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Front. Astron. Space Sci., 13 November 2017 |

Catalog of 3 < z < 5.5 Quasar Candidates Selected among XMM-Newton Sources and Its Spectroscopic Verification

Georgii Khorunzhev1*, Sergey Sazonov1, Rodion Burenin1 and Maxim Eselevich2
  • 1High Energy Astrophysics, Space Research Institute, Russian Academy of Sciences, Moscow, Russia
  • 2Laboratory of Infrared Methods in Astrophysics, Institute of Solar-Terrestrial Physics, Russian Academy of Sciences, Irkutsk, Russia

We have compiled a catalog of 903 quasar candidates (including known quasars) at 3 < z < 5.5 selected among X-ray sources from the XMM-Newton serendipitous survey (3XMM-DR4 catalog). We used photometric SDSS, 2MASS, and WISE data to select the objects. The surface number density of objects in our sample exceeds that in the SDSS spectroscopic quasar sample at the same redshifts by a factor of 1.5. We have performed spectroscopic observations of a subsample of new quasar candidates using a new low- and medium-resolution spectrograph at the 1.6-m AZT-33IK telescope (Mondy, Russia) and demonstrated that the purity of these candidates is about 65%. We have discovered one of the most distant (z = 5.08) X-ray selected quasars.

1. Introduction

Searching for quasars at z > 3 is one of the key elements of studying the growth history of supermassive black holes and the evolution of massive galaxies in the Universe. For this purpose, multiwavelength observations of a large number of quasars are needed. Particularly valuable are X-ray observations: the intensive X-ray emission is directly connected with powerful processes of accretion onto the black hole. In order to improve our fairly poor knowledge of the evolution of the X-ray luminosity function of quasars at z > 3, larger X-ray samples of distant quasars are needed.

We have made an attempt to obtain a large sample of luminous X-ray quasars at z > 3 in the fields of the serendipitous XMM-Newton 3XMM-DR4 (Watson et al., 2009) survey at Galactic latitudes |b| > 20° using photometric data from SDSS (Eisenstein et al., 2011), 2MASS (Skrutskie et al., 2006) and WISE (Wright et al., 2010). The 3XMM-DR4/SDSS overlapping area is ~300 sq. deg, exceeding by several times the areas covered by known samples of distant X-ray quasars such as Champ (Kalfountzou et al., 2014) and XMM-XXL (north) (Menzel et al., 2016). The sample of Kalfountzou et al. (2014) covers ≈33 sq. deg and includes 87 X-ray selected optically bright (detected by SDSS) quasars. Half of them have photometric redshifts only. The spectroscopic program of (Menzel et al., 2016) has revealed 61 X-ray objects at zspec > 3 in a 18 sq. deg subfield of XMM-XXL (north). Our sample thus provides an opportunity to study a population of rare high-luminosity, distant quasars and complements recent studies of less luminous high-redshift quasars conducted using deep small-area surveys (e.g., Onoue et al., 2017; Ricci et al., 2017; Vito et al., 2017).

It should be noted that SDSS spectroscopic data provide redshift measurements for 33 thousand optically selected quasars at zspec > 3 (Alam et al., 2015) over 11,000 sq. deg in the sky. However, existing wide-area X-ray surveys are too shallow to study these objects. It is only the forthcoming all-sky X-ray survey by eROSITA aboard the Spektrum-Roentgen-Gamma observatory (Pavlinsky et al., 2011; Merloni et al., 2012) that will be capable of finding X-ray counterparts for a substantial fraction of known quasars at zspec > 3.

Based on broadband photometry, we obtained photometric redshift estimates (zphot) and compiled a catalog of 903 candidates for distant quasars with zphot > 2.75 (Khorunzhev et al., 2016). The catalog includes 515 known quasars (with spectroscopic redshifts, of which 266 are at zspec > 3) and 388 new quasar candidates. Most (80%) of the spectroscopic redshifts have been obtained by SDSS (Alam et al., 2015). The remaining zspec have been gathered from various sources (Flesch, 2015). The new quasar candidates selected by zphot constitute a substantial addition to the spectroscopic sample. Thus, if most of these candidates prove to be quasars at z > 3, the existing 3XMM-DR4/SDSS sample of distant quasars will be enhanced by a factor of ~1.5.

Spectroscopic verification is necessary to determine the accuracy of our zphot estimates and assess the purity of our selection of quasar candidates. We have thus started a spectroscopy identification program for the new quasar candidates (Khorunzhev et al., 2017). Some initial results of this program are reported below.

