Compact [C II] emitters around a C IV absorption complex at redshift 5.7

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.