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I. Georgantopoulos NATIONAL OBSERVATORY OF ATHENS
A. Georgakakis, O. Giannakis, S. Kitsionas, A. Akylas, D. Gaga, M. Plionis, V.
Kolokotronis, S. Basilakos
G.C. Stewart, M.G. Watson University of Leicester
E. Hatziminaoglou European Southern Observatory
T. Shanks, M. Vallbe University of Durham
B.J. Boyle AAO
Overview
XMM-Newton/2df survey NORTH
9 adjacent fields, 5-10 ksec exposure, ~2 deg2
200 sources in 0.5-8 keV
in areas covered by both the 2-df and SDSS surveys
The survey main goals are :
a. Nature of various classes of AGN
b. groups and poor clusters of galaxies (Basilakos et al. 2003 MNRAS submitted)
c. Galaxies (Georgakakis et al. 2003 MNRAS astro-ph/)
Motivation (why bother ?)
1.
Explore the bright X-ray sky especially at hard energies
where the ASCA and BeppoSAX surveys lack spatial resolution
Bridge the gap between the Chandra surveys and bright fluxes
2. Further explore the soft X-ray sky.
With a 10 ksec exposure we reach ~3x10-15 (0.5-2 keV) similar to the PSPC
exposure in the LH (with better spatial positions and broad band spectra)
3.Find BRIGHT NEARBY counterparts of the enigmatic sources detected
in the deep Chandra fields.
Introduction
ROSAT showed that the soft (0.5-2 keV) XRB is dominated by BL AGN
In harder energies (2-10 keV) the logN-logS shows an excess of sources
Relative to the ROSAT logN-logS (obscured AGN ?). Couldn’t tell
Chandra surveys have shown that the X-ray background consists of various classes
of sources:
1. QSOs
2. Obscured AGN
3. Optically ‘passive’ galaxies
4. Optically faint sources
5. Star-forming galaxies at very faint fluxes
(Classes 3,4 may take into account the scarcity of obscured AGN)
The nature of these new populations is hard to explore mainly due
to the fact that these are quite faint
Outstanding issues
What is the nature of ‘passive’ X-ray galaxies (XBONG) ?
Can the optically faint sources be obscured AGN at high z ?
Where are the type-1.9/2 QSOs ? (only 3 detected in Chandra surveys)
A number of Type-1 QSOs present large amounts of absorption ?
The last two points present great interest for AGN unification models
(and XRB population synthesis models)
Survey Description
18 shallow 5-10 ksec pointings in the NGP/SGP covering ~4 deg2.
WHY THERE ?
Extended optical follow-up :
Spectroscopic 2-df B<21 QSOs (NGP/SGP)
Photometric B=22.5, spectroscopic by the SDSS (NGP)
ADJACENT fields
They can be observed in a single 2-df exposure to go fainter
NGP details
0.5-8 keV
0.5-2
2-8keV
---------------------------------------------
215
197
61
135
36
44
134
31
32
49
4
8
(>6σ)
with optical c’parts p>98.5%
with opt. C’parts at p<98.5%
No opt. C’parts down to B=22.5
26
4
Spectroscopic Redshifts
Photometric Redshifts
63
23
2-df/SDSS/Deeper 2-df
2 sources in the 2-8 keV catalogue only 13 sources in the soft 0.5-2 keV catalogue
Hence the 0.5-8 keV total catalogue gives a fair representation of the X-ray populations
What to remember:
Spectroscopic Follow-up largely incomplete (possibly biased)
Assigning probabilities helps to reject the chance coincidences
logN-logS
At these fluxes about
50% of the XRB is resolved
in both bands
Therefore our objects
are typical of those making
the XRB
The fx/fo diagram
Dominated by AGN
(morphology suggests
both nearby and distant
AGN)
Some galaxies are observed
with low fx/fo
Spectra:
X BL AGN
• NL/ABS Galaxy
Morphology:
Galaxy
Star
• unclassified morph
 Low prob.
0.5-8 keV
Optical Ids
2-df/SDSS and deeper AAT/2-df spectroscopy
Vast majority in the 0.5-8 keV sample associated with broad-line AGN
60
Type
0.5-8 2-8
--------------------------------QSO/Sy1
54
21
NELG/Gal.
