  We present rest-frame ultraviolet (UV) images of six luminous quasars at $z
\sim 4.8$ obtained with the Hubble Space Telescope (HST). These quasars exhibit
a wide range of star formation rates (SFRs) and lie in a wide range of
environments. We carefully model and subtract the point-like quasar emission
and investigate the morphology of the underlying host galaxies at kpc scales.
The residual images allowed identification of potential companion sources,
which enabled us to explore the role of galaxy merger scenarios in the
co-evolution of the quasars and their hosts. We also search for the mechanism
driving extreme SFRs in three of the quasars. We find that the rate of
detection of potential companions to the host galaxies does not follow trends
between high- and low-SFR sources; i.e., the HST imaging suggests that both
high- and low-SFR sources are found in both dense and sparse galactic
environments. The suggested role of major mergers driving extreme SFRs cannot
be supported by the multiwavelength data in hand. Three of four companion
sources, previously revealed by sub-millimeter observations, are not detected
in the HST images of three of our quasars. An adapted high-resolution imaging
strategy focused on high-SFR sources and extended to a larger quasar sample is
required to determine the role of mergers in the processes of star formation
and supermassive black hole growth at high redshift.
