The first blazar observed at z>6
Abstract
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 (α r<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. %i.e. a radio-loud (RL) AGN with the relativistic jet pointed toward us. Assuming that this is the only blazar at this redshift in the surveyed area of sky, we derive a space density of blazars at z6 and with M1450 < -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 (=10), we can also infer a space density of the entire radio-loud AGN population at z6 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.