Compton-thick AGN in the CDFN

I. Georgantopoulos NOAA. Akylas NOA

A. Georgakakis NOA

M. Rovilos MPE

M. Rowan-Robinson Imperial College

Why are Compton thick AGN important ?

X-ray surveys are extremely efficient in detecting AGN: the sky density of AGN is 5000 deg-2 (Bauer+04) orders of magnitude higher than that in the optical (eg Wolf+ 03)

However, at very high obscurations (NH~1024 cm-2 or AV>>100) even the hard 2-10 keV X-rays cannot penetrate these columns (Compton-thick AGN).

C-T are extremely important for the peak of the X-ray background at 40 keV (Gilli et al. 2007)

Spitzer mid-IR surveys are claiming the detection of AGN which are not detected in X-ray

The French/Italian Simbol-X, Japanese NeXT have as their main science goals the detection of these sources at very high energies.

Simbol-X

Present sample

Plot adapted from Gilli et al.

Talk Outline

A) X-ray spectral analysis in the CDF-N

B) X-ray Stacking analysis of mid-IR selected AGN not detected individually in the X-ray

X-ray spectral analysis

The sample

Selected all CDFN sources in the 2-10 keV band with flux > 1.e-15 cgs ie an order of mag above the flux limit

Two reasons: i) very good photon statistics spectra ii) Large number of redshifts (spec and photo)

222 sources of which 190 have redshift (107 spect) The photo-z have been derived using the code of Babbedge et al. using the IRAC Mags where available.

Median number of photons ~200

Method

C-T AGN can be detected either:

a) Absorption turn-over below 10 keV which shifts at lower energies at high redshifts because of K-correction

b) a flat spectrum Γ~1 (eg Matt 1996)

Spectral fits

a) single power-law model (WA*PO) with the NH free and Γ=1.8 b) single power-law model with the photon index free and the NH fixed

to the Galactic.

Then one can compare the values of chi-2 for the two models

We require Γ<1.2 (at the 90% upper limit) and Δχ2 > 5 corresponding to a probability value of

about 2σ that the ABSORBED model is better than the FLAT one [Likelihood ratio test Mushotzky 1982]

9 Compton thick sources

8 flat spectrum C-T candidates PLUS 1 for which we can see the spectral turnover directly at z=2

1 has no redshift (not in GOODS), 6/8 spectroscopic Redshifts

Mean redshift z=2.0

logLx=42-43 (observed)

Four of them are sub-mm sources (Alexander+2006)

ACS z-band cutouts of the C-T AGN

FeKα

The C-T logN-logS

Gilli predictions

Our points

Mid-IR selection

The Stern diagram: non X-ray detections

Is there a population

of mid-IR AGN

not detected in X-ray ?

Georgantopoulos+08 astro-ph/

AGN

GOODS IRAC sources

X-ray stacking analysis One can increase the effective exposure time by adding (stacking) the photons in areas of non-detected sources.

126 mid-IR ‘AGN’ at mean redshift z=1.4

Detection in both the soft (0.3-1.5 keV) and hard (1.5-4 keV) band but with a soft spectrum Γ~2.1 Moreover,

- Lx/LIR ~ 10-5

- Lx ~ 6x1040 cgs (soft band)

further supporting the galaxy scenario.

Optically-faint & red sources (a la Fiore selection)

Criteria:

R-[3.6] > 3.7 RED (EROs)

24 μm/R > 1000 optically FAINT

103 sources of which 20 detected in X-rays

The stacking analysis of the 83 gives Γ~0.8 or NH= 8x1022 cm-2 (@z=2) Lx/LIR ~ 10-5

These are either C-T or low luminosity absorbed AGN

Optically faint/ red sources: mid-IR diagram

Many sources outside the Stern wedge where the galaxy and Sy2 templates converge at z>2

X = x-ray detection

• = non-detection

ACS z-band cutouts of the X-ray detections in the selection ‘a la FioreGreen circle = 3.6 micron position

X-ray Absorbed sources are associated with early-type systems at z~1. Does the same hold at higher

redshifts ? X-ray obscured sources are RED at z~1Rovilos & Georgantopoulos 07

(ECDFS)

Georgakakis+08 (AEGIS)

See also Silverman+07

is there another tendency at higher z ?

RemarksCompton-thick candidate AGN can be easily detected through X-ray spectroscopy. XMM observations could then play the major role

Advertisement of the 1.3 CDFS XMM observation (PI Comastri) of the CDFS which will provide many bona-fide C-T AGN at faint fluxes

At brighter fluxes the fraction of C-T AGN is >>1/100. Analysis of the whole 2XMM catalogue should produce at least a few tens

In the future all the pieces of the puzzle will be filled at faint 2-10 keV fluxes with XEUS and observations at hard energies with Simbol-X and NEXT

THE END

Stern diagram : X-ray detections Stern+05 wedge is a good

way to sift mid-IR AGN

But not all X-ray AGN fall into the Stern wedge

see also Barmby+06