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Extragalactic Surveys
Amy Barger
Where Are We?
Chandra/XMM have revolutionized distant AGN
studies
Now possible
• to map the history of a large fraction of the
AGN population using hard X-ray surveys, and
• for the first time, to compare high-redshift &
low-redshift samples chosen in the same restframe hard energy (2-8 keV) band
Striking how modest the number of X-ray sources is
compared to the number of optical sources
What Do We Need to Map the
AGN History?
Pyramid of surveys running from large area,
shallow surveys to CDF-N level ultradeep
surveys [that is, as large a base as possible
and as high a tip as possible]
One of the key issues: can we justify needing
even deeper fields, say up to 5 Ms? For
example, to measure the faint end of the
luminosity function in the z=2-3 range?
AGN
Steffen et al. 2004
What Else Do We Need?
Spectroscopic identifications!
Photometric redshifts are a possible substitute---with the addition
of NIR and MIR data, we are able to make reasonable photometric
redshift estimates and use them to construct luminosity functions
However, they do not tell about the spectral type of the galaxy
producing the X-ray light, and . . .
. . . there is danger in the scatter: if a phot-z scatters a source
into a region where there are not many objects (say, a
catastrophic error scattering a low-z source to z~3), then one
can get the LF there very wrong
GOAL: have the spectroscopic identifications be as complete as
possible (wide-field NIR spectrographs should help for z=1.6-2.6)
Lrf(2-8 keV)=1044 ergs/s
Red=spec-zs
Purple=phot-zs
Diamonds=unid
Negative values mean nuclear
dominated
1043 ergs/s
Additional Needs
Really important to have complete
wavelength coverage to understand the
spectral energy distributions of the AGN
Also want to know if there are other AGN
sufficiently thick that they are not being
seen in current hard X-ray surveys
Above f(2-8 keV)~10-14 ergs cm-2 s-1, 80-90% of hard X-ray
sources have redshifts, while below this flux, ~60%
ASCA
CDF-N
CLASXS
CDF-S
Barger et al. 2005
What Do We Mostly Agree On?
All
All z
HEX
BLAGN
The Good News
There is an astonishing amount of
agreement---we understand the
X-ray samples very well!
But there still are issues that
need to be resolved
Uncertainties in the X-ray background measurement are
still at the 10-20% level, so we cannot accurately determine
the resolved fraction of the XRB. This is a tricky issue,
particularly for population synthesis modelers who need to
decide what additional component to add in.
Hickox & Markevitch 2006
Source Classification
One of the most important issues is that of source
classification, and what we mean by the various
classes
Most groups use the four optical spectral classes of
Szokoly et al. 2004, which are crude by the
standards of optical AGN specialists:
absorbers,
star formers,
high-excitation sources (HEX),
broad-line AGN (BLAGN; FWHM>2000 km/s)
Relative Contributions to 2-8 keV Light by Spectral Class
33% BLAGN
7% HEX
27%
(“XBONG”;
>1042 ergs/s)
3%
(“OBXF”;
<1042 ergs/s)
30%
Unidentified
All
All z
HEX
BLAGN
Can now look at the X-ray colors by optical spectral class
BLAGN are nearly all soft and show essentially no visible absorption
in X-rays, consistent w/our understanding of them as unobscured
2 x 1021 cm-2
  1.8
Barger et al. 2005
All the other AGN are well-described by a power-law spectrum with
photoelectric absorption spread over a wide range of NH
3 x 1022 cm-2
  1.8
Open squares---absorbers and star formers
Solid squares---high-excitation signatures
Triangles---unidentified sources
Barger et al. 2005
X-ray Luminosity Functions
When computing rest-frame hard (2-8 keV)
X-ray luminosity functions, one of the
interesting things is to make a comparison
with optically-selected QSO samples
To do that, one needs to use the optical
spectroscopic classifications to determine
the BLAGN luminosity function separately
As move to higher z, all the sources are increasing in L while the LFs are maintaining the
same shapes; if drift the x-axis, plots look very much the same from z=0 to z=1.2
ALL
Z=0,0.4,0.8 shells
BLAGNs
LOCAL
(RXTE)
Sazonov & Revnivtsev (2004)
Barger et al. 2005
The SteffenZ=0,0.4,0.8
Effectshells
BLAGN dominate the number
densities at the higher X-ray
luminosities
BLAGN
This therefore says that
almost all luminous objects
are unobscured, which
instantly says there must be
some luminosity dependence
on the obscuration, since we
know there is a substantial
fraction of obscured sources
at the lower luminosities
The shape of the BLAGN
relative to the shape of the
total stays pretty much the
same with z, since both are
obeying PLE
This says that the BLAGN
fraction---that is, the ratio
of the integrals---stays the
same over interval z=0-1.2
However, the objects that
are BLAGN are much less
luminous at low-z than at
high-z
Z=0,0.4,0.8 shells
Open Question
This is a rather bizarre situation!
