A Foreground Masking Strategy for [CII] Intensity Mapping Experiments
  Using Galaxies Selected by Stellar Mass and Redshift
  Intensity mapping provides a unique means to probe the epoch of reionization
(EoR), when the neutral intergalactic medium was ionized by the energetic
photons emitted from the first galaxies. The [CII] 158$\mu$m fine-structure
line is typically one of the brightest emission lines of star-forming galaxies
and thus a promising tracer of the global EoR star-formation activity. However,
[CII] intensity maps at $6 \lesssim z \lesssim 8$ are contaminated by
interloping CO rotational line emission ($3 \leq J_{\rm upp} \leq 6$) from
lower-redshift galaxies. Here we present a strategy to remove the foreground
contamination in upcoming [CII] intensity mapping experiments, guided by a
model of CO emission from foreground galaxies. The model is based on empirical
measurements of the mean and scatter of the total infrared luminosities of
galaxies at $z < 3$ and with stellar masses $M_{*} > 10^{8}\,\rm M_{\rm \odot}$
selected in $K$-band from the COSMOS/UltraVISTA survey, which can be converted
to CO line strengths. For a mock field of the Tomographic Ionized-carbon
Mapping Experiment (TIME), we find that masking out the "voxels"
(spectral-spatial elements) containing foreground galaxies identified using an
optimized CO flux threshold results in a $z$-dependent criterion $m^{\rm
AB}_{\rm K} \lesssim 22$ (or $M_{*} \gtrsim 10^{9} \,\rm M_{\rm \odot}$) at $z
< 1$ and makes a [CII]/CO$_{\rm tot}$ power ratio of $\gtrsim 10$ at $k=0.1$
$h$/Mpc achievable, at the cost of a moderate $\lesssim 8\%$ loss of total
survey volume.
