Photometric redshift requirements for lens galaxies in galaxy-galaxy lensing analysis

Reiko Nakajima
(UC Berkeley / LBNL)


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.