New insights on the torus models
from high-resolution SEDs of
obscured high-z QSOs
Margherita Bonzini
Collaborators: M. Polletta (INAF-IASF Milano), D. Weedman (Cornell University, USA),
L. Sargryan (Byurakan Astrphysical Observatory, Armenia) ,
G. Lodato (Università degli studi di Milano)
AGN9, Ferrara, 24-27 maggio 2010
Outline
• Obscured Active Galactic Nuclei (AGN)
at high redshift
• Sample and data
• Analysis:
- estimate of the host galaxy and AGN
spectral energy distributions (SEDs)
- fit of the AGN SED with clumpy and
smooth torus models
• Main results
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Obscured high-z AGN
Some observational problems:
1. Faint at optical and X-ray wavelengths
2. The obscured AGN fraction decreases with increasing luminosity (?)
(e.g. Simpson 2005, Akylas et al. 2006, Maiolino et al. 2007)
Recently discovered by the
Spitzer Space Telescope
(e.g. Weedman et al., 2006; Lacy et al., 2007; Fiore et al., 2008; Polletta et al.,2008)
Why studying them?
1. To estimate the contribution to the total energy due to
accretion processes
2. To study AGN evolution
3. To investigate the AGN and host galaxy interplay
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Sample selection
From the Spitzer survey of the
Boötes field (∼ 8 deg2)
SELECTION CRITERIA:
• F24 μm > 0.8 mJy
With HST data
• F[24 μm] /F[R] > 1000
Obscured quasars
at z ∼ 2
(Weedman et al. 2006; Dey et al. 2008;
Polletta et al. 2008)
+
HST images
(Bussmann et al. 2009, ApJ, 639, 750)
AGN9, Ferrara, 24-27 maggio 2010
30 sources
Margherita Bonzini
Objectives and method
Objectives:
• characterization of the nuclear emission of z~2 quasars
• dust distribution:
- is it organized in a torus?
- is it described by current models?
• do dust properties vary with redshift and luminosity?
Methods:
• nuclear and host galaxy optical-to-MIR SED reconstruction
using photometric and spectroscopic data
• model comparison
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Spitzer/IRS spectra
Mean redshift: 2.17 ± 0.36
10μm silicate feature in ABSORPTION
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Spectral energy distribution (SED)
• MIPS (24μm)
• IRS spectra (8-35μm )
• IRAC (3.6, 4.5, 5.8, 8μm)
Total Flux
Host Galaxy
AGN
• HST (I, V, H)
• KPNO (Bw, R, I)
(Ashby et al., 2009; Bussmann et
al., 2009)
High-resolution
HST data
AGN9, Ferrara, 24-27 maggio 2010
Separation of the host galaxy and AGN
contribution to the total flux
Margherita Bonzini
Host galaxy and AGN contribution
1. Estimate of the host galaxy
SED using a starburst
model library (Siebenmorgen &
Krügel, 2007)
CRITERIA:
- agreements with HST data
- model < total flux at all λ
Total Flux
Host Galaxy
AGN
2. Host galaxy subtraction
from the total flux
AGN SED
The UV nuclear emission is absorbed by the dust and
thermally re-emitted in the IR
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Comparison with high-resolution
MIR SEDs of local Seyfert 2 galaxies
NGC 5506
Mrk 573
Circinus
NGC 5506
Mrk 573
Circinus
NGC 5506
NGC 5506
The nuclear
emission
from
obscured
AGNs
with
Mrk 573
Mrk 573
different L and z is similar
Circinus
Circinus
(Ramos Almeida et al., 2009)
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Torus models
The dust is organized in clumps distributed in a toroidal structure around
the black hole (Krolik & Begelman, 1988).
TWO SETS OF TORUS MODELS
SMOOTH tori
CLUMPY tori
(Granato & Danese, 1994)
Φ~1
AGN9, Ferrara, 24-27 maggio 2010
(Nenkova et al., 2008)
Φ = Vc/Vtot
Φ ≪1
Margherita Bonzini
SEDs library
UV-to-IR SEDs with 2 components:
1. Primary nuclear emission (transmitted and reflected)
2. Dust thermal emission
SMOOTH tori
7 PARAMETERS:
• dust grains properties
• spatial dust distribution
• optical depth
• inclination angle
CLUMPY tori
6 PARAMETERS:
• number of clouds
• spatial dust distribution
• optical depth
• inclination angle
Models are obtained solving the radiative transfer equations.
