Catalog of Candidates for Quasars at 3 < z < 5.5 Selected among X-Ray Sources from the 3XMM-DR4 Survey of the XMM-Newton Observatory

3.2 MB·

by  G. A. Khorunzhev

Preprint, Accepted to cAstronomy Letters, 2016, Vol. 42, № 5, pp. 277–294
Original Russian Text(cid:13)cG.A. Khorunzhev, R.A. Burenin, A.V. Mescheryakov, S.Yu. Sazonov, 2016,
(cid:13)
Pisma v Astronomicheskii Zhurnal, 2016, Vol. 42, No. 5, pp. 313-332.
CATALOG OF CANDIDATES FOR QUASARS AT 3< <5.5
Z
SELECTED AMONG X-RAY SOURCES FROM THE 3XMM-DR4
SURVEY OF THE XMM-NEWTON OBSERVATORY
c 2017 г. G.A. Khorunzhev*1, R.A. Burenin1, A.V. Mescheryakov1 and S.Yu. Sazonov1
(cid:13)
Preprint Accepted
We have compiled a catalog of 903 candidates for type 1 quasars at redshifts 3 < z < 5.5 selected
among the X-ray sources of the serendipitous XMM-Newton survey presented in the 3XMM-DR4
7
catalog (the median X-ray flux is 5 10−15 ergs−1cm−2 in the 0.5–2 keV energy band) and located
1
0 at high Galactic latitudes b > 2≈0◦ ×in Sloan Digital Sky Survey (SDSS) fields with a total area of
2 about 300 deg2. Photometr|ic|SDSS data as well infrared 2MASS and WISE data were used to select
theobjects.WeselectedthepointsourcesfromthephotometricSDSS catalogwithamagnitudeerror
n
a hδmavze(cid:48)c<al0cu.2laatneddtahceodloeprein(cid:48)−dezn(cid:48)ce<sχ0.26(z()tofofirrvsatreioliumsisnpaetcettrhaletMem-tpylpaetesstfarrosm).tFhoerltibhreasreyletchtaetdwseoucorcmesp,ilwede
J
forthesepurposesusingtheEAZYsoftware.Basedonthesedata,wehaverejectedtheobjectswhose
0
spectral energy distributions are better described by the templates of stars at z = 0 and obtained a
3
sample of quasars with photometric redshift estimates 2.75<z <5.5. The selection completeness
phot
] of known quasars at zspec > 3 in the investigated fields is shown to be about 80%. The normalized
A median absolute deviation (∆z = zspec zphot ) is σ∆z/(1+zspec)= 0.07, while the outlier fraction is
| − |
G η = 9%, when ∆z/(1+zspec) > 0.2. The number of objects per unit area in our sample exceeds the
number of quasars in the spectroscopic SDSS sample at the same redshifts approximately by a factor
.
h of 1.5. The subsequent spectroscopic testing of the redshifts of our selected candidates for quasars at
p
3<z <5.5 will allow the purity of this sample to be estimated more accurately.
-
o DOI: 10.1134/S1063773716050042
r
t
s Keywords: quasars, X-ray surveys, photometric redshift
a
[

1
v INTRODUCTION In their recent paper, Kalfountzou et al. (2014)
6 Searching for quasars at z (cid:38) 3 is one of the most constructed the X-ray luminosity function for
0
important elements of studying the growth history quasars at z > 3 based on a catalog of 209 X-ray
8
8 of supermassive black holes and the evolution of sources detected in various sky fields with a total
0 mas- sive galaxies in the Universe. To improve areaof 33deg2.Thiscatalogconsistsoftwoparts:
≈
1. the available constraints on the models of the X- С-COSMOS is a Chandra X-ray survey with an
0 ray luminosity function for quasars at z (cid:38) 3 areaof0.9deg2,122sourceswithfluxes12 10−16 <
7 requires collecting a large X-ray sample of distant FX < 2 10−15 ergs−1cm−2 (Elvis et×al., 2009)
1 quasars at high redshifts (see, e.g., Aird et al., and ChaM×P is a "serendipitous"survey based on
v: 2015).Bright and dis tant quasars are rare objects, CHANDRApointingswitharea33deg2,87sources
Xi with X-ray surveys of large area and sufficient withfluxes3×10−15 < FX < 3×10−14 ergs−1cm−2
depth being needed for their search. Photometric (Kim et al., 2007). The C-COSMOS survey has
r
a and spectroscopic support at optical wavelengths is deep photometric (using a set of medium-band
required to work with X-ray sources. An increasing filters) and spectroscopic coverage and, for this
role of the methods for estimating the photometric reason, 90%identificationcompletenessofobjects
≈
redshift to determine z and to classify the sources at z > 3. The spectroscopic ChaMP program
is traceable in present-day works on this subject (44 objects) and various photometric relations (43
matter Assef et al. (2011); Civano et al. (2012); objects) were used to measure and estimate the
Kalfountzou et al. (2014).
