  The physical conditions of the circumgalactic medium are probed by
intervening absorption-line systems in the spectrum of background quasi-stellar
objects out to the epoch of cosmic reionization. A correlation between the
ionization state of the absorbing gas and the nature of the nearby galaxies has
been suggested by the sources detected either in Lyalpha or [C ii] 158 m near
to respectively highly-ionized and neutral absorbers. This is also likely
linked to the global changes in the incidence of absorption systems of
different types and the process of cosmic reionization. Here we report the
detection of two [C ii]-emitting galaxies at redshift $z \sim 5.7$ that are
associated with a complex high-ionization C iv absorption system. These objects
are part of an overdensity of galaxies and have compact sizes (< 2.4 kpc) and
narrow line widths (FWHM $\sim$ 62--64 km s-1). Hydrodynamic simulations
predict that similar narrow [C ii] emission may arise from the heating of small
($\lesssim$ 3 kpc) clumps of cold neutral medium or a compact photodissociation
region. The lack of counterparts in the rest-frame ultraviolet indicates severe
obscuration of the sources that are exciting the [C ii] emission. These results
may suggest a connection between the properties of the [C ii] emission, the
rare overdensity of galaxies and the unusual high ionization state of the gas
in this region.
