A model of spectral line broadening in signal forecasts for
  line-intensity mapping experiments
  Line-intensity mapping observations will find fluctuations of integrated line
emission are attenuated by varying degrees at small scales due to the width of
the line emission profiles. This attenuation may significantly impact estimates
of astrophysical or cosmological quantities derived from measurements. We
consider a theoretical treatment of the effect of line broadening on both the
clustering and shot-noise components of the power spectrum of a generic
line-intensity power spectrum using a halo model. We then consider possible
simplifications to allow easier application in analysis, particularly in the
context of inferences that require numerous, repeated, fast computations of
model line-intensity signals across a large parameter space. For the CO Mapping
Array Project (COMAP) and the CO(1-0) line-intensity field at $z\sim3$ serving
as our primary case study, we expect a $\sim10\%$ attenuation of the
spherically averaged power spectrum on average at relevant scales of
$k\approx0.2$-$0.3$ Mpc$^{-1}$, compared to $\sim25\%$ for the interferometric
Millimetre-wave Intensity Mapping Experiment (mmIME) targeting shot noise from
CO lines at $z\sim1$-$5$ at scales of $k\gtrsim1$ Mpc$^{-1}$. We also consider
the nature and amplitude of errors introduced by simplified treatments of line
broadening, and find that while an approximation using a single effective
velocity scale is sufficient for spherically-averaged power spectra, a more
careful treatment is necessary when considering other statistics such as higher
multipoles of the anisotropic power spectrum or the voxel intensity
distribution.
