The first estimate of radio jet proper motion at z>5

TL;DR

This study measures the proper motion of a radio jet in a high-redshift blazar, providing the first direct evidence of jet movement at z>5, and finds superluminal speeds consistent with relativistic models.

Contribution

First direct measurement of radio jet proper motion in a z>5 blazar, confirming relativistic jet speeds at high redshift.

Findings

Jet components show apparent motion up to 0.1 mas/yr.

Superluminal speeds exceed 10c for some jet components.

Results support relativistic jet models at high redshift.

Abstract

The extremely high redshift (z=5.3) radio source SDSS J102623.61+254259.5 (J1026+2542) is among the most distant and most luminous radio-loud active galactic nuclei (AGN) known to date. Its one-sided radio jet structure on milli-arcsecond (mas) and ~10-mas scales typical for blazars was first imaged at 5 GHz with very long baseline interferometry (VLBI) in 2006. Here we report on our dual-frequency (1.7 and 5 GHz) imaging observations performed with the European VLBI Network (EVN) in 2013. The prominent jet structure allows us to identify individual components whose apparent displacement can be detected over the time span of 7.33 yr. This is the first time when jet proper motions are directly derived in a blazar at z>5. The small values of up to ~0.1 mas/yr are consistent with what is expected in a relativistic cosmological model if redshift is a measure of distance. The apparent superluminal jet speeds, considered tentative because derived from two epochs only, exceed 10c for three different components along the jet. Based on modeling its spectral energy distribution, J1026+2542 is known to have its X-ray jet oriented close to the line of sight, with significant Doppler boosting and a large bulk Lorentz factor (Gamma~13). The new VLBI observations, indicating ~2.3 x 10^12 K lower limit to the core brightness temperature, are consistent with this picture. The spectral index in the core region is -0.35.