Probing higher-order primordial non-Gaussianity with galaxy surveys
  With a radio continuum galaxy survey by the Square Kilometre Array (SKA), a
photometric galaxy survey by Euclid and their combination, we forecast future
constraints on primordial non-Gaussianity. We focus on the potential impact of
local-type higher-order nonlinear parameters on the parameter estimation and
particularly the confirmation of the inflationary consistency inequality.
Nonstandard inflationary models, such as multifield models, introduce the
scale-dependent stochastic clustering of galaxies on large scales, which is a
unique probe of mechanism for generating primordial density fluctuations. Our
Fisher matrix analysis indicates that a deep and wide survey provided by SKA is
more advantageous to constrain $\tau_{\rm NL}$, while Euclid has a strong
constraining power for $f_{\rm NL}$ due to the redshift information, suggesting
that the joint analysis between them is quite essential to break the degeneracy
between the nonlinear parameters. The combination of the full SKA and Euclid
will achieve the precision level needed to confirm the consistency inequality
even for $f_{\rm NL}\approx 1.5$ and $\tau_{\rm NL}\approx 17$, though it is
still hard for a single survey to confirm it when $f_{\rm NL}\lesssim 2.7$.