1Here and below, all X-ray fluxes and luminosities are
*Электронный адрес: [email protected] given in the 0.5–2 keV energy band.
277
278 Khorunzhev et al.
redshifts of ChaMP sources. Thus, objects with redshift estimates based on optical data increases,
firmly established redshifts may be deemed to because the jump produced by the bright Lyα line
constitute about 70% of the sample of quasars at and the Lyα forest in the quasar spectrum fall
z>3 from Kalfountzou et al.(2014). into the optical band. The addition of infrared
The observations of numerous astrophysical WISE photometry increases the purity of the
objects with the XMM-Newton X-ray telescope sample of distant quasar candidates and gives an
accumulated for the last 15 years collectively advantage over the selection methods based only
represent a unique "serendipitous" X-ray sky on SDSSphotometry.
survey (Watson et al., 2009) with a total area In this paper, we present the sample of
of about 800 deg2 and a sensitivity of candidates for quasars at z > 3 obtained
5 10−15 ergs−1cm−2 (the 3XMM-DR4 version≈; by searching for such objects among the X-
×
Watsonet al.,2009,2013).Itishopedthatasample ray sources of the serendipitous XMM-Newton
of quasars at z > 3 selected by their X-ray emission survey, 3XMM-DR4, using SDSS, 2MASS and
thatexceedsthesampleofKalfountzouet al.(2014) WISE data. The properties of this sample are
byseveraltimeswillbeobtainedonthebasisofthis discussed. Subsequently, we are going to use
survey. This is the goal of our work. this sample to construct the X-ray luminosity
Photometric and spectroscopic support is function for quasars z > 3. Here, we use
required to work with the X-ray sources from the the following cosmological parameters: Ω0=0.3,
3XMM-DR4 catalog. The solution of the problem Λ0=0.7, H0=70 kms−1Mpc−2.
is facilitated by publicly accessible surveys: the
optical photometric and spectroscopic Sloan
THE SAMPLE OF X-RAY SOURCES WITH
Digital Sky Surveys (SDSS, Alam et al., 2015;
OPTICAL AND INFRARED PHOTOMETRY
Aihara et al., 2011; Eisenstein et al., 2011) with
In our work, we used data from the 3XMM-
areas of 14000 and 10000 deg2, respectively; the
≈ ≈ DR42 (Watson et al., 2013). Our sample consists
infrared photometric 2MASS (Cutri et al., 2003)
of 129541 point (SC_Extent=0) X-ray sources at
and WISE (Wright et al., 2010) all-sky surveys.
Galactic latitudes |b|>20◦ and has an area of
The spectra were taken and the redshifts were
300 deg2, which is determined by the area of
measured for a large number of distant quasars ≈
the overlap between the 3XMM-DR4 survey and
as part of the spectroscopic SDSS programs.
the SDSS; the area is calculated below. We work
Therefore, it is natural to use the spectroscopic
with the 0.5–2 keV X-ray flux (the sum of columns
sample of SDSS objects (Alam et al. 2015) as
SC_EP2_FLUX and SC_EP3_FLUX in the 3XMM-DR4
the first step: 60000 quasars with zspec > 3 on
10000 deg2.(cid:39)However, the spectroscopic SDSS catalog).
