Constraints on Primordial non-Gaussianity from Future HI Intensity
  Mapping Experiments
  The primordial non-Gaussianity induces scale-dependent bias of the \hi with
respect to the underlying dark matter, which exhibits features on the very
large scales of the 21-cm power spectrum potentially observable with \hi
intensity mapping observations. We forecast the prospective constraints on the
four fundamental shapes of primordial non-Gaussianity (local, equilateral,
orthogonal, and enfolded), with the current and future \hi intensity mapping
experiments, BINGO, FAST, and SKA-I. With the current configuration of the
experiments and assumed one-year observation time, we find that the SKA-I will
provide tighter constraints on the local shape of primoridal non-Gaussianity
than Planck. The results are $(\sigma_{f^{\rm local}_{\rm NL}},\sigma_{f^{\rm
equil}_{\rm NL}},\sigma_{f^{\rm orth}_{\rm NL}},\sigma_{f^{\rm enfold}_{\rm
NL}})_{\rm SKA-I}=(0.54, 86, 25, 43)$, $(\sigma_{f^{\rm local}_{\rm
NL}},\sigma_{f^{\rm equil}_{\rm NL}},\sigma_{f^{\rm orth}_{\rm
NL}},\sigma_{f^{\rm enfold}_{\rm NL}})_{\rm BINGO}=(17, 100, 128, 164)$,
$(\sigma_{f^{\rm local}_{\rm NL}},\sigma_{f^{\rm equil}_{\rm
NL}},\sigma_{f^{\rm orth}_{\rm NL}},\sigma_{f^{\rm enfold}_{\rm NL}})_{\rm
FAST}=(9.5, 44, 75, 94)$. If the lower frequency band of FAST can be used, the
constraint on local-type primordial non-Gaussianity will be
$\sigma_{f_\mathrm{NL}}\sim1.62$ which is better than Planck. In addition, if
the observation time for FAST could be extended to two years, the constraint on
the equilateral shape of primordial non-Gaussianity would be improved to
$\sigma_{f_\mathrm{NL}}\sim32$. Similarly, if the observational time of SKA-I
could be extended to two years, the constraint on local and orthogonal shapes
could be improved to $0.43$ and $20$, respectively, achieving better
constraints than Planck.
