S. Kitsionas1,2 - E. Hatziminaoglou3 - A. Georgakakis1 - I. Georgantopoulos1
1 - Institute of Astronomy & Astrophysics,
National Observatory of Athens,
I. Metaxa and V. Pavlou str.,
P. Penteli, 15236 Athens, Greece
2 -
Astrophysikalisches Institut Potsdam,
An der Sternwarte 16, 14482 Potsdam, Germany
3 -
Instituto de Astrofisica de Canarias, C/ Vía Làctea,
s/n, 38205 La Laguna, Tenerife, Spain
Received 27 August 2004 / Accepted 3 December 2004
Abstract
In this paper we present photometric redshift estimates for a sample
of X-ray selected sources detected in the wide-field (
), bright [
]
XMM-Newton/2dF survey. Unlike deeper
X-ray samples comprising a large fraction of sources with colours
dominated by the host galaxy, our bright survey primarily probes the
QSO X-ray population. Therefore photometric redshift methods employing
both galaxy and QSO templates need to be used. We employ the
photometric redshift technique of Hatziminaoglou et al.
(2000) using 5-band photometry from the SDSS. We separate our X-ray
sources according to their optical profiles into point-like
and extended. We apply QSO and galaxy templates to the
point-like and extended sources respectively. X-ray sources
associated with Galactic stars are identified and discarded from our
sample on the basis of their unresolved optical light profile, their low
X-ray-to-optical flux ratio and their broad-band colours that are best
fit by stellar templates. Comparison of our
results with spectroscopic redshifts available
allows calibration of our method and estimation of
the photometric redshift accuracy. For
70 per cent of the
point-like sources photometric redshifts are correct within
(or
75 per cent have
),
and the rms scatter is estimated to be
.
Also, in our X-ray selected
point-like sample we find that about 7 per cent of the sources
have optical colours redder than those of optically selected
QSOs. Photometric redshifts for these systems using existing QSO
templates are most likely problematic.
For the optically extended objects the photometric redshifts work only
in the case of red (
g - r > 0.5 mag) sources yielding
and
for 73 and 93 per cent respectively.
The results above are consistent with earlier findings from the application of combined galaxy/QSO photometric redshift techniques in the Chandra Deep Field North.
However, we find that the above photometric redshift technique does not
work in the case of extended sources with blue colours (g-r<0.5).
Although these form a significant fraction of the extended sources
(
), they cannot be fit successfully by QSO or galaxy templates, or any linear combination of the two.
Key words: techniques: photometric - quasars: general - galaxies: active - galaxies: distances and redshifts - X-rays: galaxies
In the past few years, X-ray surveys carried out by the XMM-Newton and the Chandra observatories have significantly improved our understanding of X-ray selected AGNs (Hasinger et al. 2001; Brandt et al. 2001; Giacconi et al. 2001; Barger et al. 2002; Brandt et al. 2002; Mainieri et al. 2002; Georgantopoulos et al. 2004). Moreover, serendipitous studies using the large volume of archival data provided by these missions are well underway, aiming to further push our knowledge of both the X-ray properties and the evolution of AGNs (Green et al. 2004; Kim et al. 2004a, 2004b).
To maximise their scientific impact, the above large-scale projects require redshift information for a large number of X-ray sources. For example the recently released XMM-Newton Serendipitous Source Catalogue comprises about 50 000 sources, with more archival observations being accumulated. Spectroscopic follow-up programs are underway (e.g. Barcons et al. 2002) but they are expensive in telescope time and completion may take years. Moreover, many sources are expected to be optically faint, rendering spectroscopic observations difficult even with 10-m class telescopes.
The issues above have motivated the development of photometric redshift techniques that use multiwaveband imaging to estimate the redshifts of extragalactic sources. These methods have become increasingly popular over the last few years because of their effectiveness and the relatively small investment in observing time that they require compared to spectroscopy. For normal galaxies in particular, with optical colours dominated by stellar processes, photometric redshifts have proven exceptionally successful, achieving accuracies that allow detailed statistical studies (e.g. Budavari et al. 2003; Csabai et al. 2003).
