Catastrophic Photo-z Errors and the Dark Energy Parameter Estimates with Cosmic Shear
Abstract
We study the impact of catastrophic errors occurring in the photometric redshifts of galaxies on cosmological parameter estimates with cosmic shear tomography. We consider a fiducial survey with 9-filter set and perform photo-z measurement simulations. It is found that a fraction of 1% galaxies at zspec~0.4 is misidentified to be at zphot~3.5. We then employ both chi2 fitting method and the extension of Fisher matrix formalism to evaluate the bias on the equation of state parameters of dark energy, w0 and wa, induced by those catastrophic outliers. By comparing the results from both methods, we verify that the estimation of w0 and wa from the fiducial 5-bin tomographic analyses can be significantly biased. To minimize the impact of this bias, two strategies can be followed: (A) the cosmic shear analysis is restricted to 0.5<z<2.5 where catastrophic redshift errors are expected to be insignificant; (B) a spectroscopic survey is conducted for galaxies with 3<zphot<4. We find that the number of spectroscopic redshifts needed scales as Nspec fcata× A where fcata=1% is the fraction of catastrophic redshift errors (assuming a 9-filter photometric survey) and A is the survey area. For A=1000 deg2, we find that Nspec>320 and 860 respectively in order to reduce the joint bias in (w0,wa) to be smaller than 2σ and 1σ. This spectroscopic survey (option B) will improve the Figure of Merit of option A by a factor × 1.5 thus making such a survey strongly desirable.