  Current and future generations of intensity mapping surveys promise dramatic
improvements in our understanding of galaxy evolution and large-scale
structure. An intensity map provides a census of the cumulative emission from
all galaxies in a given region and redshift, including faint objects that are
undetectable individually. Furthermore, cross-correlations between line
intensity maps and galaxy redshift surveys are sensitive to the line intensity
and clustering bias without the limitation of cosmic variance. Using the Fisher
information matrix, we derive simple expressions describing sensitivities to
the intensity and bias obtainable for cross-correlation surveys, focusing on
cosmic variance evasion. Based on these expressions, we conclude that the
optimal sensitivity is obtained by matching the survey depth, defined by the
ratio of the clustering power spectrum to noise in a given mode, between the
two surveys. We find that mid- to far-infrared space telescopes could benefit
from this technique by cross-correlating with coming galaxy redshift surveys
such as those planned for the Nancy Grace Roman Space Telescope, allowing for
sensitivities beyond the cosmic variance limit. Our techniques can therefore be
applied to survey design and requirements development to maximize the
sensitivities of future intensity mapping experiments to tracers of galaxy
evolution and large-scale structure cosmology.
