On Removing Interloper Contamination from Intensity Mapping Power
  Spectrum Measurements
  Line intensity mapping experiments seek to trace large scale structure by
measuring the spatial fluctuations in the combined emission, in some convenient
spectral line, from individually unresolved galaxies. An important systematic
concern for these surveys is line confusion from foreground or background
galaxies emitting in other lines that happen to lie at the same observed
frequency as the "target" emission line of interest. We develop an approach to
separate this "interloper" emission at the power spectrum level. If one adopts
the redshift of the target emission line in mapping from observed frequency and
angle on the sky to co-moving units, the interloper emission is mapped to the
wrong co-moving coordinates. Since the mapping is different in the line of
sight and transverse directions, the interloper contribution to the power
spectrum becomes anisotropic, especially if the interloper and target emission
are at widely separated redshifts. This distortion is analogous to the
Alcock-Paczynski test, but here the warping arises from assuming the wrong
redshift rather than an incorrect cosmological model. We apply this to the case
of a hypothetical [CII] emission survey at z~7 and find that the distinctive
interloper anisotropy can, in principle, be used to separate strong foreground
CO emission fluctuations. In our models, however, a significantly more
sensitive instrument than currently planned is required, although there are
large uncertainties in forecasting the high redshift [CII] emission signal.
With upcoming surveys, it may nevertheless be useful to apply this approach
after first masking pixels suspected of containing strong interloper
contamination.
