Postinflationary contamination of local primordial non-Gaussianity in galaxy power spectra

The scale-dependent galaxy bias is an important signal to be extracted in constraining local primordial non-Gaussianity (flocal NL ) from observations of large-scale structure. Constraints so obtained rely on the assumption that horizon-scale features in the galaxy power spectrum are exclusively due to primordial physical mechanisms. Yet, postinflationary effects can induce modulations to the galaxy number density that appear as horizon-scale, scale-dependent galaxy bias. We investigate the effect of two such sources of scale-dependent galaxy bias-the free-streaming of light relics and fluctuations in the background of ionizing radiation-on precision measurements of local primordial non-Gaussianity flocal NL from galaxy power spectrum measurements. Using the SPHEREx survey as a test case survey reaching sigma oflocal NL thorn less than or similar to 1, we show that ignoring the scale-dependent galaxy bias induced by free-streaming particles can negatively bias the inferred value of flocal NL by similar to 0.1-0.3 sigma. Ignoring the effect of ionizing radiation fluctuations can negatively bias the inferred value of flocal NL by similar to 1 sigma. The extent of inaccuracies in parameter-inference so incurred depends on the source populations and the ranges of scales used in the analysis, as well as the value of the neutrino mass and the modeling of the impact of ionizing radiation. If these sources of scale-dependent galaxy bias are included in the analysis, forecasts for flocal NL are unbiased but degraded.