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Seeking type 2 QSO amongst
bright X-ray selected EXOs
Agnese Del Moro
University of Leicester, UK
In collaboration with:
M.G. Watson (UoL), S. Mateos (UoL), M.Akiyama (University
of Tohoku, Japan)
Agnese Del Moro, Torino 2008
• What are EXOs?
• Sample selection
• Multiwavelength properties
• Optical identifications
• Summary and conclusions
• Future works
Agnese Del Moro, Torino 2008
Dominant AGN population have X-ray to optical flux ratio Fx/Fopt ~ 1
EXOs are objects with extreme X-ray to optical ratio (and no optical r
band detection, Koekemoer et al. 2004):
 Fx/Fopt > 10
What can they be?
rare galactic objects (INS, X-Ray Binaries, CV, ULX…)
highly variable objects
BL Lacs
high-z clusters
high-z AGN
highly obscured AGN
Why are they important?
 Constrain the fraction of EXOs powered by AGN
 A large population of type 2 AGN is required by synthesis models of the
X-ray Background (CXRB, Comastri et al.1995; Gilli et al. 2007)
Agnese Del Moro, Torino 2008
Our sample
• Cross-match with 2XMMp and SDSS DR5 catalogues:
- Joint area ~75 sq.deg.
- ~20,000 secure matches based on Likelihood Ratio analysis (Sutherland
and Saunders 1992)
- X-ray point sources
- Fx ≥ 10-13 erg cm-2sec-1 (0.2-12 keV)
- Fx/Fopt> 30  Fx/Fopt> 10 in the 2-10
keV band
Agnese Del Moro, Torino 2008
Advantages of our approach:
• X-ray fluxes 10-1000 times brighter than
samples based on deep surveys
• Large sky area
• Good X-ray data  Γ and NH
• Optical/NIR analysis is possible at these
Mignoli et al.2004
 130 sources (~10% of the AGN)
 16 “known” objects
 114 new objects
 ~30% of them have no detection in SDSS
Agnese Del Moro, Torino 2008
Hardness Ratios
HR= (H-S)/(H+S)
HR2: S=0.5-1 keV H=1-2 keV
HR3: S=1-2 keV
Known Objects
H=2-4.5 keV
New Objects
Agnese Del Moro, Torino 2008
Near-IR properties
UKIRT UFTI and WFCAM data have been obtained for ~35% of the
Optical/NIR colour analysis:
- Our sources have typically red
- 3 of them have r–K > 5 typical of
- the fraction of EROs is a lower
limit; it may increase by ≤30%
Agnese Del Moro, Torino 2008
Optical identifications
Optical spectra have been obtained with Subaru FOCAS for 16 of our
From preliminary spectral analysis:
• 10 Broad Line AGN
• 5 Narrow Line AGN
• 1 Not identified
• Redshifts 0.4 < z < 1.6
 Luminosities typical of QSOs
 Unexpected large fraction of
type 1 AGN
 5 new type 2 QSO candidates
Agnese Del Moro, Torino 2008
Type 2 QSOs
2XMMp J0922
From optical spectrum: NL AGN
z = 0.57
Γ = 1.54
NH = 3.2 x 1022
Lum (2-10 keV) ≈ 3 x 1044 erg/sec
2XMMp J1253
From optical: NL AGN
z = 0.94
Γ = 1.52
NH = 2.2 x 1022
Lum (2-10 keV) = 3.4 x 1044 erg/sec
Agnese Del Moro, Torino 2008
Summary and Conclusions
We selected a sample of high X-ray-to-optical ratio X-ray bright
sources from the cross-match between 2XMMp and SDSS DR5
* We have obtained one of the largest X-ray selected EXO sample
* Evidence of absorption from X-ray HR2 colour for > 33% objects
* Typical red optical/NIR colours, reaching also values typical for
EROs for up to ~30% of the analysed sub-sample
* Discovery of 5 new type 2 QSO candidate from optical spectra 
2 genuine TYPE 2 QSOs
* Unexpected large population of type 1 QSOs
Agnese Del Moro, Torino 2008
Ongoing and future…
• New optical spectra coming soon from VLT-FORS2
 extend our analysis and increase the sample of identified
• Detailed X-ray spectral analysis to constrain X-ray absorption
• Detailed optical spectral analysis to investigate properties of lines
and optical reddening
• Further investigations on the type 1 population amongst our sample
• Development of an SED model in order to study the global
properties of the sample and to investigate the AGN-host galaxy
Agnese Del Moro, Torino 2008
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