“false-color” 0.5-7 keV X-ray image of the Bootes field
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“false-color” 0.5-7 keV X-ray image of the Bootes field
A large population of mid-infrared selected, obscured AGN in the Bootes field
Ryan C. Hickox
Harvard-SmithsonianCenter for Astrophysics
Extragalactic SurveysWorkshop
Cambridge, MA8 November, 2006
Hickox et al. 2006, submitted to ApJ
CollaboratorsChandra (CfA)C. JonesW. FormanS. MurrayA. KenterR. Narayan
Spitzer (JPL/Caltech/CfA)P. EisenhardtM. BrodwinV. GorjianD. SternM. Pahreand the IRAC Shallow Survey Team
Optical spectroscopy (OSU/Arizona)K. KochanekD. EisensteinR. Cooland the AGES Team
Optical photometry (NOAO)B. JanuzziA. DeyK. BrandM. Brownand the NDWFS Team
Data from Bootes surveyOptical photometry: NOAO DWFS photometry Bw,R,I,J,K (Januzzi & Dey 1999)
Infrared: Spitzer IRAC Shallow Survey (Eisenhardt et al. 2004)
~30,000 sources are detected in all four IRAC bands.
X-ray: Chandra XBootes survey (Murray et al. 2004, Kenter et al. 2005), > 3000 X-ray sources
Optical spectroscopy: AGES survey w/ MMT/Hectospec
~2000 broad line AGNs, 80 narrow-line AGNs, and ~20,000 normal galaxies.
Multiwavelength studies of AGN
(Manners 2002)
(Urry & Padovani 1995)
BUT, NOT
REALLY!
Multiwavelength studies of AGN
X-rays: ~3000 in Bootes sample
Optical/UV ~2000 in sample
Infrared: ~2000 in sample
(Manners 2002)
(Urry & Padovani 1995)
Focus on high-z objects: redshifts come from spectroscopic (AGES) or photometric (IRAC + Bw, R, I, Brodwin et al. 2006) estimators. We include objects in the sample with z>0.7
Finding obscured AGN(Urry & Padovani 1995)
X-rays and infrared are especially useful for identifying OBSCURED (Type 2) AGN, which are predicted from:
-> unified models (e.g. Antonucci 1993, Urry & Urry & Padovani 1995)
-> merger fueling models (e.g. Hopkins et al. 2006)
-> cosmic X-ray background synthesis (e.g., Setti & Woljer 1989, Treister et al. 2005, Gilli et al. 2006)Much recent work has studied obscured AGN(e.g. X-ray: Alexander et al. 2005, Mainieri et al. 2005, Treister et al. 2005, Barger et al. 2005, Ptak et al. 2005, optical: Zakamska et al. 2003, infrared: Lacy et al. 2004, Martinez-Sansigre et al. 2005, Polleta et al. 2006, Brown et al. 2006, Alonso-Herrero 2006)
Infrared selection of AGN
Dust heated by AGN
Colder dust/PAHs
(Glikman et al. 2005)
(Lagache et al. 2004)
z=0.3
IRAC bands
Infrared selection of AGN
z=0.3
Infrared selection of AGN
z=0.3
Infrared selection of AGN
z=0.3
Infrared selection of AGN
z=0.3
Infrared selection of AGN
z=0.3
Infrared selection of AGN
z=0.3
Infrared selection of AGN
Infrared selection of AGN
Following Stern et al. (2005)See also: e.g., Lacy et al. (2004),
Eisenhardt et al. (2004), Alonso-Herrero et al. (2006)
Optical/UV vs. IR luminosity
~ L* for z=1 ellipticals
(e.g. Ilbert et al. 2006)
Constant UV/IR spectral shape
Unobscured?IRAGN 1s~750 objects
Obscured?IRAGN 2s~650 objects
Hickox et al. 2006, ApJ submitted
Are IRAGN 2s obscured?We test this picture with three predictions:
Test 1: X-ray properties of IRAGN 2s should be consistent with absorbed AGNs: high X-ray luminosities, absorbed spectral shapes
Test 2: Obscuring dust in the IRAGN 2s should be sufficient to obscure ALL the optical/UV light
Test 3: IRAGN 2s should have optical morphologies and colors similar to normal galaxies
