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Primordial non-Gaussianity of local type is predicted to lead to enhancedhalo clustering on very large scales. Photometric quasars, which can be seenfrom cosmological redshifts z>2 even in wide-shallow optical surveys, arepromising tracers for constraining non-Gaussianity using this effect. However,large-scale systematics can also mimic this signature of non-Gaussianity. Inorder to assess the contribution of systematic effects, we cross-correlateoverdensity maps of photometric quasars from the Sloan Digital Sky Survey(SDSS) Data Release 6 (DR6) in different redshift ranges. We find that the mapsare significantly correlated on large scales, even though we expect the angulardistributions of quasars at different redshifts to be uncorrelated. Thisimplies that the quasar maps are contaminated with systematic errors. Weinvestigate the use of external templates that provide information on thespatial dependence of potential systematic errors to reduce the level ofspurious clustering in the quasar data. We find that templates associated withstellar density, the stellar color locus, airmass, and seeing are majorcontaminants of the quasar maps, with seeing having the largest effect. Usingtemplate projection, we are able to decrease the significance of thecross-correlation measurement on the largest scales from 9.2-sigma to5.4-sigma. Although this is an improvement, the remaining cross-correlationsuggests the contamination in this quasar sample is too great to allow acompetitive constraint on fNL by correlations internal to this sample. The SDSSquasar catalog exhibits spurious number density fluctuations of ~2% rms, and weneed a contamination level less than 1% (0.6%) in order to measure values offNL less than 100 (10). Properly dealing with these systematics will beparamount for future large scale structure surveys that seek to constrainnon-Gaussianity.

Systematic effects in large-scale angular power spectra of photometric quasars and implications for constraining primordial nongaussianity