The large and small scale properties of the intergalactic gas in the
  Slug Ly-alpha nebula revealed by MUSE HeII emission observations
  With a projected size of about 450 kpc at z~2.3, the Slug Ly-alpha nebula is
a rare laboratory to study, in emission, the properties of the intergalactic
gas in the Cosmic Web. Since its discovery, the Slug has been the subject of
several spectroscopic follow-ups to constrain the properties of the emitting
gas. Here we report the results of a deep MUSE integral-field spectroscopic
search for non-resonant, extended HeII1640 and metal emission. Extended HeII
radiation is detected on scales of about 100 kpc, but only in some regions
associated with the bright Ly-alpha emission and a continuum-detected source,
implying large and abrupt variations in the line ratios across adjacent regions
in projected space. The recent detection of associated H-alpha emission and
similar abrupt variations in the Ly-alpha kinematics, strongly suggest that the
HeII/Ly-alpha gradient is due to large variations in the physical distances
between the associated quasar and these regions. This implies that the overall
length of the emitting structure could extend to physical Mpc scales and be
mostly oriented along our line of sight. At the same time, the relatively low
HeII/Ly-alpha values suggest that the emitting gas has a broad density
distribution that - if expressed in terms of a lognormal - implies dispersions
as high as those expected in the interstellar medium of galaxies. These results
strengthen the possibility that the density distribution of intergalactic gas
at high-redshift is extremely clumpy and multiphase on scales below our current
observational spatial resolution of a few physical kpc.
