The effect of finite halo size on the clustering of neutral hydrogen.
Abstract
Post-reionisation 21cm intensity mapping experiments target the spectral line of
neutral hydrogen (HI) resident in dark matter haloes. According to the halo model, these
discrete haloes trace the continuous dark matter density field down to a certain scale, which is
dependent on the halo physical size. The halo physical size defines an exclusion region which
leaves imprints on the statistical properties of HI. We show how the effect of exclusion due to
the finite halo size impacts the HI power spectrum, with the physical boundary of the host
halo given by the splashback radius. Most importantly, we show that the white noise-like
feature that appears in the zero-momentum limit of the power spectrum is exactly cancelled
when the finite halo size is taken into consideration. This cancellation in fact applies to
all tracers of dark matter density field, including galaxies. Furthermore, we show that the
exclusion due to finite halo size leads to a sub-Poissonian noise signature on large scales,
consistent with the results from N-body simulations.