ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: implications for spectral line intensity mapping at millimeter wavelengths and CMB spectral distortions

TL;DR

This study estimates the mean sky brightness temperature from distant galaxy line emissions at millimeter wavelengths using ALMA data, informing spectral distortion measurements of the CMB and large-scale structure studies.

Contribution

It provides direct, model-independent measurements of mean line brightness from high-redshift galaxies, relevant for CMB spectral distortion and intensity mapping.

Findings

Mean brightness at 99 GHz: 0.94 ± 0.09 μK

Mean brightness at 242 GHz: 0.55 ± 0.033 μK

Results are lower limits, excluding undetected faint galaxies.

Abstract

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 specrtal 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 cannot be detected individually in the current blind survey.