An upper limit to [C II] emission in a z ~= 5 galaxy

Low-ionization-state far-infrared (FIR) emission lines may be useful diagnostics of star-formation activity in young galaxies, and at high redshift may be detectable from the ground. In practice, however, very little is known concerning how strong such line emission might be in the early Universe. We attempted to detect the 158 micron [C II] line from a lensed galaxy at z = 4.926 using the Caltech Submillimeter Observatory. This source is an ordinary galaxy, in the sense that it shows high but not extreme star formation, but lensing makes it visible. Our analysis includes a careful consideration of the calibrations and weighting of the individual scans. We find only modest improvement over the simpler reduction methods, however, and the final spectrum remains dominated by systematic baseline ripple effects. We obtain a 95 per cent confidence upper limit of 33 mJy for a 200 km/s full width at half maximum line, corresponding to an unlensed luminosity of 1x10^9 L_sun for a standard cosmology. Combining this with a marginal detection of the continuum emission using the James Clerk Maxwell Telescope, we derive an upper limit of 0.4 per cent for the ratio of L_CII/L_FIR in this object.