Test 1: X-ray luminositiesAverage X-ray fluxes from
stacking analysis
For both IRAGN 1s, and IRAGN 2s, average LX~1043-1044
ergs s-1, typical of Seyferts/quasars
2-7 keV
0.5-2 keV
1044
1043
1044
1043
Test 1: X-ray spectral shapesAverage X-ray
fluxes from stacking analysis
Hardness ratio:HR=(HB-SB)/(HB+SB)crude measure of X-ray spectrum
HR~-0.5 for IRAGN 1s
corresponds to a spectrum typical of
Type 1 AGNs(photon index Γ=1.8)
HR~-0.2 for IRAGN 2s
corresponds to moderate
obscuration, NH~3x1022 cm-2
Test 2: Obscuration by dust
For a typical range of gas to dust ratios (e.g., Fall & Pei 1989),for NH=3x1022 cm-2, τUV = 2.5-40
So even for the moderate X-ray absorption, the obscuring material can be optically thick in the rest-frame UV for a reasonable dust-to-gas ratio.
VLT/ESO
Test 3: Optical morphologies of IRAGN
IRAGN 2are galaxydominated
IRAGN 1are nucleardominated
Test 3: Optical colors of IRAGN
templates: elliptical, Sb, quasar
IRAGN 2colors match galaxies
IRAGN 1colors match quasar
Are IRAGN 2s obscured?Predictions:
Test 1: X-ray properties of IRAGN 2s should be consistent with absorbed AGNs: high X-ray luminosities, absorbed spectral shapes
Test 2: Obscuring dust in the IRAGN 2s should be sufficient to obscure ALL the optical/UV light
Test 3: IRAGN 2s should have optical morphologies and colors typical of normal galaxies
Are IRAGN 2s obscured?Predictions:
Test 1: X-ray properties of IRAGN 2s should be consistent with absorbed AGNs: high X-ray
luminosities, absorbed spectral shapes YES
Test 2: Obscuring dust in the IRAGN 2s should be
sufficient to obscure ALL the optical/UV light YES
Test 3: IRAGN 2s should have optical morphologies and colors similar to normal galaxies
YES
Comparison to other obscured AGN samples
We have found ~650 candidate obscured AGN at
0.7<z<3. This represents one of the largest
populations of luminous, obscured AGN identified to
date
A few important questions-> How many of these are actually AGN? How many are we missing (i.e. what are the completeness and contamination)?
-> Why the bimodality in obscuration at high luminosity?
-> What is the underlying number density of distant Type 2 AGN, and their ratio to Type 1 AGNs?
-> What are the optical spectral properties of the IRAGN 2s?
-> How does this sample overlap with the absorbed AGN from other selection techniques (Chandra Deep Fields, etc.)?
unabsorbed
Quite extincted
Why so few objects?
Summary (a poem)Observations in a great wide field of nine whole square degreesShow us thousands upon thousands of new active galaxies
We can pick them out with Chandra, or in the optical instead,But we can also use their colors in the Spitzer infrared
Our technique selects a lot of broad-line AGN, that's sureBut we also find a lot sources that are quite obscured
The X-ray, optical and infrared all do agree:This sample has a lot of obscured objects at high z!
But are many even more obscured, and missing from our total? And is the distribution in AV really bimodal?
Such fascinating questions loom for us in coming days!It’s quite exciting times for all these AGN surveys
Test 3: Optical colors of IRAGN
Test 1: X-ray luminosities(X-ray non detected)
Average X-ray fluxes from
stacking analysis
2-7 keV
0.5-2 keV1043
1044
1043
1042
1042
Test 1: X-ray hardness(X-ray non detected)
Average X-ray fluxes from
stacking analysis
Tests of photo-zs
IRAGN 2s
Optical spectral templates