Spectral Confusion for Cosmological Surveys of Redshifted CII
  Observations
  Far infrared cooling lines are ubiquitous features in the spectra of star
forming galaxies. Surveys of redshifted fine-structure lines provide a
promising new tool to study structure formation and galactic evolution at
redshifts including the epoch of reionization as well as the peak of star
formation. Unlike neutral hydrogen surveys, where the 21 cm line is the only
bright line, surveys of red-shifted fine-structure lines suffer from confusion
generated by line broadening, spectral overlap of different lines, and the
crowding of sources with redshift. We use simulations to investigate the
resulting spectral confusion and derive observing parameters to minimize these
effects in pencil-beam surveys of red-shifted far-IR line emission. We generate
simulated spectra of the 17 brightest far-IR lines in galaxies, covering the
150 to 1300 micron wavelength region corresponding to redshifts 0 < z < 7, and
develop a simple iterative algorithm that successfully identifies the 158
micron [CII] line and other lines. Although the [CII] line is a principal
coolant for the interstellar medium, the assumption that the brightest observed
lines in a given line of sight are always [CII] lines is a poor approximation
to the simulated spectra once other lines are included. Blind line
identification requires detection of fainter companion lines from the same host
galaxies, driving survey sensitivity requirements. The observations require
moderate spectral resolution 700 < R < 4000 with angular resolution between 20
arcsec and 10 armin, sufficiently narrow to minimize confusion yet sufficiently
large to include a statistically meaningful number of sources.
