Search for C ii emission on cosmological scales at redshift Z ∼ 2.6

We present a search for C II emission over cosmological scales at high redshifts. The CII line is a prime candidate to be a tracer of star formation over large-scale structure since it is one of the brightest emission lines from galaxies. Redshifted C II emission appears in the submillimeter regime, which means it could potentially be present in the higher frequency intensity data from the Planck satellite is used to measure the cosmic infrared background (CIB). We search for CII emission over redshifts z = 2−3. 2 in the Planck 545 GHz intensity map by cross-correlating the three highest frequency Planck maps with spectroscopic quasars and CMASS galaxies from the Sloan Digital Sky Survey III, which we then use to jointly fit CII intensity, CIB parameters, and thermal Sunyaev–Zeldovich (SZ) emission. We report a measurement of an anomalous emission I_ν=6. 6^ (+5. 0) _ (−4. 8) ×10⁴ Jy sr^ (−1) at 95 per cent confidence, which could be explained by C II emission, favouring collisional excitation models of C II emission that tend to be more optimistic than models based on C II luminosity scaling relations from local measurements; however, a comparison of Bayesian information criteria reveals that this model and the CIB & SZ model are equally plausible. Thus, more sensitive measurements will be needed to confirm the existence of a large-scale CII emission at high redshifts. Finally, we forecast that intensity maps from Planck cross-correlated with quasars from the Dark Energy Spectroscopic Instrument would increase our sensitivity to CII emission by a factor of 5, while the proposed Primordial Inflation Explorer could increase the sensitivity further.