Deep and Blind Far-Infrared Line Surveys

Far-infrared spectral lines such [C II] 157 um, [O I] 63 um, and [O III] 88 um probe gas conditions from which new stars form and are well understood in the local Universe from Herschel and SOFIA observations. To-date, ALMA has detected [C II] in over 100 hundred galaxies at z>4, but only 10 galaxies between z~0.5-3 with costly Band 9 programs requiring the best observing conditions. Far-infrared lines are an untapped tool for studying galaxy formation up to and during the peak of the cosmic star-formation rate density. Collectively, far-infrared lines constrain cold gas densities, radiation field strengths, and cooling rates in addition to being calibrated tracers of total dust-obscured star-formation rates and the gas reservoir. Moreover, [C II] and [O III] can be the brightest emission lines originating from star-forming galaxies, making them excellent sources of spectroscopic redshifts. A space-based, cryogenically cooled telescope covering ~50-300 um would fill a much-needed gap in capabilities between the limits of JWST and ALMA with significant scientific payoff. For example, deep and blind far-infrared line surveys would overcome bias imposed during target selection for ALMA follow-up, yield robust far-infrared line luminosity functions, and increase the number of far-infrared line-detected galaxies by two orders of magnitude between z~0.5-3, enabling detailed studies of the gas which gave rise to most of the stars in the Universe today. I estimate that over 1000 galaxies up to z~2 would be detected in 2-4 far-infrared emission lines by surveying 0.1deg2 to a depth of 1.5e-19 W/m2. These transformative capabilities are within the baseline specifications of the PRIMA probe mission concept under development for NASA probe mission call.