  We present kpc-scale ALMA and HST imaging of the quasar PJ308-21 at
$z$=$6.2342$, tracing dust, gas (via the [CII] 158 $\mu$m line) and young
stars. At a resolution of $\sim0.3''$ ($\approx1.7$ kpc), the system is
resolved over $>4''$ ($>$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 [CII] emission
of the entire system stretches over $>$1500 km/s along the line of sight. All
the components of the system are observed in dust, [CII], and rest-frame UV
emission. The inferred [CII] luminosities [(0.9-4.6)$\times 10^9$ L$_\odot$],
dust luminosities [(0.15-2.6)$\times10^{12}$ L$_\odot$], and rest-frame UV
luminosities [(6.6-15)$\times10^{10}$ 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.
