We present direct estimates of the mean sky brightness temperature in observing bands around 99GHz and 242GHz due to line emission from distant galaxies .
These values are calculated from the summed line emission observed in a blind , deep survey for spectral line emission from high redshift galaxies using ALMA ( the ’ ASPECS ’ survey ) .
In the 99 GHz band , the mean brightness will be dominated by rotational transitions of CO from intermediate and high redshift galaxies .
In the 242GHz band , the emission could be a combination of higher order CO lines , and possibly [ CII ] 158 \mu m line emission from very high redshift galaxies ( z \sim 6 to 7 ) .
The mean line surface brightness is a quantity that is relevant to measurements of spectral distortions of the cosmic microwave background , and as a potential tool for studying large-scale structures in the early Universe using intensity mapping .
While the cosmic volume and the number of detections are admittedly small , this pilot survey provides a direct measure of the mean line surface brightness , independent of conversion factors , excitation , or other galaxy formation model assumptions .
The mean surface brightness in the 99GHZ band is : T _ { B } = 0.94 \pm 0.09 \mu K. In the 242GHz band , the mean brightness is : T _ { B } = 0.55 \pm 0.033 \mu K. These should be interpreted as lower limits on the average sky signal , since we only include lines detected individually in the blind survey , while in a low resolution intensity mapping experiment , there will also be the summed contribution from lower luminosity galaxies that can not be detected individually in the current blind survey .