Ubiquitous giant Ly $\alpha$ nebulae around the brightest quasars at
  $z\sim3.5$ revealed with MUSE
  Direct Ly $\alpha$ imaging of intergalactic gas at $z\sim2$ has recently
revealed giant cosmological structures around quasars, e.g. the Slug Nebula
(Cantalupo et al. 2014). Despite their high luminosity, the detection rate of
such systems in narrow-band and spectroscopic surveys is less than 10%,
possibly encoding crucial information on the distribution of gas around quasars
and the quasar emission properties. In this study, we use the MUSE
integral-field instrument to perform a blind survey for giant Ly $\alpha$
nebulae around 17 bright radio-quiet quasars at $3<z<4$ that does not suffer
from most of the limitations of previous surveys. After data reduction and
analysis performed with specifically developed tools, we found that each quasar
is surrounded by giant Ly $\alpha$ nebulae with projected sizes larger than 100
physical kpc and, in some cases, extending up to 320 kpc. The circularly
averaged surface brightness profiles of the nebulae appear very similar to each
other despite their different morphologies and are consistent with power laws
with slopes $\approx-1.8$. The similarity between the properties of all these
nebulae and the Slug Nebula suggests a similar origin for all systems and that
a large fraction of gas around bright quasars could be in a relatively "cold"
(T$\sim$10$^4$K) and dense phase. In addition, our results imply that such gas
is ubiquitous within at least 50 kpc from bright quasars at $3<z<4$
independently of the quasar emission opening angle, or extending up to 200 kpc
for quasar isotropic emission.
