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

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti, 93/3, 40129 Bologna, Italy
e-mail: stefano.marchesi@inaf.it
² Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA
³ Department of Physics, University of Miami, Coral Gables, FL 33124, USA
⁴ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
⁵ Centro de Astrobiologia (CSIC-INTA), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁷ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁸ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Alfonso Corti 12, 20133 Milan, Italy
⁹ AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ Johns Hopkins University – Center for Astrophysical Sciences, 3400 N. Charles Street, Baltimore, MD 21218, USA
¹¹ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
¹² Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
¹³ ICREA, Pg. Luís Companys 23, 08010 Barcelona, Spain
¹⁴ Department of Physics and Astronomy, Tufts University, Medford, MA 02155, USA
¹⁵ International Research Fellow of Japan Society for the Promotion of Science (Invitational Fellowships for Research in Japan – Short-term), Tokyo, Japan
¹⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50125 Firenze, Italy
¹⁸ Scuola Normale Superiore, Piazza dei Cavalieri 9, Pisa, Italy

Received: 28 May 2021
Accepted: 15 September 2021

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 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.