database: the latest implementation of CLUMPY, the fit algorithms, the Markov Chain Monte Carlo sampler, and the Bayesian estimator. In collaboration with observing groups we have applied our methods to a multitude of real-life AGN
On the 10-micron silicate feature in Active Galactic Nuclei
The 10-micron silicate feature observed with Spitzer in active galactic
nuclei (AGN) reveals some puzzling behavior. It (1) has been detected in
emission in type 2 sources, (2) shows broad, flat-topped emission peaks shifted
toward long wavelengths in several type 1 sources, and (3) is not seen in deep
absorption in any source observed so far. We solve all three puzzles with our
clumpy dust radiative transfer formalism. (1) We present the spectral energy
distribution (SED) of SST1721+6012, the first type 2 quasar observed to show a
clear 10-mic silicate feature in emission. We constructed a large database of
clumpy torus models and performed extensive fitting of the observed SED,
constraining several of the torus parameters. We find that the source
bolometric luminosity is ~3*10^12 L_sun. Our modeling suggests that <35% of
objects with tori sharing characteristics and geometry similar to the best fit
would have their central engines obscured. This relatively low obscuration
probability can explain the clear appearance of the 10-mic emission feature in
SST1721+6012 together with its rarity among other QSO2. (2) We also fitted the
SED of PG1211+143, one of the first type 1 QSOs with a 10-mic silicate feature
in emission. Among similar sources, this QSO appears to display an unusually
broadened feature whose peak is shifted toward longer wavelengths. Although
this led to suggestions of non-standard dust chemistry in these sources, our
analysis fits such SEDs with standard galactic dust; the apparent peak shifts
arise from radiative transfer effects. (3) We find that the distribution of
silicate feature strengths among clumpy torus models closely resembles the
observed distribution, and the feature never occurs deeply absorbed. (abridged)Comment: 11 pages, 9 figures, 4 tables, accepted for publication in ApJ; minor
revision, added reference in Section
A Tale of Three Galaxies: Deciphering the Infrared Emission of the Spectroscopically Anomalous Galaxies IRAS F10398+1455, IRAS F21013-0739 and SDSS J0808+3948
The \textit{Spitzer}/Infrared Spectrograph spectra of three spectroscopically
anomalous galaxies (IRAS~F10398+1455, IRAS~F21013-0739 and SDSS~J0808+3948) are
modeled in terms of a mixture of warm and cold silicate dust, and warm and cold
carbon dust. Their unique infrared (IR) emission spectra are characterized by a
steep \simali5--8\mum emission continuum, strong emission bands from
polycyclic aromatic hydrocarbon (PAH) molecules, and prominent silicate
emission. The steep \simali5--8\mum emission continuum and strong PAH
emission features suggest the dominance of starbursts, while the silicate
emission is indicative of significant heating from active galactic nuclei
(AGNs). With warm and cold silicate dust of various compositions ("astronomical
silicate," amorphous olivine, or amorphous pyroxene) combined with warm and
cold carbon dust (amorphous carbon, or graphite), we are able to closely
reproduce the observed IR emission of these %spectroscopically anomalous
galaxies. We find that the dust temperature is the primary cause in regulating
the steep 5--8\mum continuum and silicate emission, insensitive to the
exact silicate or carbon dust mineralogy and grain size as long as
a\simlt1\mum. More specifically, the temperature of the \simali5--8\mum
continuum emitter (which is essentially carbon dust) of these galaxies is
250--400\K, much lower than that of typical quasars which is
640\K. Moreover, it appears that larger dust grains are preferred in
quasars. The lower dust temperature and smaller grain sizes inferred for these
three galaxies compared with that of quasars could be due to the fact that they
may harbor a young/weak AGN which is not maturely developed yet.Comment: 31 pages, 14 figures, accepted for publication in Ap
Subaru Spectroscopy and Spectral Modeling of Cygnus A
We present high angular resolution (0.5) MIR spectra
of the powerful radio galaxy, Cygnus A, obtained with the Subaru telescope. The
overall shape of the spectra agree with previous high angular resolution MIR
observations, as well as previous Spitzer spectra. Our spectra, both on and off
nucleus, show a deep silicate absorption feature. The absorption feature can be
modeled with a blackbody obscured by cold dust or a clumpy torus. The deep
silicate feature is best fit by a simple model of a screened blackbody,
suggesting foreground absorption plays a significant, if not dominant role, in
shaping the spectrum of Cygnus A. This foreground absorption prevents a clear
view of the central engine and surrounding torus, making it difficult to
quantify the extent the torus attributes to the obscuration of the central
engine, but does not eliminate the need for a torus in Cygnus A
The Meaning of \u3cem\u3eWISE\u3c/em\u3e Colours – I. The Galaxy and its Satellites
Through matches with the Sloan Digital Sky Survey (SDSS) catalogue we identify the location of various families of astronomical objects in WISE colour space. We identify reliable indicators that separate Galactic/local from extragalactic sources and concentrate here on the objects in our Galaxy and its closest satellites. We develop colour and magnitude criteria that are based only on WISE data to select asymptotic giant branch (AGB) stars with circumstellar dust shells, and separate them into O-rich and C-rich classes. With these criteria we produce an all-sky map for the count ratio of the two populations. The map reveals differences between the Galactic disc, the Magellanic Clouds and the Sgr Dwarf Spheroidal galaxy, as well as a radial gradient in the Large Magellanic Cloud (LMC) disc. We find that the C:O number ratio for dusty AGB stars increases with distance from the LMC centre about twice as fast as measured for near-IR selected samples of early AGB stars. Detailed radiative transfer models show that WISE colours are well explained by the emission of centrally heated dusty shells where the dust has standard properties of interstellar medium (ISM) grains. The segregation of different classes of objects in WISE colour space arises from differences in properties of the dust shells: those around young stellar objects have uniform density distributions while in evolved stars they have steep radial profiles