We present kpc–scale ALMA and HST imaging of the quasar PJ308–21 at z = 6.2342 , tracing dust , gas ( via the [ C ii ] 158 \mu m line ) and young stars .
At a resolution of \sim 0.3 ^ { \prime \prime } ( \approx 1.7 kpc ) , the system is resolved over > 4 ^ { \prime \prime } ( > 20 kpc ) .
In particular , it features a main component , identified to be the quasar host galaxy , centered on the accreting supermassive black hole ; and two other extended components on the West and East side , one redshifted and the other blueshifted relative to the quasar .
The [ C ii ] emission of the entire system stretches over > 1500 km s ^ { -1 } along the line of sight .
All the components of the system are observed in dust , [ C ii ] , and rest–frame UV emission .
The inferred [ C ii ] luminosities [ ( 0.9–4.6 ) \times 10 ^ { 9 } L _ { \odot } ] , dust luminosities [ ( 0.15–2.6 ) \times 10 ^ { 12 } L _ { \odot } ] , and rest–frame UV luminosities [ ( 6.6–15 ) \times 10 ^ { 10 } L _ { \odot } ] , their ratios , and the implied gas/dust masses and star formation rates [ 11–290 M _ { \odot } yr ^ { -1 } ] are typical of high–redshift star–forming galaxies .
A toy model of a single satellite galaxy that is tidally stripped by the interaction with the quasar host galaxy can account for the observed velocity and spatial extent of the two extended components .
An outflow interpretation of the unique features in PJ308–21 is not supported by the data .
PJ308–21 is thus one of the earliest galaxy mergers imaged at cosmic dawn .