Measuring ultra-large scale effects in the presence of 21cm intensity
  mapping foregrounds
  \textsc{Hi} intensity mapping will provide maps of the large-scale
distribution of neutral hydrogen (\textsc{Hi}) in the universe. These are prime
candidates to be used to constrain primordial non-Gaussianity using the Large
Scale Structure of the Universe as well as to provide further tests of
Einstein's theory of Gravity (GR). But \textsc{Hi} maps are contaminated by
foregrounds, which can be several orders of magnitude above the cosmological
signal. Here we quantify how degenerated are the large-scale effects ($f_{\rm
NL}$ and GR effects) with the residual foregrounds. We conclude that a joint
analysis does not provide a catastrophic degradation of constraints and
provides a framework to determine the marginal errors of large scale-effects in
the presence of foregrounds. Similarly, we conclude that the macroscopical
properties of the foregrounds can be measured with high precision.
Notwithstanding, such results are highly dependent on accurate forward
modelling of the foregrounds, which incorrectly done catastrophically bias the
best fit values of cosmological parameters, foreground parameterisations, and
large-scale effects.