2. Sample Selection

K16 considered point SDSS sources at Galactic latitudes |b| > 20° that have an X-ray counterpart in 3XMM-DR41. In addition, we used near- (2MASS) and medium-infrared (WISE) photometry if available. To keep sources with reliable photometry and to get rid of M-dwarfs, we applied the following condition:

δmz<0.2 & iz<0.6 ,    (1)

where i′ and z′ are the PSF magnitudes in the appropriate SDSS bands and δmz<0.2 is the corresponding error. This is a well-known technique (e.g., Richards et al., 2002; Wu et al., 2012; Skrzypek et al., 2015) for separating M-dwarfs from distant quasars. Stars have i′ − z′ > 0.8, while quasars have i′ − z′ < 0.4. This color remains approximately constant up to a redshift of ≈5.5, until the Lyα line passes from i′ to z′. The color of quasars then becomes i′ − z′ ≈ 2.

We then performed a broadband energy distribution fitting using the EAZY software (Brammer et al., 2008) to obtain photometric redshift estimates. We made two fitting iterations for each object using libraries of quasar (by various authors) and star (Pickles, 1998) templates. Those objects with

χstar2/χqso2>1 &zphot>2.75,    (2)

finally constituted our catalog of 903 candidates for distant quasars selected by photometric redshift. At the same time, our procedure missed 63 known quasars at zspec > 3 (see Figure 1). Note that we lowered the threshold of selection from zphot = 3.0 to 2.75 to achieve reasonable selection completeness at z ~ 3.


Figure 1. Scatter plot of zspec relative to zphot for point objects (having an error δzpsf<0.2 and spectroscopic SDSS redshift zspec < 5.5). Light gray dots denote all SDSS sources. Black symbols are K16 objects with known redshift at the time of catalog compilation. Red symbols are objects whose spectra were obtained afterwards at AZT-33IK. The dash–dotted lines limit the area of zphot catastrophic outliers. The vertical dotted line at zphot = 2.75 denotes the K16 low zphot boundary of selection. The horizontal dotted line (zspec = 3.0) denotes our target zspec boundary for distant quasars.

The completeness of our catalog in the investigated fields relative to existing spectroscopic catalogs of quasars (SDSS DR12 Alam et al. 2015 and The Half Million Quasars Flesch 2015) with zspec > 3 is about 80%. The normalized median absolute deviation of photometric redshift estimates for the spectroscopically confirmed quasars (Δz = |zspeczphot|) is σΔz/(1+zspec) = 0.07, while the catastrophic outlier fraction is η = 9% (when Δz/(1+zspec) > 0.2).

3. Spectroscopic Verification

We have performed a quasi-random spectroscopic survey of quasar candidates from the K16 catalog at the 1.6-m AZT-33IK telescope (Kamus et al., 2002; Denisenko et al., 2009) equipped with the low- and medium-resolution ADAM spectrograph (Afanasiev et al., 2016; Burenin et al., 2016). A number of additional observations were conducted with the SCORPIO I (Afanasiev and Moiseev, 2005) spectrograph at the 6-m BTA telescope.

3.1. Observations at the AZT-33IK Telescope

The AZT-33IK telescope is located at the Sayan Solar Observatory of the Institute of Solar-Terrestrial Physics, the Siberian branch of the Russian Academy of Sciences, and has a primary mirror diameter of 1.6 m. The low- and medium-resolution ADAM spectrograph was produced at the Special Astrophysical Observatory of the Russian Academy of Sciences and was installed on AZT-33IK in 2015. The quantum efficiency of the entire system (telescope mirror, spectrograph, grating and CCD array) reaches 50% (Burenin et al., 2016). The ADAM spectrograph allows the spectra of objects with an apparent magnitude R ~ 19.5 to be taken with an exposure time of half an hour. If necessary and under good weather conditions, a magnitude I ~ 21 can be reached with an exposure time of two hours.

By now, we have obtained the spectroscopic redshifts for 48 quasar candidates, i.e more than 10% of such objects in the K16 catalog. Approximately 60% of the observed objects proved to be quasars at zspec ≳ 2.5, of which 16 have zspec > 3. We found 4 new spectroscopically confirmed quasars at zspec > 4, of which one, 3XMM J125329.4+305539 is at zspec = 5.08.

3.2. Discovery of a Quasar at z = 5.08

3XMM J125329.4+305539 was first reported (Khorunzhev et al., 2016) as a probable quasar at zphot = 4.64 in the K16 catalog, and there was no information about this source in other photometric catalogs of quasar candidates2. Its SDSS apparent magnitude is i′ ≃21.0. Its 0.5–2 keV flux is 1.5 × 10−15 erg/s/cm2 and the corresponding X-ray luminosity is 4 × 1044 erg/s (without k-correction).

