Evidence for C ii diffuse line emission at redshift z ∼ 2.6

C ii is one of the brightest emission lines from star-forming galaxies and is an excellent tracer for star formation. Recent work measured the C ii emission line amplitude for redshifts 2 < z < 3.2 by cross-correlating Planck High Frequency Instrument emission maps with tracers of overdensity from the Baryon Oscillation Spectroscopic Sky Survey, finding I_ C\, II=6.6+5.0-4.8× 10⁴ Jy sr−1 at 95\,per cent confidence level. In this paper, we present a refinement of this earlier work by improving the mask weighting in each of the Planck bands and the precision in the covariance matrix. We report a detection of excess emission in the 545 GHz Planck band separate from the cosmic infrared background (CIB) present in the 353–857 GHz Planck bands. This excess is consistent with redshifted C ii emission, in which case we report b_ C \, III_ C \, II =2.0+1.2-1.1× 10⁵ Jy sr−1 at 95\ per cent confidence level, which strongly favours many collisional excitation models of C ii emission. Our detection shows strong evidence for a model with a non-zero C ii parameter, though line intensity mapping observations at high spectral resolution will be needed to confirm this result.