Far-IR/Submillimeter Spectroscopic Cosmological Surveys: Predictions of
  Infrared Line Luminosity Functions for z<4 Galaxies
  Star formation and accretion onto supermassive black holes in the nuclei of
galaxies are the two most energetic processes in the Universe, producing the
bulk of the observed emission throughout its history. We simulated the
luminosity functions of star-forming and active galaxies for spectral lines
that are thought to be good spectroscopic tracers of either phenomenon, as a
function of redshift. We focused on the infrared (IR) and sub-millimeter
domains, where the effects of dust obscuration are minimal. Using three
different and independent theoretical models for galaxy formation and
evolution, constrained by multi-wavelength luminosity functions, we computed
the number of star-forming and active galaxies per IR luminosity and redshift
bin. We converted the continuum luminosity counts into spectral line counts
using relationships that we calibrated on mid- and far-IR spectroscopic surveys
of galaxies in the local universe. Our results demonstrate that future
facilities optimized for survey-mode observations, i.e., the Space Infrared
Telescope for Cosmology and Astrophysics (SPICA) and the Cerro Chajnantor
Atacama Telescope (CCAT), will be able to observe thousands of z>1 galaxies in
key fine-structure lines, e.g., [SiII], [OI], [OIII], [CII], in a
half-square-degree survey, with one hour integration time per field of view.
Fainter lines such as [OIV], [NeV] and H_2 (0-0)S1 will be observed in several
tens of bright galaxies at 1<z<2, while diagnostic diagrams of active-nucleus
vs star-formation activity will be feasible even for normal z~1 galaxies. We
discuss the new parameter space that these future telescopes will cover and
that strongly motivate their construction.
