Fig. 2: Landscape of millimetre line monopoles in brightness temperature.
From: Cosmic CO and [C II] backgrounds and the fuelling of star formation over 12 Gyr
The mean sky monopoles Tb versus observed frequency for the CO and [C II] backgrounds, measured in this work up to z = 4.2 and extrapolated to z = 10, are shown as curves colour-coded by redshift. For comparison, the black curve indicates the updated CIB dust-continuum monopole, fitted jointly with the lines in this work, with their sum closely matching the previous continuum-only fit3. The spectral distortion of the thermal SZ effect background is shown in red, with the dashed segment indicating the decrement (y ≈ 1.22 × 10−6)20. The amplitudes of all these components are now empirically constrained, with shaded bands indicating 68% uncertainties. These lines trace the three-dimensional LSS of the universe, while the continua trace it in projection; together they constitute the main extragalactic foreground of the CMB. Filled grey regions mark atmospheric transmission windows at a relatively dry site49, highlighting more accessible low-frequency bands from the ground. This figure establishes key benchmarks for the far-infrared to millimetre line backgrounds, replacing previously divergent model forecasts with direct measurements and defining amplitude baselines and sensitivity targets for future LIM experiments.
