Origins Space Telescope: Predictions for far-IR spectroscopic surveys

We illustrate the extraordinary potential of the (far-IR) Origins Survey Spectrometer (OSS) on board the Origins Space Telescope (OST) to address a variety of open issues on the co-evolution of galaxies and AGNs. We present predictions for blind surveys, each of 1000 h, with different mapped areas (a shallow survey covering an area of 10 deg$^{2}$ and a deep survey of 1 deg$^{2}$) and two different concepts of the OST/OSS: with a 5.9 m telescope (Concept 2, our reference configuration) and with a 9.1 m telescope (Concept 1, previous configuration). In 1000 h, surveys with the reference concept will detect from $\sim 1.9 \times 10^{6}$ to $\sim 8.7 \times 10^{6}$ lines from $\sim 4.8 \times 10^{5}$-$2.7 \times 10^{6}$ star-forming galaxies and from $\sim 1.4 \times 10^{4}$ to $\sim 3.8 \times 10^{4}$ lines from $\sim 1.3 \times 10^{4}$-$3.5 \times 10^{4}$ AGNs. The shallow survey will detect substantially more sources than the deep one; the advantage of the latter in pushing detections to lower luminosities/higher redshifts turns out to be quite limited. The OST/OSS will reach, in the same observing time, line fluxes more than one order of magnitude fainter than the SPICA/SMI and will cover a much broader redshift range. In particular it will detect tens of thousands of galaxies at $z \geq 5$, beyond the reach of that instrument. The polycyclic aromatic hydrocarbons lines are potentially bright enough to allow the detection of hundreds of thousands of star-forming galaxies up to $z \sim 8.5$, i.e. all the way through the re-ionization epoch. The proposed surveys will allow us to explore the galaxy-AGN co-evolution up to $z\sim 5.5-6$ with very good statistics. OST Concept 1 does not offer significant advantages for the scientific goals presented here.