“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