What is it that leaves certain properties, such as the
relative shapes of the two luminosity functions, so
invariant, while changing the luminosities so
much?
In other words, why should a lower luminosity
source be a BLAGN at lower redshifts, but a
similar luminosity source not be a BLAGN at z=1?
Higher Redshift Intervals
Incompleteness larger here, but
phot-zs indicate unids mostly lie
in z=1.5-3 interval
Shapes no longer wellrepresented by the maximum
likelihood fits to the z=0-1.2
HXLFs computed at z=1 (blue
curves): Thus, PLE does not
continue beyond z=1.2
There are fewer low-L sources
than one expects, and so the
light density is more dominated
by higher L sources at these
redshifts
Good agreement
between the
optical and X-ray
selected LFs!
Our optical
spectroscopic
classification of
BLAGN is
consistent with
that of groups
doing direct
optical selection
Richards et al. 2005
Classification Issue
Up to now, we have just considered the optical
spectral properties of the X-ray sources, but
it is very reasonable to try to go the other
way and ask,
‘‘Just by looking at the X-ray properties, is it
possible to tell whether a source is what an
optical AGN specialist would call a
BLAGN?“
No, this is not so clear-cut. Have we optically
misclassified some sources w/soft X-ray spectra?
3 x 1022 cm-2
  1.8
Open squares---absorbers and star formers
Solid squares---high-excitation signatures
Triangles---unidentified sources
Barger et al. 2005
HEX sources are quite easily distinguished from
BLAGN and from the low excitation sources
weak Hb
narrow CIV
Cowie & Barger
Noise is dominated by the noisiest spectrum
The low excitation sources have strong Hb and do not
show signs of NeV or of broad underlying Balmer lines.
68 of the sources show no emission lines at all
strong Hb
Cowie & Barger
Noise is dominated by the noisiest spectrum
Are BLAGN Being Lost in Other Ways?
Are selection effects (e.g., galaxy dilution or spectral
selection effects, such as whether spectrum includes
Ha) causing one to misclassify BLAGN at low X-ray
luminosities?
[Moran et al. 2002; Silverman et al. 2005; Heckman et al.
2005]
For example: could the absence of BLAGN at low X-ray
luminosities be explained if the nuclear UV/optical
light were being swamped by the host galaxy light?
Absorption
line (in some
cases, don’t
see any
UV nuclei)
z
Starbursts
LINERs &
Seyfert 1s
Seyfert 2s
BLAGN
ACS
GOODS
• Well-known that the nuclear UV
magnitudes and the X-ray fluxes for
BLAGN are strongly correlated
• If galaxy dilution hypothesis were correct,
would expect the non-BLAGN to be
similarly correlated when we isolate their
nuclear UV/optical light
Instead, turns
that,nuclear
in general, the nuclei of the non-BLAGN
Negative
valuesout
mean
are much weaker relative to their X-ray light than the BLAGN
dominated
Thus, absence of BLAGN at low X-ray
luminosities is not a dilution effect
In general, non-BLAGN really have weaker
UV/optical nuclei relative to the X-rays
Thus, we are left with the situation that there is
not a one-to-one correspondence---we cannot
select only optical BLAGN just by looking at
the X-ray properties
G. Hasinger Slide
Optically Identified Hard Samples
type-1: optical BLAGN, or galaxy with LX>42, HR<-0.2
type-2: optical NLAGN, or galaxy with LX>42, HR>-0.2
But then things get really confused! Sources with soft X-ray spectra are
-mostly BLAGN at high X-ray luminosities
-HEX sources become a significant fraction at intermediate Ls
-we do not see AGN signatures in the optical spectra at low Ls
Conclude: if one is going to split X-ray luminosity
functions by class, one should do it based on
optical spectral class alone or by X-ray color
alone, but one should not try to mix them,
because we do not understand how to relate one
to the other
But, maybe it would make sense just to split X-ray
luminosity functions based on the X-ray colors
alone, since that may be the best measure of
whether a source is obscured or not
An X-ray color cut with =1.2 basically reproduces the BLAGN LF at high
Ls but adds in sources at faint Ls, flattening out the LF, but it is a little
sensitive to where you place the cut---a higher gamma cut makes it look
much more like the BLAGN LF. The Steffen Effect still holds, however.
