High-redshift quasars (HRQs) are important for cosmology and galaxy studies as they can provide information of accretion processes on supermassive black holes (SMBH) and active galactic nuclei (AGN) evolution in the early Universe. Among the HRQs, the radio-loud subsample constitutes an attractive group since the radio jets are directly related to the SMBH activity.

The radio structures of the radio-loud subsample can be studied with the highest-angular resolution on milli-arcsecond scales via Very Long Baseline Interferometry (VLBI) technique. Until now, more than 100 quasars have been detected at z > 5 while the number of radio-loud ones is about 10.

Recently, an international research group led by Prof. AN Tao from Shanghai Astronomical Observatory (SHAO) of Chinese Academy of Sciences has studied radio-loud HRQs, and found one promising target CGRaBS J0906+6930 (hereafter J0906+6930) in the early Universe. This study has been published in Monthly Notices of the Royal Astronomical Society.

J0906+6930 was discovered as the most distant radio blazar in 2004, with a redshift of z=5.47. The radio flux of J0906+6930 at 15 GHz is over 100 mJy which makes it the best candidate for VLBI imaging at z > 5.

In order to verify the physical nature of J0906+6930, AN Tao' s group has conducted a new VLBI observation using the KVN and VERA Array (KaVA) in January, 2016, which obtains a compact source image with the flux density of 80 mJy at 22 GHz. This observation was served as the first application of KaVA for imaging studies of high-z quasars.

"By collecting and analyzing previous VLBI data of J0906+6930, we investigate the detected variability, compact core-jet structure and high flux density. All these evidence confirms its nature as a balzar," ZHANG Yingkang, the first author of the paper, said. "A peaked spectrum around 10 GHz is found, which suggest that the source could be a newly born quasar in the early universe."

The evolution of SMBHs in early stages of the Universe has been a hot topic in astrophysics frontier for a long time. The detection of z > 5 radio-loud quasars demonstrated that SMBHs have already existed just after the Epoch of Reionization and are producing prominent electromagnetic radiation. The high radio flux density and compact core-jet structure make J0906+6930 an excellent laboratory for studying radio activity of AGN at the youngest age of the Universe.

This research group is now planning for follow-up high-resolution VLBI observations of this target, aiming at studying spectrum evolution and jet proper motion. The unique radio properties of J0906+6930 will surely provide more valuable information about the baby Universe.

Figure: The re-analyzed image of J0906+6930 from VLBA database at 15 GHz. (Image by SHAO)