(cid:39) We cross-correlated the catalog of X-ray sources
is based on a fairly complex method for the
with the data from the photometric (data release
selection of quasar candidates whose completeness
10) and spectroscopic (data release 12) SDSS
and purity depend on many factors: the redshift,
(Aihara et al., 2011; Ahn et al., 2012; Alam et al.,
the morphological properties of objects, the quality
2015; Eisenstein et al., 2011). Within the 2σ X-
of photometry, etc. (Ross et al., 2013). The purity
ray source position error (if it was less than 3(cid:48)(cid:48),
of the samples of distant quasar candidates for the
then within 3(cid:48)(cid:48)) we searched for all optical SDSS
SDSS does not exceed 50% (Ross et al., 2012). At
sources.Asaresult,weproducedacatalogof64714
the same time, it is unclear how the completeness
X-ray sources that have an optical counterpart
and purity of the selection of candidates for the
among the SDSS objects. At the same time,
spec- troscopic SDSS program behave as a function
2489 (4%) X-ray sources have more than one
of the X-ray flux.
optical counterpart. In the case of an ambiguous
The technique of comparing the spectral energy
correspondence, we estimated z for all the
distribution of a source in the optical and infrared phot
possible optical counterparts of a given X-ray
bands with the template spectra of quasars, stars,
source. It turns out that there are spectroscopic
and galaxies can be used for the selection of quasar
datafor12823( 20%ofthesample)X-raysources
candidates at z > 3 among the X-ray sources ≈
in the SDSS. We deemed the spectroscopic redshift
of the 3XMM-DR4 survey. Such a technique not
measurement reliable at the flag zWarning=0. It
only gives the probability that the object is a
turns out that 390 sources in the 3XMM-DR4
quasar but also allows its photometric redshift
estimatezphot tobeobtained.Inthecaseofquasars 2http://heasarc.gsfc.nasa.gov/W3Browse/
at high redshifts, z > 3, the reliability of their xmm-newton/xmmssc.html
ASTRONOMY LETTERS Vol. 42 №5 2016
Quasars of XMM-Newton at 3 < z < 5.5 279
cataloghavez > 3andzWarning=0;280ofthem
spec
have a magnitude measurement error δmz(cid:48) < 0.2
(see below).
We additionally made a cross-correlation with
the catalog of quasars from Flesch (2015). This
catalog contains the spectroscopically confirmed X-
rayquasarsthedataonwhichwerepublishedbefore
January 25, 2015, and the sample of reliable (the
probabilitythat>99%willturnouttobeaquasar)
photometric candidates from Bovy и др. (2012);
Richards et al. (2009). We added 49 objects with
z > 3 from the catalog of Flesch (2015), five of
them with photometric z estimates(Richards et al.,
2009). As a result, we obtained a spectroscopic
sample of 329 quasars with redshifts greater than
3 (280 from the spectroscopic SDSS sample and
Fig. 1. The solid line indicates the fraction of X-ray sources
49 from Flesch (2015)). This sample is required to
in the 3XMM-DR4 catalog that were detected in the
check the completeness of our sample of quasars photometric SDSS (δmz(cid:48) <0.2) or that have a counterpart
(see below). in the GSC as a function of the X-ray flux. The dashed
line (at low X-ray fluxes) indicates the fraction of quasars
We searched for infrared counterparts in the
atz>3fromCOSMOS (Kalfountzouet al.,2014)detected
2MASS (Cutri et al., 2003) and WISE (Wright
in the photometric SDSS. The dots indicate the detection
et al., 2010) catalogs. The search was made within completeness function for X-ray quasars at z > 3 in the
6arcsecoftheX-raysourceposition.Ifwefoundno SDSS composedfromthesetwodependencesandusedbelow
versus the X-ray flux.
infrared counterpart for an optical source, then we
set 7σ and 5σ limits for the 2MASS photometric
J, H, Ks bands and WISE photometric w1, w2,
by processing survey plates from the Palomar
w3, w4 bands, respectively, using the photometric
(northern hemisphere), Siding Spring (southern
data for adjacent sources. For the 2MASS and
hemisphere), and several more observatories. The
WISE sources that were not detected in all bands,
GSC contains 19millionsourceswithmagnitudes
we used the upper limits from the corresponding
∼
from 6 to 16 (note that the sources brighter than
catalogs. We assumed that the 2MASS sources
magnitude 15 are overexposed in the SDSS), some
always corresponded to the sources with WISE
of which were astrometrically classified as stars.