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Comparison between best-fit models
SMOOTH tori
SMOOTH models
reproduce continuum
+
silicate absorption
CLUMPY tori
CLUMPY models are NOT
able to reproduce the
silicate absorption
Smooth models better fit
obscured QSO SEDs
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
SMOOTH tori parameters distributions
Optically thick structures along the line of sight
NO constraints on geometrical parameters
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Torus size
Smooth torus models with a deep Si absorption
correspond to spatially extended structures
(~100 pc ).
BUT: high-resolution measurements suggest
compact structures (≲ 10 pc )
( e.g. Tristram et al. 2007; Meisenheimer et al. 2007)
Possible explanation:
compact torus + cold dust absorber
(see also Polletta et al. 2008, Martinez-Sansigre et al 2009)
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Conclusions
•
IR continuum:
•
Si absorption:
-
similar in AGNs with different z and L
well reproduced by both smooth and clumpy torus models
Not reproduced by clumpy torus models but only by the smooth
ones
Importance of using both photometric and spectroscopic data to
describe optical-to-IR AGN SEDs
•
Smooth dust density structures better fit obscured QSO SEDs
•
Deep Si absorption features correspond to extended structures
(in contrast with high resolution measurements)
Possible cold dust component separated from the torus
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Conclusions
•
The IR continuum of AGNs with different z and L is similar
•
The clumpy model are not able to reproduce the deep Si
absorption observed
•
The SEDs are well reproduced by smooth tori
•
Deep Si absorptions correspond to extended structures (in
contrast with high resolution measurements)
•
Possible cold dust component separated from the torus
•
Importance of using both photometric and spectroscopic
data to describe optical-to-IR AGN SEDs
AGN9, Ferrara, 24-27 maggio 2010
Margherita Bonzini
Obscured AGN at high redshift and
luminosity
The fraction of obscured AGN decreases at high luminosities
(from>80% at L~1042 erg/s to 38% at 1045 erg/s)
(e.g. Hasinger et al. 2004, Simpson 2005, La Franca et al. 2005, Treister &
Urry 2006, Akylas et al. 2006, Tozzi et al. 2006, Maiolino et al. 2007)
Many high-z obscured quasars have been recently
discovered by the Spitzer Space Telescope
(e.g. Weedman et al., 2006; Lacy et al., 2007; Polletta et al.,2008)
• Dust properties vs luminosity
• Test of current torus models
IMPRS workshop, Garching, Feb 9-11, 2010
Margherita Bonzini
Fitting the AGN SED
Granato & Danese, 1994
Nenkova et al., 2002
METHOD:
Minimization of the merit
function:
METHOD:
BayesCLUMPY = code for
the Bayesian analysis of AGN
SEDs using clumpy models.
1
 
N
2

i
( Fi ,obs  Fi ,mod ) 2
 i2
(Asensio Ramos & Ramos Almeida,
2009, ApJ, 696, 2075)
N = number of detections
Fi = fluxes
OUTPUT:
Best fit values and
probability distribution
of the model parameters
OUTPUT:
probability distribution of
the model parameters
(See also e.g. Efstathiou & Rowan-Robinson 1995; van Bemmel & Dullemond 2003;
Hönig et al. 2006)
IMPRS workshop, Garching, Feb 9-11, 2010
Margherita Bonzini
Fitting results (1): MIR continuum & Si
(10 μm) feature
Granato & Danese
IMPRS workshop, Garching, Feb 9-11, 2010
Nenkova et al.
Margherita Bonzini
Fitting results (2)
Granato & Danese
IMPRS workshop, Garching, Feb 9-11, 2010
Nenkova et al.
Margherita Bonzini
Granato & Danese best-fit parameters
About 2/3 of the objects require highly inclined tori with high
optical depth
IMPRS workshop, Garching, Feb 9-11, 2010
Margherita Bonzini