2021A&A...656A.117M

We publicly release the spectroscopic and photometric redshift catalog of the sources detected with Chandra in the field of the z = 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 of f_0.5 - 2_ = 6 x 10^–17 erg s^–1 cm^–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 of σ_NMAD = 1.48 x median(||z_phot - z_spec||/(1 + z_spec)) = 0.065. We use these new spectroscopic and photometric redshifts to study the properties of the Chandra J1030 field. We observe several peaks in our spectroscopic redshift distribution between z = 0.15 and z = 1.5, and find that the sources in each peak are often distributed across the whole Chandra field of view. This confirms that X-ray-selected AGNs can efficiently track large-scale structures over physical scales of several megaparsecs. Finally, we computed the Chandra J1030 z > 3 number counts: while the spectroscopic completeness of our sample is limited at high redshift, our results point towards a potential source excess at z >= 4, which we plan to either confirm or reject in the near future with dedicated spectroscopic campaigns.