photometry. When a WISE source was offset from
an SDSS source by more than 2 arcsec, then we The fraction of X-ray sources in the 3XMM-DR4
additionally considered the case where the source catalog for which there are optical counterparts
had only SDSS photometry when calculating z . in the photometric SDSS and the GSC drops
phot
In this case, the upper limits in the 2MASS with decreasing X-ray flux from 80%
and WISE filters were taken to be equal to the at FX 10−14 ergs−1cm−2 to ≈20% at
WISE/2MASS magnitudes of the controversial FX 10≈−15 ergs−1cm−2. At lower X≈-ray fluxes,
≈
source. the fraction of nearby galaxies (z < 1) in the
Thus, we have a flux measurement or an upper sample of X-ray sources increases (Lehmer et al.,
limit on the flux in 12 broad bands (u(cid:48), g(cid:48), r(cid:48), i(cid:48), z(cid:48), 2012) ; therefore, no further reduction in the
J, H, Ks, w1, w2, w3, w4) for the X-ray sources. detection completeness of objects in the SDSS
To calculate the fluxes in all photometric bands, we occurs.
usedthemagnitudesdesignedtomeasurethefluxes
from point sources, which the distant quasars are. The quasars at z > 3 constitute a small fraction
In addition, the search was made only among the of all X-ray sources, and the fraction of objects
point SDSS sources. with optical counterparts for them can differ from
There is no photometry for bright stars in that for all X-ray sources. This assumption can be
the SDSS. To take into account their influence tested using the representative sample of quasars
on our quasar selection method, we searched for with reliably measured z > 3 from the deep
the counterparts of XMM-Newton X-ray sources С-COSMOS X-ray survey that was talked about
within 6 arcsec in the Guide Star Catalog (GSC, above. Out of 122 objects in this sample, 32
Lasker et al., 1990). This catalog was produced have spectroscopic redshifts z , 75 have reliable
spec
Preprint of ASTRONOMY LETTERS Vol. 42 №5 2016
280 Khorunzhev et al.
photometric redshifts z (measured with a set complete template library.
phot
of medium-band filters), and 15 have no optical
counterpart to mi(cid:48) 28 and, therefore, are also For each object, we made separate iterations
≈
deemed probable candidates for quasars at z > for the libraries of quasar and star templates.
3 (Kalfountzou et al., 2014; Civano et al., 2011). When fitting by the quasar templates, we corrected
Figure 1 shows how the fraction of objects from the SDSS photometry for extinction in our
this sample detected in the photometric SDSS Galaxy using its values for each object from the
changes with X-ray flux. The fraction of objects photometric SDSS catalog. For the star templates,
with optical counterparts detected in the SDSS the SDSS photometry was not corrected for
among the quasars at z > 3 turns out to be lower extinction in our Galaxy. The best value of z
phot
than that for all X-ray sources at fluxes below was sought on the grid of redshifts [0.01,7] with a
2 10−15erg s−1 cm−2 by several times. 0.01 step by finding the best-fitting template from
×Below, when taking into account the influence of the library of quasars according to the χ2 test.
object detection incompleteness in the photometric We also took into account the intergalactic ab-
SDSS on our final sample of candidates for sorption by neutral hydrogen (Madau, 1995). he
quasars at z > 3, we use an approximate smooth intergalactic absorption allows for the influence of
photometriccompletenessfunctionoftheX-rayflux the Lyα forest on the spectral energy distribution
composed from the dependence for the detection of a quasar. At redshifts z > 3, Lyα forest absorbs
completeness of quasars at z > 3 at fluxes below a substantial fraction of light on the blue side
2 10−15 erg s−1 cm−2 and the corresponding of the Lyα line, and this fraction increases with
×
dependence for all X-ray sources at fluxes above redshift.Atsuchredshifts,theLyαlinefallsintothe
5 10−15 erg s−1 cm−2 (see Fig. 1). photometric g(cid:48) band, and the presence of a jump in
×
аличие скачка поглощения Lyα-forest absorption
PHOTOMETRIC DATA FITTING BY can already be determined from u(cid:48) g(cid:48) color. At
−
SPECTRAL TEMPLATES redshifts above z 3.5, the Lyα line moves into
In our work, to calculate the χ2 при values when ther(cid:48) band,andth≈epresenceofanabsorptionjump
is determined from the g(cid:48) r(cid:48). This feature in the
fittingthespectralenergydistributionintheoptical
−
spectra of quasars at redshifts z > 3 allows one to
and infrared bands by various spectral templates
increase the identification reliability of such objects
and to obtain the photometric redshift estimates,
and to obtain more reliable photometric estimates
we used the EAZY (Brammer et al., 2008). This
of their redshifts (see below).
