Tomographic Intensity Mapping Detections of Cosmic [CII] and CO Line Backgrounds

We provide empirical tomographic measurements of the cosmic [CII] and CO line backgrounds arising from the aggregate emission of galaxies and potentially diffuse gas in large-scale structure. Previously, only the most luminous galaxies were detected in these lines, revealing just the tip of the iceberg. Using the intensity mapping method, we measure the total backgrounds integrated over all sources and report the cosmic mean. These measurements are based on detections of line excesses above the cosmic infrared background (CIB) continuum—at 6σ significance for CO and 3σ for [CII]—using CIB–galaxy correlations over 0 < z < 4, originally measured in Chiang et al. (2025). We use these redshift-dependent, multi-band correlation amplitudes to isolate the line contributions, which are released here.

The [CII] background corresponds to the 158-micron fine-structure line, while the CO background includes nine rotational transitions from J=1–0 to J=9–8, starting at 115.27 GHz, with each successive line spaced by the same frequency interval.

The file contains the monopole brightness temperatures of the [CII] and CO lines in microkelvin (µK) as functions of redshift (0 < z < 12) and observed frequency (GHz). Best-fit values represent posterior medians, with lower and upper bounds denoting the 68% credible interval.

This data release serves as a baseline for future line intensity mapping experiments and forecasts, and as a reference for comparison with galaxy surveys in astrophysical studies.