Abstract
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.
Highlights
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
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)
We found 4 new spectroscopically confirmed quasars at zspec > 4, of which one, 3XMM J125329.4+305539 is at zspec = 5.08
Summary
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. 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). Quasar Candidates Catalog K16 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. The catalog includes 515 known quasars (with spectroscopic redshifts, of which 266 are at zspec > 3) and 388 new quasar candidates.
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