Apart from this source, there are only 3 optically bright (i.e., with reliable SDSS photometry) X-ray quasars at zspec > 5.0, which are not XMM-Newton observational targets, in the 3XMM-DR4 catalog. Thus, 3XMM J125329.4+305539 is one of the brightest and most distant X-ray quasars at zspec > 5.0 suitable for constructing the X-ray luminosity function at such redshifts.

3.3. Purity and Selection Completeness

We can estimate the purity of the K16 catalog using the obtained quasi-random spectroscopic sample. Consider the following photometric redshift intervals: 2.75 ≤ zphot < 4, 4 ≤ zphot < 5 and 5 ≤ zphot < 5.5. By purity we mean the ratio of the number of true quasars (|zphotzspec|/(1 + zspec) < 0.2) to that of all objects with available spectra. Here, the value 0.2 reflects the scatter (3 standard deviations) of zphot relative to zspec for all of the known and spectroscopically confirmed quasars in the K16 catalog. The purity of the spectroscopic sample calculated in this way is shown in Figure 2 (circles).


Figure 2. Circles with Poissonian errorbars indicate the purity of the quasar candidates whose spectra have been taken at AZT-33IK or BTA. The arrows indicate the estimated lower limit for the purity of the K16 catalog relative to the objects with known (from literature or SDSS) spectroscopic redshifts.

For comparison, the arrows in Figure 2 indicate the lower limit on the purity of the K16 catalog estimated at the time of its compilation (before our AZT-33IK observations). This limit was deduced as the ratio of the number of true quasars with known spectroscopic redshifts and |zphotzspec|/(1 + zspec) < 0.2 to the total number of objects in the catalog. Recall that the new candidates without spectroscopic redshifts accounted for about 40% of the K16 catalog.

Based on these preliminary results, we can expect ~250 quasars at z ≳ 3 to be firmly identified upon completion of the spectroscopy of all new quasar candidates from the K16 catalog. This would significantly increase the sample of known distant X-ray quasars.

4. Conclusion

The obtained spectra of dosens of quasars at z ~ 3 and especially the discovery of one of the most distant X-ray selected quasars (3XMM J125329.4+305539) at zspec = 5.08 demonstrate that the AZT-33IK telescope of the Sayan Solar Observatory equipped with the new ADAM spectrograph is well suited for identification of distant quasars. This telescope is planned to be one of the instruments employed for optical support of the upcoming all-sky X-ray survey by the Spektrum-Roentgen-Gamma observatory (with its eROSITA and ART-XC telescopes, Pavlinsky et al., 2011; Merloni et al., 2012).

We have demonstrated that the existing sample of distant X-ray quasars can be substationally increased using publicly available X-ray (XMM-Newton) and optical photometry (SDSS) data, complemented by a relatively low-cost spectroscopic identification program.

We are continuing our spectroscopic verification program for K16 candidates using the AZT-33IK and BTA telescopes and intend to obtain a complete spectroscopic sample at least for brighter X-ray sources from the K16 catalog.

Author Contributions

GK provided key contributions to the compilation of the catalog, spectroscopic observations, data analysis and writing of this paper. SS supervised the research at all stages. RB contributed to the aquisition and processing of spectroscopic data. ME organized spectroscopic observations at AZT-33IK.


The observations at the 6-m BTA telescope were financially supported by the Ministry of Education and Science of the Russian Federation (contract no. 14.619.21.0004, project identifier RFMEFI61914X0004).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


This research was partially supported by the Program of the President of the Russian Federation for support of leading scientific schools (grant NSh-10222.2016.2).

This research is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. Also, we used SDSS-III, 2MASS, WISE data. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The Two Micron All Sky Survey is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. The Wide-field Infrared Survey Explorer is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.



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Keywords: active galactic nuclei, quasars, X-ray surveys, photometric redshifts, spectroscopy, XMM-Newton, SDSS

Citation: Khorunzhev G, Sazonov S, Burenin R and Eselevich M (2017) Catalog of 3 < z < 5.5 Quasar Candidates Selected among XMM-Newton Sources and Its Spectroscopic Verification. Front. Astron. Space Sci. 4:37. doi: 10.3389/fspas.2017.00037

Received: 31 August 2017; Accepted: 24 October 2017;
Published: 13 November 2017.

Edited by:

Mauro D'Onofrio, Università degli Studi di Padova, Italy

Reviewed by:

Paola Severgnini, Brera Astronomical Observatory, Italy
Fabio La Franca, Università degli Studi Roma Tre, Italy

Copyright © 2017 Khorunzhev, Sazonov, Burenin and Eselevich. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Georgii Khorunzhev,