AGN9, Ferrara, 24-27 maggio 2010
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
Margherita Bonzini
• 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
Obscured high-z AGN
Margherita Bonzini
Some observational problems:1. Faint at optical and X-ray wavelengths2. 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?3. To estimate the contribution to the total energy due to
accretion processes4. To study AGN evolution5. To investigate the AGN and host galaxy interplay
AGN9, Ferrara, 24-27 maggio 2010
Sample selection
Margherita Bonzini
SELECTION CRITERIA:
• F24 μm > 0.8 mJy
• F[24 μm] /F[R] > 1000
Obscured quasars at z ∼ 2
(Weedman et al. 2006; Dey et al. 2008; Polletta et al. 2008)
With HST data
(Bussmann et al. 2009, ApJ, 639, 750)
+HST images
30 sources
From the Spitzer survey of the Boötes field (∼ 8 deg2)
AGN9, Ferrara, 24-27 maggio 2010
Objectives and method
Margherita Bonzini
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
Spitzer/IRS spectra
Margherita Bonzini
10μm silicate feature in ABSORPTION
Mean redshift: 2.17 ± 0.36
AGN9, Ferrara, 24-27 maggio 2010
Spectral energy distribution (SED)
Margherita Bonzini
Host GalaxyAGN
• MIPS (24μm)
• IRS spectra (8-35μm )
• IRAC (3.6, 4.5, 5.8, 8μm)
• HST (I, V, H)
• KPNO (Bw, R, I)
(Ashby et al., 2009; Bussmann et al., 2009)
Separation of the host galaxy and AGN contribution to the total flux
High-resolution HST data
Total Flux
AGN9, Ferrara, 24-27 maggio 2010
Host galaxy and AGN contribution
Margherita Bonzini
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
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
Total Flux
Host GalaxyAGN
AGN9, Ferrara, 24-27 maggio 2010
Comparison with high-resolution MIR SEDs of local Seyfert 2 galaxies
Margherita Bonzini
(Ramos Almeida et al., 2009)
NGC 5506Mrk 573Circinus
NGC 5506Mrk 573Circinus
NGC 5506Mrk 573Circinus
NGC 5506Mrk 573Circinus
The nuclear emission from obscured AGNs with different L and z is similar
AGN9, Ferrara, 24-27 maggio 2010
Torus models
Margherita Bonzini
The dust is organized in clumps distributed in a toroidal structure around the black hole (Krolik & Begelman, 1988).
CLUMPY tori (Nenkova et al., 2008)
SMOOTH tori(Granato & Danese, 1994)
Φ ~ 1 Φ ≪1
TWO SETS OF TORUS MODELS
Φ = Vc/Vtot
AGN9, Ferrara, 24-27 maggio 2010
SEDs library
Margherita Bonzini
CLUMPY tori SMOOTH tori
7 PARAMETERS:• dust grains properties• spatial dust distribution• optical depth• inclination angle
6 PARAMETERS:• number of clouds • spatial dust distribution• optical depth• inclination angle
Models are obtained solving the radiative transfer equations.
UV-to-IR SEDs with 2 components:1. Primary nuclear emission (transmitted and reflected)2. Dust thermal emission
AGN9, Ferrara, 24-27 maggio 2010
Comparison between best-fit models
Margherita Bonzini
CLUMPY models are NOT able to reproduce the
silicate absorption
Smooth models better fit obscured QSO SEDs
SMOOTH models reproduce continuum
+ silicate absorption
SMOOTH tori
CLUMPY tori
AGN9, Ferrara, 24-27 maggio 2010
SMOOTH tori parameters distributions
Margherita Bonzini
NO constraints on geometrical parameters
Optically thick structures along the line of sight
AGN9, Ferrara, 24-27 maggio 2010
Torus size
Margherita Bonzini
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
Conclusions
Margherita Bonzini
• IR continuum:- similar in AGNs with different z and L- well reproduced by both smooth and clumpy torus models
• Si absorption: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
Conclusions
Margherita Bonzini
• 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
Margherita BonziniIMPRS workshop, Garching, Feb 9-11, 2010
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)
• Dust properties vs luminosity• Test of current torus models
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)
IMPRS workshop, Garching, Feb 9-11, 2010
Fitting the AGN SED
Nenkova et al., 2002Granato & Danese, 1994
i i
ii FF
N 2
2mod,obs,2 )(1
METHOD:Minimization of the merit function:
METHOD:BayesCLUMPY = code for the Bayesian analysis of AGN SEDs using clumpy models.(Asensio Ramos & Ramos Almeida, 2009, ApJ, 696, 2075)
OUTPUT:probability distribution of the model parameters
Margherita Bonzini
OUTPUT:Best fit values and probability distribution of the model parameters
(See also e.g. Efstathiou & Rowan-Robinson 1995; van Bemmel & Dullemond 2003; Hönig et al. 2006)
N = number of detectionsFi = fluxes
IMPRS workshop, Garching, Feb 9-11, 2010
Fitting results (1): MIR continuum & Si (10 μm) feature
Nenkova et al.
Margherita Bonzini
Granato & Danese
IMPRS workshop, Garching, Feb 9-11, 2010
Fitting results (2)
Margherita Bonzini
Granato & Danese
Nenkova et al.
Margherita BonziniIMPRS workshop, Garching, Feb 9-11, 2010
Granato & Danese best-fit parameters
About 2/3 of the objects require highly inclined tori with high optical depth