Reiko Nakajima
(UC Berkeley / LBNL)
Abstract:
Weak gravitational lensing is a valuable probe of galaxy formation and
cosmology. However, quantifying its signal to mass requires redshift
information of lens and source. In this talk, I will discuss the use
of photometric redshifts (photo-z) in weak gravitational lensing, in
particular for applications with galaxy-galaxy lensing.
We have used the ZEBRA template-based method to estimate the redshifts
from SDSS DR8 photometry, for both lens and source catalogs. A
heterogeneous set of spectroscopic surveys (zCOSMOS, EGS, VVDS,
PRIMUS+DEEP2) were used to calibrate the photo-zs, which were modified
to accurately represent the SDSS photometric sample. We find the
photo-z errors to be large (sigma_{\Delta z / (1+z)} ~ 0.1, averaged
over all sample). The photo-z errors affect the estimates of lens
galaxy absolute luminosity and stellar mass, which determine the
galaxy-galaxy lensing stacking bin width. While the photo-z bias in
the lensing signal can be large, its uncertainty can be small---after
correcting for the bias, the uncertainty in the lensing signal can be
as small as 3 per cent, for the case when lens redshifts are known.