software steps through a grid of redshifts and at
each z finds the best-fitting synthetic template by The Template Library
minimizing the χ2 value:
The targets of our search are type 1 quasars
N (withoutanysignificantintrinsicabsorption)atz >
χ2 = (cid:88)filt (Tz,i,j −Fj)2 , (1) 3. We used the spectral templates of quasars from
(δFj)2 the EAZY (Brammer et al., 2008) and LePHARE
j=1
(Ilbert et al., 2006) libraries as well as from other
where N is the number of photometric data libraries (Assef et al., 2010; Richards et al., 2006;
filt
points (filters), T is the synthetic flux of Krawczyk et al., 2013). The templates that we
z,i,j
template i at redshift z in filter j, F — is the modified (Krawczyk et al., 2013) were added to the
j
observed flux in filter j and δF — is the flux error library.Inthesetemplates,wetookintoaccountthe
j
that takes into account the photometric error in contribution of the quasar’s emission lines obtained
filter j. from the template (Vanden Berk et al., 2001).
The EAZY software implements an algorithm Besides,weextendedtheaveragespectrumofSDSS
that allows one to seek a superposition of several quasars (Vanden Berk et al., 2001) to the infrared
spectral templates, for example, the spectra of (λ > 8000 A˚) using the template of a type 1 active
a galaxy and a quasar. However, we use only a galactic nucleus (AGN) (Assef et al., 2010) and
one-template fit in our work, because we search addedtheresultingtemplatetoourlibrary.Wealso
for bright quasars whose emission dominates over added the templates of an elliptical galaxy (Assef
the emission from the host galaxy. The quasars et al., 2010) and a type 2 quasar (Polletta et al.,
are characterized by a large scatter of spectra and 2007). As in (Dahlen et al., 2013), we applied the
strong variability (see e.g., Richards et al., 2006); set of extinctions E(B V) = 0.01, 0.05, 0.5 and
−
therefore, it is necessary to have a sufficiently 1.0. according to the law of (Calzetti et al., 2000)
ASTRONOMY LETTERS Vol. 42 №5 2016
Quasars of XMM-Newton at 3 < z < 5.5 281
106
105 1
104
2
103 3
4
5
102
6
101 7
8
10
100 9
11
Fν 10 1
−
ν
10 2
−
12
10 3
−
13
10 4
−
10 5
−
10 6
−
10 7
−
14
10 8
−102 103 104 105 106
Wavelength,Angstrom
Fig. 2. Templates from the library of quasars in relative units νF : 1 — the average spectrum of type 1 quasars (Assef
ν
et al., 2010), 2 — the average spectrum of SDSS quasars (Brammer et al., 2008), 3 — the average spectrum of luminous
type 1 quasars (Krawczyk et al., 2013), 4 — the average spectrum of type 1 quasars (Krawczyk et al., 2013) supplemented
by emission lines with extinction E(B−V) = 0.01, 5 —the average spectrum of type 1 quasars (variant 2) (Assef et al.,
2010), 6 — the average spectrum of type 1 quasars (Krawczyk et al., 2013), supplemented by emission lines with extinction
E(B−V)=0.05,7—theaveragespectrumoftype1quasars(Krawczyket al.,2013),supplementedbyemissionlineswith
extinctionE(B−V)=0.1,8—thetemplateofatype1quasarextendedtotheinfraredVandenBerket al.(2001),9—the
average spectrum of type 1 quasars (Krawczyk et al., 2013) truncated to fit only the optical part, аппроксимации только
оптической части, 10 — template 1 (Assef et al., 2010) with extinction E(B−V) = 0.5 , 11 — template 1 (Assef et al.,
2010) with extinction E(B −V) = 1.0, 12 — the composite quasar template from the spectra of the deep VIMOS-VLT
survey (Gavignaud et al., 2006; Ilbert et al., 2006), 13 —t he spectrum of an obscured type 2 quasar Polletta et al. (2007),
14 — the spectrum of an elliptical E0 galaxy (Assef et al., 2010).
