Far-infrared lines hidden in archival deep multi-wavelength surveys: Limits on [CII]-158 μm at

Context. Singly ionized carbon [CII] is theorized to be the brightest emission line feature in star-forming galaxies, and hence an excellent tracer of the evolution of cosmic star formation. Archival maps from far-infrared and submillimeter surveys potentially contain the redshifted [CII]-158 μm, hidden in the much-brighter continuum emission. Aims. We present a search for aggregate [CII]-158 μm line emission across the predicted peak of star formation history by tomographically stacking a high-completeness galaxy catalog on broadband deep maps of the COSMOS field and constraining residual excess emission after subtracting the continuum spectral energy distribution (SED). Methods. The COSMOS equatorial 2deg2 patch has been mapped by Spitzer, Herschel, and SCUBA2/JCMT. Using the high precision UV-O-IR photometry catalog COSMOS2020, we performed unbiased simultaneous stacking of ∼360 000 photometric redshifts on these confusion-limited maps to resolve the sub-THz radiation background. By subtracting a continuum SED model with conservative uncertainty estimation and completeness correction through comparison to the COBE/FIRAS monopole spectrum, we obtain tomographic constraints on the sky-averaged [CII]-158 μm signal within the three SPIRE maps: 11.8 ± 10.2, 11.0 ± 8.7, 9.6 ± 9.8, and 9.2 ± 6.6 kJy/sr at redshifts z ∼ 0.65, ∼1.3, ∼2.1, and ∼2.6, respectively, corresponding to 1 − 1.4σ significance in each bin. Results. Our 3σ upper limits are in tension with past z ∼ 2.6 results from cross-correlating SDSS-BOSS quasars with high-frequency Planck maps, and indicate a much less dramatic evolution (∼×7.5) of the mean [CII] intensity across the peak of star formation history than collisional excitation models or frameworks calibrated to the tentative PlanckxBOSS measurement. We discuss this tension, particularly in the context of in-development surveys (TIM, EXCLAIM) that will map this [CII] at high redshift resolution. Conclusions. Having demonstrated stacking in broadband deep surveys as a complementary methodology to next-generation spectrometers for line intensity mapping, these novel methods can be extended to upcoming galaxy surveys such as Euclid, as well as to place upper limits on fainter atomic and molecular lines.