Post on 12-Jan-2016
description
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…