Preprint of ASTRONOMY LETTERS Vol. 42 №5 2016
282 Khorunzhev et al.
using tabulated data from the LePHARE software until the Lyα line passes from i(cid:48) to z(cid:48). The color of
package (Ilbert et al., 2006) to the templates. thequasarsthenbecomesi(cid:48) z(cid:48) 2.Thequasarsat
We tested the template library on the z (cid:38) 5.5 have reliable photom−etr≈y only in one SDSS
spectroscopic SDSS-DR12 sample and eliminated band, and, in that case, the method of determining
those templates that more often erroneously z by template fitting works poorly. Therefore,
phot
classified a nearby object as an object at z > 3 to increase the purity of our sample of quasar
than correctly classified distant quasars from candidates, we used a constraint on the sensitivity
the template library. Thereafter, 14 templates in the z(cid:48) SDSS band and the i(cid:48) z(cid:48) color:
−
remained, which were subsequently used to search
for quasars at z > 3. These templates are presented δmz(cid:48) < 0.2 & i(cid:48) z(cid:48) < 0.6 , (2)
−
in Fig. 2.
The galaxy and obscured quasar templates are where δmz(cid:48) < 0.2 is the error in the apparent
needed only to eliminate the nearby objects whose magnitude of a point source in the z(cid:48) SDSS filter.
accurateredshiftisofnointeresttous.Inthiscase, The constraint δmz(cid:48)<0.2 help to reduce the
no additional templates of galaxies (star-forming selection effects in searching for quasars. The
ones, with a different contribution of the active spectral energy distribution of quasars rises from
nucleus, etc.) are required to be applied, because λ 1 µm to Lyα. The relations between the
≈
thiswillincreasethenumberofoutliersanddegrade threshold sensitivities of the filters in the SDSS
the completeness and purity of the final sample of are such that by imposing the constraint (2), we
quasars at z > 3 (Simm et al., 2015; Salvato et al., guarantee the detection com- pleteness of type 1
2009). quasars not only in the z(cid:48) filter but also in bluer
For the templates of stars, we used the library of SDSS filters.
spectra from Pickles (1998). It includes the spectra After applying the photometric constraints, the
of spectral types O, A, B, F, G, K, M. For some of quasar candidates are selected according to the
the templates, there is a breakdown into luminosity condition:
classes: i — supergiants, ii — luminous giants,
iii — giants, iv — subgiants, v — main-sequence χ2star/χ2qso > 1 & zphot > 2.75 (3)
stars. The library spectra were constructed only
to a wavelength of 2.5 µm Pickles (1998). Since whereχ2 isthesmallestχ2 valueforthetemplate
star
the WISE filters are longer-wavelength ones, each from the library of stars, and χ2 is the primary
qso
spectrum from this library was extrapolated by minimum of the χ2 distribution for the quasar
Planck’s law for black- body radiation to a templates.
wavelength of 30 µm. Such assumption works well The conditions χ2 /χ2 > 1 and i(cid:48) z(cid:48)<0.6
star qso
for single main-sequence stars hotter than M0. eliminate 97% of the stars among the X-ray−sources
of the 3XMM-DR4 catalog. This was tested on
Criteria for the Selection of Quasar Candidates
stars for which the SDSS spectra are available
3 < z < 5.5
(see Fig. 3).
Since the quasars are star-like objects, we There exists a scatter of z relative to z .
phot spec
consider only the point SDSS sources when Hence, some of the objects with z > 3 will have
spec
searching for distant quasars. In this case, the z (cid:46) 3;therefore,wechosethelowerlimitz >
phot phot
contamination of the sample by M-type stars 2.75 when compiling the catalog of candidates for
presents a significant problem. Many of them are quasars at z > 3.
detected in the XMM-Newton X-ray survey owing We additionally checked our sample of quasar
to their coronal activity. During flares, the ratio of candidates to eliminate, where possible, the source
the X-ray and optical/near-IR fluxes for M dwarfs misidentification errors. We excluded 20 objects
can be the same as that for quasars. from the sample: two stars with the SDSS spectra,
The SDSS sensitivity in the blue filters is in- several faint sources against the background of
sufficient to separate distant quasars with i(cid:48) (cid:38)20.5 a nearby bright galaxy, faint sources near a
from stars. M dwarfs can be separated from distant bright star, and objects with erroneously measured
quasars only by the i(cid:48) z(cid:48) (Richards et al., 2002; redshifts.