For AGN-dominated systems with featureless continua, photometric
redshift estimates are more challenging. Nevertheless, significant
effort has been expended in this direction in recent years. Budavari
et al. (2001) and Richards et al. (2001) used Sloan Digital Sky
Survey (SDSS) data to estimate photometric redshifts of optically selected QSOs; their results are accurate within
for 70 per cent of their sample.
For X-ray selected samples, Gonzalez & Maccarone (2002) have
estimated photometric redshifts for X-ray sources to the limit
.
Using
galaxy templates these authors find good agreement between
photometric and spectroscopic redshifts only for systems
dominated by light from the host galaxy rather than the central
engine. This class of sources is the dominant
population, representing about 90 per cent of their X-ray selected
sample to the flux limit above. Similarly, Mobasher et al. (2004) used
optical data from the GOODS survey to estimate photometric redshifts for X-ray selected
sources in the Chandra Deep Field North [
]. Although for most of
these X-ray faint sources galaxy templates provide adequate
photometric redshift estimates, they fail for powerful QSOs.
Babbedge et al. (2004) recently presented a combined galaxy-QSO
template approach to derive photometric redshifts for the
X-ray sources in the Chandra Deep Field North (CDFN).
The use of combined galaxy-QSO templates is
imperative in the case of X-ray surveys at brighter limits, which comprise a
significant fraction of powerful AGNs with optical light dominated by
the central engine. In this paper we address this issue, providing
photometric redshifts for X-ray sources detected in a wide-field
(
), shallow
[
]
XMM-Newton survey near the North Galactic Pole region (part of
the XMM-Newton/2dF survey; Georgakakis et al. 2003, 2004). The
strength of this sample is that it overlaps with the SDSS
(York et al. 2000) and hence, high quality 5-band
photometry is available, allowing photometric redshift estimates. Also
spectroscopic redshifts are available for a sub-sample of the X-ray
source population from the 2dF QSO Redshift Survey (2QZ; Croom et al. 2001), the SDSS survey (York et al. 2000; Stoughton et al. 2002) as well as our own spectroscopic campaign. The spectroscopic redshifts allow both calibration of the photometric
redshift method and estimation of its accuracy.
In the following section we give a detailed description of both the X-ray and optical (photometric and spectroscopic) data used in the present study. In Sect. 3 we give a brief summary of the photometric redshift method while Sect. 4 presents our results. These are discussed in Sect. 5. Our conclusions are summarised in Sect. 6.
The X-ray data are from the XMM-Newton/2dF survey, a wide-area,
shallow [
]
X-ray sample near the North and the South Galactic Pole regions. The X-ray data reduction,
source extraction, flux estimation and catalogue generation
have been described in detail by Georgakakis et al. (2003, 2004).
In the present study we concentrate on the XMM-Newton/2dF survey
sub-sample near the North Galactic Pole region. This is because of the
wealth of homogeneous follow-up optical photometric and spectroscopic
observations available in this region. A total of 291 X-ray
sources have been detected in the 0.5-8 keV spectral band above the threshold.
The optical photometric data are from the SDSS Early Data Release
(EDR; Stoughton et al. 2002). The SDSS is an ongoing imaging and
spectroscopic survey that aims to cover about 10 000 deg2 of the
sky. Photometry is performed in 5 bands (ugriz; Fukugita et al. 1996) to a limiting magnitude of
mag. These data are used for the optical identification of the X-ray sample using the method described by Downes et al. (1986). We propose optical
counterparts for 193 out of 291 X-ray sources.
