text: 291,187,1292715732061626368,786855300322172928,Alkistis Pourtsidou,"Paper alert (with a delay due to holiday season!) -- in <LINK> led by @CunningtonSD we studied the degeneracy between primordial non-gaussianity (PNG) and foreground removal systematics for intensity mapping experiments [thread]. Foreground removal methods remove some signal (unless one is very conservative, but then the error budget becomes much larger due to the residuals), especially on the large scales, where PNG signatures are expected to lie! With simulated data and MCMC we studied the effects of this on the precision accuracy with which we can probe the f_NL local parameter with a large SKA1-MID @SKA_telescope survey, using FG removal methods that are currently used in real data analyses. If we ignore the possibility of FG removal effects on the signal, we get *extremely biased* (wrong) estimates of f_NL, as shown in the purple contour where our fiducial f_NL = 0. The other contours correspond to the unrealistic cases where FG removal is 0 or perfectly known. <LINK> To add some realism, we devised a model with 1 free, nuisance parameter, which is marginalised over to account for FG removal properly. We found that it works well, and that we can recover unbiased estimates (but the f_NL uncertainties increase a lot!). <LINK> This result has implications also for cross-correlations and multi-tracer methods, and it means that further work is required to get the precision we need to be competitive. We are thinking about solutions -- more, hopefully, soon! As an aside, note that we have made suites of simulated data and power spectrum and MCMC codes available at our group's repository <LINK> -- @CunningtonSD @psahds also see <LINK>",https://arxiv.org/abs/2007.12126,"Potential evidence for primordial non-Gaussianity (PNG) is expected to lie in the largest scales mapped by cosmological surveys. Forthcoming 21cm intensity mapping experiments will aim to probe these scales by surveying neutral hydrogen (HI) within galaxies. However, foreground signals dominate the faint 21cm emission, meaning foreground cleaning is required to recover the cosmological signal. The effect this has is to damp the HI power spectrum on the largest scales, especially along the line-of-sight. Whilst there is agreement that this contamination is potentially problematic for probing PNG, it is yet to be fully explored and quantified. In this work we carry out the first forecasts on $f_\text{NL}$ that incorporate simulated foreground maps that are removed using techniques employed in real data. Using an MCMC analysis, we demonstrate that foreground cleaned data recovers hugely biased values ($f_\text{NL} = -102.1_{-7.96}^{+8.39}$ [68% CL]) on our $f_\text{NL}=0$ fiducial input. Introducing a model with fixed parameters for the foreground contamination allows us to recover unbiased results ($f_\text{NL} = -2.94_{-11.9}^{+11.4}$). However, it is not clear that we will have sufficient understanding of foreground contamination to allow for such rigid models. Treating the main parameter $k_\parallel^\text{FG}$ in our foreground model as a nuisance parameter and marginalizing over it, still recovers unbiased results but at the expense of much larger errors ($f_\text{NL} = 0.75^{+40.2}_{-44.5}$), that can only be reduced by imposing the Planck 2018 prior. Our results show that significant progress on understanding and controlling foreground removal effects is necessary in order to study PNG with HI intensity mapping. ","The degeneracy between primordial non-Gaussianity and foregrounds in