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 X-ray field, and reaches a
0.5-2 keV flux limit $f_{\rm 0.5-2}$=6$\times$10$^{-17}$ erg s$^{-1}$
cm$^{-2}$. By using two independent methods, we measure a photometric redshift
for 243 objects, while 123 (51%) sources also have a spectroscopic redshift,
110 of which coming 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 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
$\sigma_{NMAD}$=1.48||$z_{phot}$-$z_{spec}$||/(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
evidence confirms that X-ray selected AGN can efficiently track large-scale
structures over physical scales of several Mpc. Finally, we computed the
Chandra J1030 $z>$3 number counts: while the spectroscopic completeness at
high-redshift of our sample is limited, our results point towards a potential
source excess at $z\geq$4, which we plan to either confirm or reject in the
near future with dedicated spectroscopic campaigns.