−
Wu et al., 2012; Skrzypek et al., 2015): the stars
Examples of Objects from the Catalog
have i(cid:48) z(cid:48)>0.8, while the quasars have i(cid:48) z(cid:48)<0.4.
The co−lor for the quasars i(cid:48) z(cid:48) < 0.4−remains Figures 4 and 5 provide several examples of
−
approximately constant up to a redshift of 5.5, photometric data fitting by quasar and star
≈
ASTRONOMY LETTERS Vol. 42 №5 2016
Quasars of XMM-Newton at 3 < z < 5.5 283
Fig. 3. Criteria for selection by the i(cid:48)−z(cid:48) SDSS(left) and χ2 /χ2 >1 (right) versus redshift. The solid horizontal lines
star qso
indicate the boundaries of the selection criteria. The dots indicate 329 quasars with zspec > 3. The symbol (cid:63) indicates 66
spectroscopically confirmed stars from SDSS DR12 that have a minimum of the χ2(z ) distribution at z > 3. The
phot phot
circles mark the stars with the same color i(cid:48)−z(cid:48)<0.6 as that for the quasars.
templates for known quasars (for which z many of such objects can be eliminated using the
spec
is available) or stars. Figures 4 and 5 present, photometric constraints (2).
respectively, the cases where one of the quasar The object 3XMMJ094109.9+344902 lies at
spectral templates describes well the photometric z = 2.643. There is a WISE source more
spec
data points and gives a nearly correct estimate of than 2 arcsec away from the SDSS source. Two
z and where quasar template fitting leads to a photometric redshift estimates were made for this
phot
misidentification and gives an unreliable estimate object: by taking into account the fluxes measured
of z . The corresponding χ2(z). distributions are in WISE and 2MASS filters and using these
phot
shown in the insets. values only as upper limits on the infrared flux
The objects 3XMMJ151147.1+071406 (zspec = (see above). In the former case, zphot = 2.62
3.481), 3XMMJ001115.2+144601 (z = 4.964), (template 12 was used in fitting) and the object
spec
and 3XMMJ004054.6 091527 (z = 5.020) are doesnotenterintothecatalogofquasarcandidates,
spec
exampleswheretheph−otometricredshiftestimation because zphot<2.75. In the latter case, zphot = 3.00
methodworkswell.Inthiscase,thesignaldetection (template 1 was used) and, as a result, the object
in infrared filters guarantees that the object is not enters into the catalog. Such cases degrade the
a star. catalog purity.
The object 3XMMJ153322.7+324351 is an ex-
ample of a poor redshift determination: z = THE CATALOG AND ITS PROPERTIES
phot
3.01. The true value measured in the SDSS is According to the selection rules described above,
zspec = 1.897. wecompiledacatalogof903candidatesforquasars
The object 3XMMJ080630.4+153241 is an M- at 3 < z < 5.5. These include 21 3XMM-
type star. It was not included in our catalog DR4 pointing targets. The catalog is presented
of quasar candidates, because i(cid:48) z(cid:48)>0.6 and in the Appendix in the form of a table where
χ2 /χ2 > 1 for this object.−If an object the coordinates, X-ray fluxes, photometric and
qso star
with a similar color relation is fainter by two spectroscopic (if available) redshifts, the best
apparent magnitudes, i.e., near the SDSS detection quasarandstartemplatesaregiven.Theproperties
threshold, then the sensitivity of the WISE and of the catalog and its spectroscopic subsample are
2MASS surveyswillbeinsufficientforitsdetection. reflected in Fig. 6.