Spectroscopic data for the optically brighter X-ray sources are from the SDSS and the 2QZ. The SDSS will obtain spectra for over 1 million objects, including galaxies brighter than r=17.7 mag, luminous red galaxies to z =0.45 and colour selected QSOs (York et al. 2000; Stoughton et al. 2002; Richards et al. 2002). The 2QZ is a large-scale spectroscopic campaign that fully exploits the capabilities of the 2dF multifibre spectrograph on the 4m Anglo-Australian Telescope (AAT). This project provides high quality spectra, redshifts and spectral classifications for 23 000 optically selected bj<20.85 mag QSOs (Croom et al. 2001). In addition to publicly available spectroscopic data from the 2QZ and the SDSS spectroscopic surveys, we use redshift measurements from our own on-going spectroscopic campaign of the XMM-Newton/2dF sources. Part of these data is presented by Georgakakis et al. (2004).
Photometric redshifts are estimated using the method described by
Hatziminaoglou et al. (2000; HMP00). In brief, this is
based on a standard minimisation technique where the
photometric redshift of an object is estimated by comparing its
multiband photometry with model QSO Spectral Energy Distribution (SED)
templates, shifted in redshift space and integrated through the
bandpass throughput functions. The redshift is allowed to vary in
the range 0-6 with a linear step of 0.1. For each template and
at each redshift, the
probability of each source is
calculated. Once a local minimum in
is found, a
refined search for a better solution is made around it, with a step
of 0.01. More than one local minimum might exist, as in all template
fitting techniques. The photometric redshift of the source is then
given as the redshift corresponding to the global minimum of
these
values.
The QSO templates are produced by varying the optical power-law
spectral index of simulated QSO spectra between 0 and 1, while keeping
the UV spectral index constant at 1.76 (Wang et al. 1998). We note, however, that varying the UV spectral index does not modify our results and conclusions. Emission lines (Ly,
Ly
,
CIV, [CIII] 1909, MgII, SiIV, H
,
H
and H
)
as well as the blue bump centred at 3000
are also included in the QSO-template SEDs. The Ly
forest has
been modelled following Madau (1995). No reddening has been applied
to the QSO-template SEDs.
Using the spectroscopic data available for our sample, we attempt to fine-tune the spectral line properties (i.e. equivalent widths and relative intensities) of the QSO-template SEDs. This is in order to calibrate the HMP00 method on X-ray selected samples. We conclude that the spectral line profiles used by HMP00 are also optimal for our X-ray selected AGN sample.
The template SED library of the HMP00 technique also comprises stellar spectra for a range of spectral types including white dwarfs (Pickles 1998). These are used to identify candidate stars (Hatziminaoglou et al. 2002) within our X-ray selected sample. The observed mean spectra of four different galaxy types (E/S0, Sbc, Scd, Im) from Coleman et al. (1980; CWW) are also used in this paper to estimate photometric redshifts for X-ray sources dominated by light from the host galaxy.
For the photometric redshift estimation we employ the SDSS 5-band photometry using PSF magnitudes (psfMag SDSS parameter) for optically unresolved sources and Petrosian magnitudes (petroMag SDSS parameter) for optically extended systems.
We apply the method described in the section above to the sub-sample of X-ray sources with spectroscopic data available to assess the success rate of our technique. The spectroscopic sub-sample comprises 71 optically unresolved (e.g. point-like morphology) systems and 26 sources with extended optical light profile. The morphological classification has been obtained from the SDSS. This is reliable at the 95 per cent confidence level at the magnitude limit r=21 mag (York et al. 2000; Stoughton et al. 2002). At fainter magnitudes the star/galaxy classification becomes less robust.
We treat extended and point-like sources separately using galaxy and QSO templates respectively. X-ray sources with unresolved optical light profile are most likely associated with distant powerful AGNs where the light from the central engine dominates the optical emission. QSO-template SEDs are clearly more appropriate for these sources. The X-ray sources with extended optical light profile are most likely relatively nearby galaxies where the central source does not dominate their optical light. Therefore, galaxy templates are likely appropriate for these systems.