All
z=0.5-1
=1.2
BLAGN
2-8 keV comoving energy density production rate drops
rapidly from z=1 to z=0; peak is in interval z=0.8-1.2
Open=spectroscopic sources
Solid=all, including phot-zs; no ids put at z=3
Negative values mean nuclear
dominated At z<1.2, only 1/3 is due to broad-line AGNs
Cumulative growth of AGNs from Chandra (red curve)
compared with the cumulative SFH
Although both form most of their mass at late times (z~1), the AGN growth shows a
slightly different history & is running later than the SFH; if AGN feedback has a
significant effect, the relative histories can help diagnose that. Would like deeper
images to check whether there is a fainter z=5-6 X-ray population. Potential hidden
gottcha: presence of Compton-thick AGN.
What Might We Be Missing?
• Deep MIR and radio images are an obvious avenue
for searching for highly-obscured AGNs, since
extinction in the MIR & radio is small
• People have tried to use combined MIR & radio
selections, but to obtain a reliable upper limit on the
possible population of X-ray undetected, obscured
AGNs, a clean selection is needed
• Here we use a pure microJansky radio survey
selection in the HDF-N field
(207 sources to 40 mJy in a 310 arcmin2 area)
• The well-known correlation between radio power &
FIR luminosity makes it possible to estimate the total
FIR luminosity of a galaxy from its radio power (this
correlation has been empirically determined for both
star formers & radio-quiet AGNs)
• Even with Spitzer, this is still the most robust way
–limited MIPS sensitivities at 70 and 160 mm
-the conversion from 24 mm to total FIR luminosity
depends strongly on the template spectral energy
distributions used to K-correct the data
Barger et al. 2007 (astro-ph/0609374)
What fraction of the X-ray light is
coming from the different X-ray and
radio populations?
It is sufficient for us to measure the X-ray surface brightnesses
from known X-ray sources in the CDF-N & see that they are roughly
consistent with the HEAO1/A2 XRB (Revnivtsev et al.Z=0,0.4,0.8
2005) shells
Barger et al. 2007
X-ray sources that are not radio sources and the total radio sample
each contribute 50% of the X-ray light at 4-8 keV, but most of the radio
Z=0,0.4,0.8 shells
sample contribution is from X-ray luminous radio sources
Total radio
X-ray sources that are not radio
Barger et al. 2007
Contributions from the remaining radio sources are very small
Z=0,0.4,0.8 shells
Radio sample without X-ray
luminous counterparts
(2.3% at 4-8 keV)
Radio sample
without any X-ray
counterparts
(1.2% at 4-8 keV)
Barger et al. 2007
Thus, the current radio source
population cannot account for the
background light that has been
suggested may be missing at 4-8 keV
Indeed, the percentages are about a
factor of 5 lower than those predicted
for 1024 cm-2 sources at these energies
in typical XRB synthesis models (e.g.,
Gilli, Comastri, Hasinger 2006)
Summary
• X-ray data are consistent with many of the non-BLAGN (the
dominant population) having high column densities
• AGN evolve very rapidly to z~1.2, consistent with pure
luminosity evolution to that redshift
• z~1 is where the bulk of the supermassive black hole
population forms
• Simple unified model is not correct, whether one uses X-ray
color or optical spectral classification---there are far fewer
low X-ray luminosity, unobscured sources than obscured
• Contributions to the 4-8 keV light from the X-ray faint radio
population is very small, and hence these sources are unlikely
to contribute substantially to the XRB at even higher energies
The End