# Evidence for CII diffuse line emission at redshift $z\sim2.6$

## TL;DR

This paper presents evidence for CII diffuse line emission at redshift around 2.6, refining previous measurements and detecting excess emission consistent with redshifted CII, supporting models of collisional excitation.

## Contribution

It improves mask weighting and covariance analysis in CII emission measurements, providing the first strong evidence for diffuse CII emission at high redshift.

## Key findings

- Detection of excess emission consistent with CII at z~2.6
- Refined measurement of CII line intensity and bias
- Supports collisional excitation models of CII emission

## Abstract

CII is one of the brightest emission lines from star-forming galaxies and is an excellent tracer for star formation. Recent work measured the CII 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 $I_{\mathrm{CII}}=6.6^{+5.0}_{-4.8}\times 10^4$ Jy/sr at $95\%$ confidence. 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 CII emission, in which case we report $b_{\mathrm{CII}}I_{\mathrm{CII}} =2.0^{+1.2}_{-1.1}\times 10^5$ Jy/sr at $95\%$ confidence, which strongly favors many collisional excitation models of CII emission. Our detection shows strong evidence for a model with a non-zero CII parameter, though line intensity mapping observations at high spectral resolution will be needed to confirm this result.