Dark Galaxy Candidates at Redshift ~3.5 Detected with MUSE
  Recent theoretical models suggest that the early phase of galaxy formation
could involve an epoch when galaxies are gas-rich but inefficient at forming
stars: a "dark galaxy" phase. Here, we report the results of our MUSE (Multi
Unit Spectroscopic Explorer) survey for dark galaxies fluorescently illuminated
by quasars at $z>3$. Compared to previous studies which are based on deep
narrow-band (NB) imaging, our integral field survey provides a nearly uniform
sensitivity coverage over a large volume in redshift space around the quasars
as well as full spectral information at each location. Thanks to these unique
features, we are able to build control samples at large redshift distances from
the quasars using the same data taken under the same conditions. By comparing
the rest-frame equivalent width (EW$_{0}$) distributions of the Ly$\alpha$
sources detected in proximity to the quasars and in control samples, we detect
a clear correlation between the locations of high EW$_{0}$ objects and the
quasars. This correlation is not seen in other properties such as Ly$\alpha$
luminosities or volume overdensities, suggesting the possible fluorescent
nature of at least some of these objects. Among these, we find 6 sources
without continuum counterparts and EW$_{0}$ limits larger than
$240\,\mathrm{\AA}$ that are the best candidates for dark galaxies in our
survey at $z>3.5$. The volume densities and properties, including inferred gas
masses and star formation efficiencies, of these dark galaxy candidates are
similar to previously detected candidates at $z\approx2.4$ in NB surveys.
Moreover, if the most distant of these are fluorescently illuminated by the
quasar, our results also provide a lower limit of $t=60$ Myr on the quasar
lifetime.
