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The scale-dependent bias of galaxy density contrasts is an important signalto be extracted in constraining local primordial non-Gaussianity ($f_{\rmNL}^{\text{local}}$) from observations of large-scale structure. Constraints soobtained rely on the assumption that horizon-scale features in the galaxy powerspectrum are exclusively due to primordial physical mechanisms. Yet,post-inflationary effects can induce modulations to the galaxy number densitythat appear as horizon-scale, scale-dependent bias. We investigate the effectof two such sources of scale-dependent bias - the free-streaming of lightrelics and fluctuations in the background of ionising radiation - on precisionmeasurements of local primordial non-Gaussianity $f_{\rm NL}^{\text{local}}$from galaxy power spectrum measurements. Using the SPHEREx survey as a testcase survey reaching $\sigma(f_{\rm NL}^{\rm local}) \lesssim 1$, we show thatignoring the scale-dependent bias induced by free-streaming particles cannegatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $\sim0.1-0.3\sigma$. Ignoring the effect of ionising radiation fluctuations cannegatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $ \sim1\sigma$. The range of biases depends on the source populations and the rangesof scales used in the analysis, as well as the value of the neutrino mass andthe modelling of the impact of ionising radiation. If these sources ofscale-dependent bias are included in the analysis, forecasts for $f_{\rmNL}^{\rm local}$ are unbiased but degraded.

Post-inflationary Contamination of Local Primordial Non-Gaussianity in Galaxy Power Spectra