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

December 2021 • 2021A&A...656A.117M

Authors • Marchesi, S. • Mignoli, M. • Gilli, R. • Peca, A. • Bolzonella, M. • Nanni, R. • Annunziatella, M. • Balmaverde, B. • Brusa, M. • Calura, F. • Cassarà, L. P. • Chiaberge, M. • Comastri, A. • Cusano, F. • D'Amato, Q. • Iwasawa, K. • Lanzuisi, G. • Marchesini, D. • Morishita, T. • Prandoni, I. • Rossi, A. • Tozzi, P. • Vignali, C. • Vito, F. • Zamorani, G. • Norman, C.

Abstract • 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 f0.5 − 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 σNMAD = 1.48 × median(||zphot − zspec||/(1 + zspec)) = 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 ( or via


IPAC Authors


Takahiro Morishita

Assistant Scientist