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  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 L x ~ (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 deg 2 of the sky. We have collected all available Spitzer and Optical photometric data on 5 SWIRE fields (  43 deg 2http:// 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 deg 2 ) 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(24  m)>1.3mJy Flux 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(F 5.8 /F 3.6 ) > 0.4 and log(F 8.0 /F 4.5 ) > 0.4) Polletta+08 Obscured QSO

IR properties of the sample C-COSMOS Fiore+08b CDF-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 L IR by a factor 5 higher of C-COSMOS (2 deg 2 ) sources and factor higher of CDF-S (<0.1 deg 2 ) sources L MIR can be used as a proxy for L 2-10 keV : L MIR α L bol α L X (Steffen+06; Polletta+08; Piconcelli+07)

The sample X-ray 25/44 have a positive X-ray detection in the keV band For 23 is possible to calculate the N H from: HR (11 sources) Spectral analysis (12 sources) Our sample consists of 44 eXtreme MIR/O flux ratios Galaxies (XMOGs) Redshifts 7 Spectroscopic: All but one(z=0.22) are 0.7≤ z sp ≤ Photometric: 19/44 from SWIRE Photometric Redshift Catalogue (Rowan-Robinson+08) For the remaining 18 XMOGs we calculated a z phot using the Fontana+00 method (e.g. also Feruglio+08) 0.54≤z phot ≤3.77

Results: N H vs Lum X Source Breakdown:. Obscured (N H >10 22 ) 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 (N H >10 22 ) 20% Unobscured 80% QSO 2s 5% We check the reliability of the HR technique in case of very low spectral counts The source falling out of the 1:1 relationship is the CT candidate  the HR underestimate the N H for CT sources? Lanzuisi et al. 2008

Results: X/O ratio 20% of hard x-ray selected sources show extreme F X /F O 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 L X XMM-LSS Garcet+07 Sources with X/O > 10 have N H >10 22

X-ray Spectra N H >1.4x10 24 L 2-10 > 1.4x10 45 z phot ~1.44 N H > 3x10 23 L 2-10 > 1.7x10 44 z spec = 2.54 N H > 2.8x10 23 L 2-10 > 1.3x10 44 z spec = 2.31

Future Perspectives We 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 F 24 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 F 24 > 4 mJy) SWIRE sources with z spec 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 F 24  m >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 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…