Revealing X-ray obscured Quasars in SWIRE sources with

extreme MIR/O

Giorgio Lanzuisi

Fabrizio Fiore

Enrico Piconcelli

Chiara Feruglio

Cristian Vignali

Accretion History of the Universe: Open Issues

La Franca et al. 2005

The resolved fraction of the CXB at > 8 keV is only 50%.

Synthesis models require a population of heavily obscured AGN

A “factor 2” SMBH relics wrt expectations based on Hard X-ray LF

In the local universe ~ 50% of Seyfert 2 are CT

But Few Compton-thick AGNs detected (Tozzi+06, Akylas+06, Alexander+08)

The LF of highly obscured AGN is still unknown

particularly at high luminosity

Relevant consequences for cosmology:

Tight link between SMBH mass density, nuclear activity and galaxy formation and evolution

Searching for Obscured AGN: Mid-IR selection The MIR band is promising since absorbed opt/UV/X-ray AGN light is thermally

re-emitted by the obscuring material at these frequencies

Martinez-Sansigre+05,+07: (combination of radio + MIR data) claimed the discovery of a distant, optically-faint QSO2 population with density ~ to optically-bright QSO population

Fiore+08: Extreme Sources with MIR/O>1000 and R-K>4.5 in the CDF-S Found a “factor 2” population of

CT AGN candidates at Lx ~ 1043

(deep pencil-beam survey: HST+VLT+Spitzer+Chandra data on the CDFS; e.g. Daddi+07 for a similar result)

A complementary study is needed to investigate the high luminosity part of the

LF of obscured AGNs

Stacked image of

undetected sources

The XSWIRE survey: a unique opportunity

Elais-N1 Elais-N2

XMM-LSS

Elais-S1

Lockman Hole

Given the low surface density of luminous QSOs,

the sampling of a large area is required The Spitzer Wide-Area Infrared Extragalactic Survey covers with medium-deep MIPS

and IRAC photometry ~50 deg2 of the sky.

We have collected all available Spitzer and Optical photometric data on 5 SWIRE

fields (http://www.oa-roma.inaf.it/~fiore/XSWIRE) 43 deg2

The X-ray coverage of the SWIRE fields is incomplete and highly inhomogeneous with exposure times ranging from 5 ks Chandra to 70 ks XMM-Newton observations (6.5 deg2)

Goal: Confirmation of the obscured

AGN nature of the MIR-selected

sources using the X-ray spectral

analysis

Sample Selection

X-ray Coverage, MIR/O>2000 and F(24m)>1.3mJyFlux limit chosen to have a sample of 44 sources

All but one source fall in the AGN region of the IRAC color-color diagram, and most

of them show extreme colors (i.e. log(F5.8/F3.6) > 0.4 and log(F8.0/F4.5) > 0.4)

Polletta+08Obscured QSO

IR properties of the sample

C-COSMOS Fiore+08bCDF-S Fiore+08a

Our selection criterium MIR/O>2000 is well suited to find high IR luminous sources

The large area of SWIRE allows to collect sources with LIR by a factor 5 higher of C-COSMOS (2 deg2) sources and factor 15-20 higher of CDF-S (<0.1 deg2) sources

LMIR can be used as a proxy for L2-10 keV: LMIR α Lbol α LX

(Steffen+06; Polletta+08; Piconcelli+07)

The sample

X-ray25/44 have a positive X-ray

detection in the 0.5-10 keV band

For 23 is possible to

calculate the NH from:

• HR (11 sources)

• Spectral analysis (12 sources)

Our sample consists of 44 eXtreme MIR/O flux ratios Galaxies (XMOGs)

Redshifts7 Spectroscopic:

All but one(z=0.22) are

0.7≤ zsp≤2.5

37 Photometric:

• 19/44 from SWIRE Photometric Redshift Catalogue

(Rowan-Robinson+08)

•For the remaining 18 XMOGs we calculated a zphot using the Fontana+00 method (e.g. also Feruglio+08)

0.54≤zphot≤3.77

Results: NH vs LumX

Source Breakdown: .

Obscured (NH>1022) 55-95%

Unobscured 5-45% QSO 2s 40-55%

18% of the detected sample are promising C-thick QSO 2s candidates

Control Sample (randomly selected):Obscured (NH>1022) 20%

Unobscured 80% QSO 2s 5%

We check the reliability of the HR technique in case of very low spectral countsThe source falling out of the 1:1 relationship is the CT candidate the HR underestimate the NH for CT sources?

Lanzuisi et al. 2008

Results: X/O ratio20% of hard x-ray selected sources show extreme FX/FO ratios (i.e. X/O > 10)

Most of EXOs are obscured AGN at z > 1 Nuclear optical light is blocked, differently from X-ray

Fiore+03 found that:X/O is a proxy of obscuration

correlation between X/O and LX

XMM-LSSGarcet+07

Sources with X/O > 10 have NH>1022

X-ray Spectra

NH >1.4x1024

L2-10 > 1.4x1045

zphot~1.44

NH > 3x1023

L2-10 > 1.7x1044

zspec= 2.54

NH > 2.8x1023

L2-10 > 1.3x1044

zspec= 2.31

Future PerspectivesWe demonstrate the efficiency of our MIR/O selection over a large area survey

to find out a large fraction of QSO2.

OPEN ISSUES:

• A fraction of sources is undetected even in medium/deep exposures (possibly CT?)

Deep Chandra/XMM exposures of the most interesting sources (Next AO XMM-Newton, Chandra)

• A complete X-ray survey of high MIR/O, high F24 sources is still lacking due to the highly inhomogeneous X-ray coverage

Chandra AO10 LP proposal submitted (PI. Fiore) to observe with deep (35-60 ks) exposures the 10 most extreme (MIR/O>2000 F24> 4 mJy) SWIRE sources

with zspec

By joining this sample to the CDFS and C-COSMOS samples we will able to determine the “bolometric” LF of both unobscured and highly obscured AGNs in several redshift bins.

Summary

We selected a sample of 44 SWIRE sources with extreme MIR/O flux

ratios (MIR/O>2000) and F24m >1.3 mJy

• Spectroscopic and Photometric redshifts are 0.7 < z < 3.7 • We are able to infer the X-ray spectral properties for 23/44

XMOGs Most (50-95%) are Obscured or Highly Obscured ~50% are QSO 2s ~20% are promising C-thick QSO 2s candidates

Lanzuisi et al. 2008

We tested that our MIR selection applied to a large area survey like

SWIRE is very efficient in collecting a large number of heavily obscured QSO2s

More detailed X-ray and optical follow-up studies have been planned…