Measuring galaxy environments in large scale photometric surveys
  The properties of galaxies in the local universe have been shown to depend
upon their environment. Future large scale photometric surveys such as DES and
Euclid will be vital to gain insight into the evolution of galaxy properties
and the role of environment. Large samples come at the cost of redshift
precision and this affects the measurement of environment. We study this by
measuring environments using SDSS spectroscopic and photometric redshifts and
also simulated photometric redshifts with a range of uncertainties. We consider
the Nth nearest neighbour and fixed aperture methods and evaluate the impact of
the aperture parameters and the redshift uncertainty. We find that photometric
environments have a smaller dynamic range than spectroscopic measurements
because uncertain redshifts scatter galaxies from dense environments into less
dense environments. At the expected redshift uncertainty of DES, 0.1, there is
Spearman rank correlation coefficient of 0.4 between the measurements using the
optimal parameters. We examine the galaxy red fraction as a function of mass
and environment using photometric redshifts and find that the bivariate
dependence is still present in the SDSS photometric measurements. We show that
photometric samples with a redshift uncertainty of 0.1 must be approximately
6-16 times larger than spectroscopic samples to detect environment correlations
with equivalent fractional errors.