Before applying the HMP00 method to the full spectroscopic catalogue we
identify and exclude from our analysis Galactic star candidates. In
particular, we first identify optically unresolved (e.g. point-like
morphology) sources with low X-ray-to-optical flux ratio (
;
total of 13), i.e. in the region of the parameter space
occupied by Galactic stars (Stocke et al. 1991). Secondly, for these
sources we apply the HMP00 technique using both QSO and
stellar templates in an attempt to identify objects with optical SEDs
consistent with those of stars. This is because the
regime also comprises a small fraction of QSOs in addition to other
classes of sources. Of the 13 sources with
a
total of 9 are best fit by stellar templates. All of them are
spectroscopically identified Galactic stars (Georgakakis et al. 2004).
The remaining 4 sources are best fit by QSO templates. This classification is consistent
with the optical spectroscopy available for these objects (Georgakakis et al. 2004). Therefore, our method based on both X-ray and optical broad-band
information is successful in identifying all the Galactic stars
within the spectroscopic sub-sample. In the remaining of this paper we shall
not consider the 9 Galactic stars identified in our spectroscopic sub-sample.
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Figure 1:
Comparison between
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Table 1: Photometric redshifts for the 88 X-ray sources with spectroscopic identification in the XMM- Newton/2dF sample.
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Figure 2:
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For the remaining 62 optically unresolved X-ray sources
we have applied the photometric redshift
technique of HMP00 using QSO templates only. The results are presented
in Table 1 and are compared with the spectroscopic redshift
measurements in Fig. 1. A total of 42 out of 62 objects (68 per cent) have photometric redshift estimates correct within
.
For this sample we estimate a fractional mean error defined as
For the 26 optically extended X-ray sources in the XMM-Newton/2dF sample with spectroscopic data we have applied the HMP00 technique using galaxy SED templates only. The photometric redshift estimates for these sources are also presented in Table 1. Figure 2 plots
as a function of g-r colour for the
spectroscopic sub-sample of extended sources (total of 26 sources), while
Fig. 3 plots spectroscopic against photometric redshift for the same sources.
It is clear from these figures that for sources with red optical colours (g-r>0.5 mag) our
method performs well with a mean error of
and rms scatter
.
For objects with blue colours (g-r<0.5 mag) however, our method is not as successful, giving
and
.
The
photometric redshifts of this blue X-ray source population cannot be
improved by using QSO or starburst templates, suggesting that their
optical light is a mix of both AGN and stellar emission.
We further explore this possibility in an attempt to improve the
photometric redshift estimates for these sources by linearly combining
QSO (total of 3; see Sect. 3) and galaxy (total of 4 from CWW)
templates. These are normalised at 5000 Å, although the choice of
the normalisation wavelength does not alter our results. For each of
the 12 QSO/galaxy SED pairs (3 QSO times 4 galaxy SEDs) we vary the normalisation of the
QSO component relative to the galaxy component between 0.1 and 0.9 in steps
of 0.2, and we then sum the weighted SED pairs. This results in 5 linear combinations for each QSO/galaxy SED pair and therefore a total of 60 combined QSO/galaxy templates. We
find that using these 60 templates does not significantly improve the
photometric redshift estimates of optically extended blue (g-r<0.5)
objects: only two additional sources are assigned photometric
redshifts accurate within
.
Finally, for the optically extended spectroscopic sub-sample there is only 1 source with r>21 mag, and therefore we cannot comment on the success rate of our method for fainter sources where the star/galaxy separation becomes less reliable. For this one source (#10 in Table 1) the photometric redshift estimate is not in good agreement with the spectroscopic redshift measurement, but this is most likely because of the blue colour of the object.
Table 2: Photometric redshifts for the 95 X-ray sources without spectroscopic identification in the XMM- Newton/2dF sample.
In this section we apply the HMP00 method to X-ray sources for which no
spectroscopic data are available. This sub-sample comprises 96 sources of
which 45 have point-like optical morphology and 51 are optically
extended. From the point-like sub-sample we exclude one
source with
that is also best fit
with a stellar template. Based on the discussion in Sect. 4.1,
this is likely to be a Galactic star. The results for the remaining 44 sources are presented
in Table 2. We also caution the reader that there are 24 point-like
X-ray sources with r>21 mag for which, as discussed above, our
method is expected to perform less well.
