Photometric redshift requirements for lens galaxies in galaxy-galaxy
  lensing analyses
  Weak gravitational lensing is a valuable probe of galaxy formation and
cosmology. Here we quantify the effects of using photometric redshifts
(photo-z) in galaxy-galaxy lensing, for both sources and lenses, both for the
immediate goal of using galaxies with photo-z as lenses in the Sloan Digital
Sky Survey (SDSS) and as a demonstration of methodology for large, upcoming
weak lensing surveys that will by necessity be dominated by lens samples with
photo-z. We calculate the bias in the lensing mass calibration as well as
consequences for absolute magnitude (i.e., k-corrections) and stellar mass
estimates, for a large sample of SDSS Data Release 8 (DR8) galaxies. The
redshifts are obtained with the template based photo-z code ZEBRA on the SDSS
DR8 ugriz photometry. We assemble and characterise the calibration samples (~9k
spectroscopic redshifts from four surveys) to obtain photometric redshift
errors and lensing biases corresponding to our full SDSS DR8 lens and source
catalogues. Our tests of the calibration sample also highlight the impact of
observing conditions in the imaging survey when the spectroscopic calibration
covers a small fraction of its footprint; atypical imaging conditions in
calibration fields can lead to incorrect conclusions regarding the photo-z of
the full survey. For the SDSS DR8 catalogue, we find sigma_z/(1+z)=0.096 and
0.113 for the lens and source catalogues, with flux limits of r=21 and r=21.8,
respectively. We also explore the systematic uncertainty in the lensing signal
calibration when using source photo-z, and both lens and source photo-z; given
the size of existing training samples, we can constrain the lensing signal
calibration (and therefore the normalization of the surface mass density) to
within 2 and 4 per cent, respectively. [ABRIDGED]
