Context : The emission and proper motion of the terminal hotspots of AGN jets can be used as a powerful probe of the intergalactic medium .
However , measurements of hotspot-advance speeds in active galaxies are difficult , especially in the young universe , due to the low angular velocities and the low brightness of distant radio galaxies .

Aims : 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 .

Methods : We use the LOw Frequency ARray ( 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 .

Results : 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 .

Conclusions : 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 .