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Figure 3:
Comparison between
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The results for the 51 optically extended sources in the spectroscopically unidentified sub-sample are also presented in Table 2. 29 of these sources have red optical colours (g-r>0.5 mag) for which our method gives acceptable photometric redshift estimates. The remaining 22 sources however, have g-r<0.5 mag and as discussed above their photometric redshifts are not reliable. Nevertheless, for the sake of completeness these are also listed in Table 2.
In this paper we describe a recipe for estimating photometric
redshifts for bright X-ray selected samples comprising both powerful
distant QSOs and less luminous lower-z Seyfert-type systems.
Firstly, we exclude from our analysis X-ray sources associated
with Galactic stars by identifying optically unresolved objects with low
X-ray-to-optical flux ratio (
)
and SEDs that
are best fit with stellar templates. The remaining X-ray sources are
split into optically point-like and extended systems. The
former are most likely associated with powerful QSOs where the central
engine dominates the optical light. The latter are low-luminosity AGNs at
moderate redshifts and therefore their optical
colours also have a significant stellar component from
the host galaxy. For optically point-like objects we use QSO templates
to estimate their photometric redshifts while for optically extended
X-ray sources we use galaxy SEDs.
This method is applied to all the spectroscopically identified X-ray sources
detected in the present XMM-Newton/2dF sample. For optically
extended sources with relatively red colours (g-r>0.5) the photometric redshift success rate is reasonably good: i) 73 per cent have
;
ii) the fraction of non-catastrophic redshifts, i.e. with
,
is 93 per cent; iii) the rms scatter is
.
The fraction of problematic photometric redshift estimates increases, however, for optically extended objects with g-r<0.5, most likely
because of contamination of their optical light by emission from the central engine. For these sources we find that 27 per cent have
and we estimate an rms scatter of
.
We find no improvement when QSO or star-forming galaxy templates, or linear combinations of QSO and galaxy SEDs, are applied to these systems. In addition the application of a distance prior (z <1) results in no significant improvement of the results, with the rms scatter
still large, i.e.
.
For point-like sources we estimate an accuracy of
for
68 per cent of the sample. About 75 per cent of these sources have
.
Figure 1 shows that
there are degeneracies that affect the efficiency of our method,
particularly for sources with
.
As discussed by Richards
et al. (2001), this is likely because of strong emission lines falling
between the SDSS filters at certain redshift ranges. Indeed, these
authors use a different method to that described here to estimate photometric redshifts for
optically selected SDSS QSOs. They find that photometric redshifts
for 70 per cent of the sources in their sample are correct within
with an rms scatter of 0.67, estimated on their whole sample (
2600 sources). Their success
rate for optically selected QSOs is better than the one obtained here for X-ray selected QSOs: 52 per cent have
and the rms scatter is estimated to be 0.72 (based on our sample of 62 sources)
. Nevertheless, similarly to our results, the accuracy of their method is also lower for sources in the range
.
They attribute these problems to the MgII, CIV and
Ly
lines falling between the u and g bands at different redshifts.
We also explore the possibility that the colours of our X-ray selected
AGNs are responsible for the problematic redshifts at
.
Richards et al. (2001) found that about 4 per cent of their optically selected AGN sample have colours redder than
those of typical QSOs at
(i.e.
)
suggesting either
reddening or high-z systems. A large fraction of these redder sources
has inaccurate photometric redshift estimates. Since X-ray selection
is least biased to obscured AGNs we also expect a non-negligible
fraction of QSOs with red colours most likely because of dust
extinction. Estimating the redshift of these systems is problematic
since their SEDs are likely to be very different to the templates used
by the HMP00 method.
Figure 4 plots u-g against g-r for all the X-ray
selected sources with unresolved optical morphology. The curves are
the colour-colour tracks of the HMP00 QSO templates in the redshift
range 0-6 for two different values of the optical power-law spectral
index, 0 and 1 (solid and long-dashed line respectively). The
regime for these models lies at
.
