The clustering of galaxies in the SDSS-III DR9 Baryon Oscillation Spectroscopic Survey: constraints on primordial non-Gaussianity

We analyse the density field of 264 283 galaxies observed by the Sloan Digital Sky Survey (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS) and included in the SDSS Data Release 9 (DR9). In total, the SDSS DR9 BOSS data include spectroscopic redshifts for over 400 000 galaxies spread over a footprint of more than 3000 deg(2). We measure the power spectrum of these galaxies with redshifts 0.43 < z < 0.7 in order to constrain the amount of local non-Gaussianity, f(NL)(local), in the primordial density field, paying particular attention to the impact of systematic uncertainties. The BOSS galaxy density field is systematically affected by the local stellar density and this influences the ability to accurately measure f(NL)(local). In the absence of any correction, we find (erroneously) that the probability that f(NL)(local) is greater than zero, P(f(NL)(local) > 0), is 99.5 per cent. After quantifying and correcting for the systematic bias and including the added uncertainty, we find -45 < f(NL)(local) < 195 at 95 per cent confidence and P(f(NL)(local) > 0) = 91.0 per cent. A more conservative approach assumes that we have only learnt the k dependence of the systematic bias and allows any amplitude for the systematic correction; we find that the systematic effect is not fully degenerate with that of f(NL)(local), and we determine that - 82 < f(NL)(local) < 178 (at 95 per cent confidence) and P(f(NL)(local) > 0) = 68 per cent. This analysis demonstrates the importance of accounting for the impact of Galactic foregrounds on f(NL)(local) measurements. We outline the methods that account for these systematic biases and uncertainties. We expect our methods to yield robust constraints on f(NL)(local) for both our own and future large-scale structure investigations.