LOFAR Measures the Hotspot Advance Speed of the High-Redshift Blazar S5
  0836+710
  Our goal is to study the termination of an AGN jet in the young universe and
to deduce physical parameters of the jet and the intergalactic medium. We use
LOFAR to image the long-wavelength radio emission of the high-redshift blazar
S5 0836+710 on arcsecond scales between 120 MHz and 160 MHz. The LOFAR image
shows a compact unresolved core and a resolved emission region about 1.5 arcsec
to the southwest of the radio core. This structure is in general agreement with
previous higher-frequency radio observations with MERLIN and the VLA. The
southern component shows a moderately steep spectrum with a spectral index of
about $\gtrsim -1$ while the spectral index of the core is flat to slightly
inverted. In addition, we detect for the first time a resolved steep-spectrum
halo with a spectral index of about $-1$ surrounding the core. The
arcsecond-scale radio structure of S5 0836+710 can be understood as an FR
II-like radio galaxy observed at a small viewing angle. The southern component
can be interpreted as the region of the approaching jet's terminal hotspot and
the halo-like diffuse component near the core can be interpreted as the
counter-hotspot region. From the differential Doppler boosting of both
features, we can derive the hotspot advance speed to $(0.01-0.036)$ c. At a
constant advance speed, the derived age of the source would exceed the total
lifetime of such a powerful FR II-like radio galaxy substantially. Thus, the
hotspot advance speed must have been higher in the past in agreement with a
scenario in which the originally highly relativistic jet has lost collimation
due to the growth of instabilities and has transformed into an only mildly
relativistic flow. Our data suggest that the density of the intergalactic
medium around this distant ($z=2.22$) AGN could be substantially higher than
the values typically found in less distant FR II radio galaxies.
