Volume 635, March 2020
|Number of page(s)||7|
|Section||Letters to the Editor|
|Published online||06 March 2020|
Letter to the Editor
The first blazar observed at z > 6
INAF – Osservatorio Astronomico di Brera, Via Brera, 28, 20121 Milano, Italy
2 DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
3 INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
4 Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
5 INAF – Osservatorio Astronomico di Brera, sede di Merate, Via E. Bianchi, 46, 23807 Merate, Italy
6 Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
7 Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway
8 INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF), Via A. Corti 12, 20133 Milano, Italy
9 INAF – Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
Accepted: 6 February 2020
We present the discovery of PSO J030947.49+271757.31, the radio brightest (23.7 mJy at 1.4 GHz) active galactic nucleus (AGN) at z > 6.0. It was selected by cross-matching the NRAO VLA Sky Survey and the Panoramic Survey Telescope and Rapid Response System PS1 databases and its high-z nature was confirmed by a dedicated spectroscopic observation at the Large Binocular Telescope. A pointed Neil Gehrels Swift Observatory XRT observation allowed us to measure a flux of ∼3.4 × 10−14 erg s−1 cm−2 in the [0.5–10] keV energy band, which also makes this object the X-ray brightest AGN ever observed at z > 6.0. Its flat radio spectrum (ανr < 0.5), very high radio loudness (R > 103), and strong X-ray emission, compared to the optical, support the hypothesis of the blazar nature of this source. Assuming that this is the only blazar at this redshift in the surveyed area of sky, we derive a space density of blazars at z ∼ 6 and with M1450 Å < −25.1 of 5.5+11.2−4.6 × 10−3 Gpc−3. From this number, and assuming a reasonable value of the bulk velocity of the jet (Γ = 10), we can also infer a space density of the entire radio-loud AGN population at z ∼ 6 with the same optical/UV absolute magnitude of 1.10+2.53−0.91 Gpc−3. Larger samples of blazars will be necessary to better constrain these estimates.
Key words: galaxies: active / galaxies: high-redshift / galaxies: jets / quasars: individual: PSO J030947.49+271757.31
© ESO 2020
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