Dark Galaxy Candidates at Redshift similar to 3.5 Detected with MUSE

Abstract

Recent theoretical models suggest that the early phase of galaxy formation could involve an epoch when galaxies
\nare gas rich but inefficient at forming stars: a “dark galaxy” phase. Here, we report the results of our Multi-Unit
\nSpectroscopic Explorer (MUSE) survey for dark galaxies fluorescently illuminated by quasars at z > 3. Compared
\nto previous studies which are based on deep narrowband (NB) imaging, our integral field survey provides a nearly
\nuniform sensitivity coverage over a large volume in redshift space around the quasars as well as full spectral
\ninformation at each location. Thanks to these unique features, we are able to build control samples at large redshift
\ndistances from the quasars using the same data taken under the same conditions. By comparing the rest-frame
\nequivalent width (EW0) distributions of the Lyα sources detected in proximity to the quasars and in control
\nsamples, we detect a clear correlation between the locations of high-EW0 objects and the quasars. This correlation
\nis not seen in other properties, such as Lyα luminosities or volume overdensities, suggesting the possible
\nfluorescent nature of at least some of these objects. Among these, we find six sources without continuum
\ncounterparts and EW0 limits larger than 240 Å that are the best candidates for dark galaxies in our survey at
\nz > 3.5. The volume densities and properties, including inferred gas masses and star formation efficiencies, of
\nthese dark galaxy candidates are similar to those of previously detected candidates at z ≈ 2.4 in NB surveys.
\nMoreover, if the most distant of these are fluorescently illuminated by the quasar, our results also provide a lower
\nlimit of t = 60 Myr on the quasar lifetime.