
We study the impact of the cosmological parameters uncertainties on the measurements of primordial nonGaussianity through the largescale nonGaussian halo bias effect. While this is not expected to be an issue for the standard Lambda CDM model, it may not be the case for more general models that modify the largescale shape of the power spectrum. We consider the socalled local nonGaussianity model, parametrized by the f(NL) nonGaussianity parameter which is zero for a Gaussian case, and make forecasts on f(NL) from planned surveys, alone and combined with a Planck CMB prior. In particular, we consider EUCLID and LSSTlike surveys and forecast the correlations among f(NL) and the running of the spectral index alpha(s), the dark energy equation of state w, the effective sound speed of dark energy perturbations c(s)(2), the total mass of massive neutrinos Mnu = Sigma m(nu), and the number of extra relativistic degrees of freedom Nnu(rel). Neglecting CMB information on f(NL) and scales k > 0.03h/Mpc, we find that, if Nnu(rel) is assumed to be known, the uncertainty on cosmological parameters increases the error on f(NL) by 10 to 30% depending on the survey. Thus the f(NL) constraint is remarkable robust to cosmological model uncertainties. On the other hand, if Nnu(rel) is simultaneously constrained from the data, the f(NL) error increases by similar to 80%. Finally, future surveys which provide a large sample of galaxies or galaxy clusters over a volume comparable to the Hubble volume can measure primordial nonGaussianity of the local form with a marginalized 1sigma error of the order Delta f(NL) similar to 2  5, after combination with CMB priors for the remaining cosmological parameters. These results are competitive with CMB bispectrum constraints achievable with an ideal CMB experiment.
