Redshift identification of X-ray-selected active galactic nuclei in the J1030 field: searching for large-scale structures and high-redshift sources

Abstract

We publicly release the spectroscopic and photometric redshift catalog of the sources detected withChandrain the field of thez= 6.3 quasar SDSS J1030+0525. This is currently the fifth-deepest extragalactic X-ray field, and reaches a 0.5–2 keV flux limit off0.5 − 2= 6 × 10−17erg s−1cm−2. Using two independent methods, we measure a photometric redshift for 243 objects, while 123 (51%) sources also have a spectroscopic redshift, 110 of which come from an INAF-Large Binocular Telescope (LBT) Strategic Program. We use the spectroscopic redshifts to determine the quality of the photometric ones, and find it to be in agreement with that of other X-ray surveys which used a similar number of photometric data points. In particular, we measure a sample normalized median absolute deviation ofσNMAD= 1.48 × median(||zphot − zspec||/(1 +zspec)) = 0.065. We use these new spectroscopic and photometric redshifts to study the properties of theChandraJ1030 field. We observe several peaks in our spectroscopic redshift distribution betweenz= 0.15 andz= 1.5, and find that the sources in each peak are often distributed across the wholeChandrafield of view. This confirms that X-ray-selected AGNs can efficiently track large-scale structures over physical scales of several megaparsecs. Finally, we computed theChandraJ1030z> 3 number counts: while the spectroscopic completeness of our sample is limited at high redshift, our results point towards a potential source excess atz ≥ 4, which we plan to either confirm or reject in the near future with dedicated spectroscopic campaigns.