10
5
Star
5
2
No-ID
102
25
50
40
30
20
10
0
BL AGN
NELG
Star
Photometric Redshifts
Using the Code of Hatziminaoglou et al. (1999)
It does not work for extended objects
where the colours are contaminated
by the galaxy
Object selection:
1. Stellar
2. High fx/fo
70% of these have Dz<0.3
(Kitsionas et al. 2003)
The Redshift Distribution
Excess number of sources at low
Redshift (even when we take into account only the type-1)
Blue= type-2
Red= type-1
Green=Photom
0.5-8 keV
2-8 keV
The 0.5-2/2-8 hardness ratios
Large columnns are observed,
much higher than those
encountered in type-1.
Still these are lower than
those predicted by population
synthesis models.
0.5-8 keV
5
2-8 keV
The column density histogram
0.5-8 keV
Surprisingly, the galaxies have
A lower N_H than the point sources.
Things get worse considering
That the point sources are further
Away and thus that the rest-frame
N_H will be even larger
Two peaks with Galactic and moderate (logN_H~21.5) column densities
0.5-8 keV
Spectra:
X BL AGN
• NL/Galaxy
Morphology:
Galaxy
 Star
Most galaxies are red but these are not necessarily hard
ie there is not a simple picture where we find many
Nearby Obscured AGN
Where are the type 1.9-2 QSOs ?
Only local type-2 AGN detected so far
exception is RXJ at z=2.35 already discovered by ROSAT
(Almaini et al. 1995, Georgantopoulos et al. 1999)
0.5-8 keV
The 0.5-8 keV sample
is dominated by the blue
AGN. In the hard band
red galaxies and blue
AGN play equal roles
2-8 keV
The colour-colour
diagram easily picks out
candidate type-2 AGN
(Note that the HR
of the red stellar objects
is not hard)
Type-1 QSOs with large N_H
Correlation with redshift also
found by Reeves & Turner
2000 (but logN_H~21 )
Corrected N_H for redshift
Population synthesis models
predict 2/3 logN_H>22
10-14 cgs 2-10 keV
An Example at z=0.8
N_H= 3 (+-2)x10^21 cm-2
Photon index =1.9 fixed
Implications for AGN unification models
The apparent scarcity of type-2 AGN but mainly the observation of type-1 AGN
with large amounts of X-ray absorption could possibly suggest that:
At high redshifts Dust cannot survive but neutral gas can.
Dust sublimates in the strong radiation field ? sublimation radius = 0.2 pc for L~1045
But then the X-ray absorbing gas must avoid ionization
For 0.2 pc, L~1045 the ionization constant is relatively neutral ξ =1 for densities
n>109 cm-3
Dust coagulation is an alternative
Optically passive galaxies
Lx> 1043 for all 3 galaxies
N_H=0
Photon index=1.9 fixed
Cf Severgnini et al. in press
Incompleteness of UVX surveys
Not significant as far as the colour selection is concerned
The optical extension may be a far bigger problem
Summary
Large number of sources at low redshift (even more pronounced at hard energies
see Jahoda et al. 1991, Lahav et al. 1993)
There is an apparent scarcity of absorbed (narrow-line) AGN at high redshift
(same in IR wavelengths)
Instead there is a number of type-1 QSOs with large column densities
Type-1 AGN are many more than type-2 ?
These have interesting implications for unification models.
Dust sublimates but the gas remains neutral and hence absorbs X-rays ?
No significant incompleteness of optical UVX surveys found due to colour
selection.
Galaxies
How we find them:
1.
2.
3.
Objects with log(fx/fopt)<-1
Extended in the optical
Additional criterion hardness
ratio
2-4 galaxies in ALL 18 fields
Z=0.05-0.09
Galaxies are quite rare:
One would need tens
Of XMM fields to form
a decent sample
Another way of
determining the properties
of galaxies is by using
stacking analysis
(>200 2df galaxies in our
fields z~0.1)
Extend at higher redshifts
using radio galaxies
Z~0.4