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

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 × 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 × 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.

A copy of the spectra and SEDs is available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/656/A117