A number of sources
are redder than u-g > 0.6 (total of 21) but these are most likely
high-z rather than dusty QSOs. Indeed, all sources with spectroscopic
identification and
have
.
Nevertheless, a striking result in Fig. 4 is the
non-negligible fraction of optically point-like X-ray sources that are
red in both the u-g and the g-r colours (7 per cent with u-g >
0.6 and
g - r > 0.6). These red colours cannot be accounted for by
the HMP00 QSO templates at any redshift and suggest the presence
of dust. Unfortunately spectroscopic redshifts are not available for
these sources to explore their nature. It is likely however, that the
photometric redshifts for these systems are problematic since their
colours are much redder than those of optically selected QSOs at any
redshift. A different set of template SEDs is probably required for
such sources.
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Figure 4:
Colour-colour diagram of the X-ray sources with point-like
optical morphology. Open circles are for sources without
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Babbedge et al. (2004) recently presented a method similar to the one described here to
estimate photometric redshifts for both QSOs and galaxies in the
ELAIS N1 and N2 fields as well as in the Chandra Deep Field North region.
The additional criteria applied by our method are the use
of the X-ray to optical flux ratios to identify stars
and the separation between blue and red galaxies. The application of these is essential in
bright X-ray samples which comprise an elevated fraction of both stars and sources having blue colours. The accuracy of the photometric redshift estimates of Babbedge et al.,
in the case of the CDFN region, is in reasonable agreement with those obtained here.
The above authors find that 83 per cent of their QSO sample
have
;
in our case this fraction amounts to 75 per cent. For the galaxies
Babbedge et al. estimate that a fraction of 90 per cent has
,
while
we find a fraction of 93 per cent for the red galaxies only. We find however that there are limitations in the application of photometric redshifts to the blue optically
resolved sources.
In this paper we explore the use of photometric redshifts
to bright X-ray samples. In particular,
we estimate photometric redshifts for a sample of 193 X-ray sources with optical counterparts in the XMM-Newton/2dF survey. This is a bright X-ray sample [
]
consisting of both powerful distant QSOs and lower luminosity nearby AGNs. We split our sample into extended and point-like sources based on
their optical light profile. We then employ the HMP00 technique to determine photometric redshifts, using galaxy and QSO templates for the extended and the point-like sources respectively. A number of stars can be immediately identified on the basis of their unresolved
optical light profile, their low X-ray-to-optical flux ratio and their SEDs
that are best fit with stellar templates.
Comparison between photometric and spectroscopic redshifts for 62 optically unresolved sources
shows that for 68 per cent the photometric redshift estimates are accurate within
.
The photometric redshifts appear to be in good agreement with
spectroscopic ones up to
.
At larger redshifts
the presence of emission lines between filters limits the
efficiency of our method. We also find a non-negligible fraction (7%)
of X-ray sources with point-like optical profiles that have colours redder
than those of optically selected QSOs. The photometric redshift
estimates for these sources are likely to be problematic.
For X-ray sources with extended optical light profiles we
find that galaxy templates provide reasonable
(73 per cent have
)
photometric
redshifts at least for sources with red colours (g-r > 0.5).
We find however that a combined galaxy-QSO template approach cannot
be successfully applied to optically extended sources with bluer
colours. These present poor photometric redshift estimates,
regardless of whether the estimates are based on galaxy or QSO templates, or any linear combination of the two.
Acknowledgements
We thank the anonymous referee for valuable comments and suggestions. This work is funded by the European Union and the Greek Ministry of Development in the framework of the Programme "Competitiveness-Promotion of Excellence in Technological Development and Research-Action 3.3.1'', Project "X-ray Astrophysics with ESA's mission XMM'', MIS-64564. The XMM-Newton/2dF survey data as well as part of the observations presented here are electronically available at http://www.astro.noa.gr/xray/.
The 2dF QSO Redshift Survey (2QZ) was compiled by the 2QZ survey team from observations made with the 2-degree Field on the Anglo-Australian Telescope.
Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the US Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, Princeton University, the United States Naval Observatory, and the University of Washington.