Systematic effects in large-scale angular power spectra of photometric
  quasars and implications for constraining primordial nongaussianity
  Primordial non-Gaussianity of local type is predicted to lead to enhanced
halo clustering on very large scales. Photometric quasars, which can be seen
from cosmological redshifts z>2 even in wide-shallow optical surveys, are
promising tracers for constraining non-Gaussianity using this effect. However,
large-scale systematics can also mimic this signature of non-Gaussianity. In
order to assess the contribution of systematic effects, we cross-correlate
overdensity maps of photometric quasars from the Sloan Digital Sky Survey
(SDSS) Data Release 6 (DR6) in different redshift ranges. We find that the maps
are significantly correlated on large scales, even though we expect the angular
distributions of quasars at different redshifts to be uncorrelated. This
implies that the quasar maps are contaminated with systematic errors. We
investigate the use of external templates that provide information on the
spatial dependence of potential systematic errors to reduce the level of
spurious clustering in the quasar data. We find that templates associated with
stellar density, the stellar color locus, airmass, and seeing are major
contaminants of the quasar maps, with seeing having the largest effect. Using
template projection, we are able to decrease the significance of the
cross-correlation measurement on the largest scales from 9.2-sigma to
5.4-sigma. Although this is an improvement, the remaining cross-correlation
suggests the contamination in this quasar sample is too great to allow a
competitive constraint on fNL by correlations internal to this sample. The SDSS
quasar catalog exhibits spurious number density fluctuations of ~2% rms, and we
need a contamination level less than 1% (0.6%) in order to measure values of
fNL less than 100 (10). Properly dealing with these systematics will be
paramount for future large scale structure surveys that seek to constrain
non-Gaussianity.
