Into the Ly{\alpha} jungle: exploring the circumgalactic medium of
  galaxies at z ~ 4-5 with MUSE
  We present a study of the galaxy environment of 9 strong HI+CIV absorption
line systems ($16.2<{\rm log}(N({\rm HI}))<21.2$) spanning a wide range in
metallicity at $z\sim4-5$, using MUSE integral field and X-Shooter
spectroscopic data collected in a $z\approx 5.26$ quasar field. We identify
galaxies within a 250 kpc and $\pm1000$ km s$^{-1}$ window for 6 out of the 9
absorption systems, with 2 of the absorption line systems showing multiple
associated galaxies within the MUSE field of view. The space density of
Ly$\alpha$ emitting galaxies (LAEs) around the HI and CIV systems is
$\approx10-20$ times the average sky density of LAEs given the flux limit of
our survey, showing a clear correlation between the absorption and galaxy
populations. Further, we find that the strongest CIV systems in our sample are
those that are most closely aligned with galaxies in velocity space, i.e.
within velocities of $\pm500$ km s$^{-1}$. The two most metal poor systems lie
in the most dense galaxy environments, implying we are potentially tracing gas
that is infalling for the first time into star-forming groups at high redshift.
Finally, we detect an extended Ly$\alpha$ nebula around the $z\approx 5.26$
quasar, which extends up to $\approx50$ kpc at the surface brightness limit of
$3.8 \times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. After scaling for
surface brightness dimming, we find that this nebula is centrally brighter,
having a steeper radial profile than the average for nebulae studied at
$z\sim3$ and is consistent with the mild redshift evolution seen from $z\approx
2$.