In that case, the star and quasar templates Theobjectsinthecatalogmostlyhavephotomet-
will describe the SDSS photometric data points ric redshifts 2.75 (cid:46) z 3.5. The objects have
phot
with comparable χ2 values, and the star may be following distributions by z≤ : 381 at 3 (cid:46) z
phot phot
≤
mistaken for a quasar at z > 3. Nevertheless, 4; 60 at 4 < z 5; 2 at z > 5.
phot phot phot
≤
Preprint of ASTRONOMY LETTERS Vol. 42 №5 2016
284 Khorunzhev et al.
Fig. 4. Examples of a satisfactory determination of the photometric redshift. Top: 3XMM J151147.1+071406 is a quasar,
zspec = 3.481, zphot = 3.22. Middle: 3XMM J001115.2+144601 is a quasar, zspec = 4.964, zphot = 4.54. Bottom:
3XMM J004054.6−091527 isaquasar,zspec =5.020,zphot =4.88.TheSDSS spectrum(blacksolidline),thephotometric
data points with their errors (red), the template of a quasar at z (blue solid line), and the template of a star (black
phot
dots) are shown. The χ2(z) distribution when fitting the photometric data by quasar templates is shown in the insets.
ASTRONOMY LETTERS Vol. 42 №5 2016
Quasars of XMM-Newton at 3 < z < 5.5 285
Fig. 5. Example of an unreliable estimation of the photometric redshift. Upper left: 3XMM J153322.7+324351 is a quasar
at zspec = 1.897. Its photometry is best described by the template of a type 1 quasar with zphot = 3.06. Upper right:
3XMM J080630.4+153241 is an M–type star with z = 5.58±0.02. The object was correctly classified as a star and
phot
was not included in the catalog of quasar candidates, because i(cid:48)−z(cid:48)>0.6 and χ2 > χ2 for it. Bottom left and right:
qso star
3XMM J094109.9+344902 isaquasar at zspec =2.643,forwhichtwophotometricredshiftestimates wereobtaineddue to
itscontroversialassociationwithaWISE:z =2.62andz =3.00).ThedesignationsarethesameasthoseinFig.4.
phot phot
Preprint of ASTRONOMY LETTERS Vol. 42 №5 2016
286 Khorunzhev et al.
Fig.6.Upperleft:thedistributionofquasarcandidatesinphotometricredshift(z ).Upperright:thedistributioninthe
phot
commonlogarithmofX-rayluminosity(inthe0.5–2keVenergybandintheobserver’sframe),Lowerleft:thedistributionin
i(cid:48) SDSS magintude.Lowerright:thedistributioninX-ray0.5–2keVflux.Thewhitecolorindicatesthedistributionsforthe
entire catalog of quasar candidates. The hatching indicates the objects with z that have known spectroscopic redshifts
phot
(withanyzspec).Thegraycolorindicatestheknownquasarswithzspec >3.Theirdistributionswereconstructedfromthe
spectroscopicredshifts(zspec).Theheightofeachcolumnisthetotalnumberofobjectsofthecorrespondingsubsamplein
a given bin along the Х axis.
The catalog contains 515 known quasars with for 388 distant quasar candidates.
reliably measured z ; 266 of them have z >3.
spec spec The median X-ray flux for the objects from the
The photometry for these 515 objects is described
catalog (903 sources) is 5.3 10−15 ergs−1cm−2 in
by the template of a quasar at zphot > 2.75 and the 0.5–2 keV energy band.×All sources with fluxes
“passes” according to the selection criteria (see
above 4 10−14 ergs−1cm−2 have spectroscopic
above). However, 63 known quasars with zspec>3 redshifts.×Because of the constraint on the error
(Flesch, 2015; Alam et al., 2015) did not satisfy the
in the z(cid:48) SDSS magnitude and the requirement
selection criteria. Their list is given in a separate
i(cid:48) z(cid:48)<0.6, most of our objects turn out to be
table in the Appendix. Their photometric data bri−ghter than magnitude 21.5 in the i(cid:48) SDSS filter.
pointsaremostoftenbetterdescribed(withsmaller
χ2)bythestartemplateorthetemplateofaquasar Thus, our catalog of candidates for distant
at lower z. Some of the objects with z >3 in the quasars can (as a result of the subsequent
spec
SDSS areclassifiedasextendedsources.Noreliable spectroscopic testing) lead to a noticeable (up to
spectroscopic redshift measurements are available a factor of 1.5) increase in the number of known X-
ray quasars at z>3 relative to the already existing
ASTRONOMY LETTERS Vol. 42 №5 2016