Redshift requirements for cosmic shear with intrinsic alignment

Silvan Fischbacher, Tomasz Kacprzak, Jonathan Blazek, Alexandre Refregier

Submitted on 4 July 2022


Intrinsic alignment (IA) modelling and photometric redshift estimation are two of the main sources of systematic uncertainty in weak lensing surveys. We investigate the impact of redshift errors and their interplay with different IA models. Generally, errors on the mean δz and on the width σz of the redshift bins can both lead to biases in cosmological constraints. We find that such biases can, however, only be partially resolved by marginalizing over δz and σz. For Stage-III surveys, δz and σz cannot be well constrained due to limited statistics. The resulting biases are thus sensitive to prior volume effects. For Stage-IV surveys, we observe that marginalizing over the redshift parameters has an impact and reduces the bias. We derive requirements on the uncertainty of σz and δz for both Stage-III and Stage-IV surveys. We assume that the redshift systematic errors on S8 should be less than half of the statistical errors, and the median bias should be smaller than 0.25σ. We find that the uncertainty on δz has to be 0.025 for the NLA IA model with a Stage-III survey. For σz, the requirement is met even for large uncertainties 0.3. For the TATT IA model, the uncertainty on δz has to be 0.02 and the uncertainty on σz has to be 0.2. For Stage-IV surveys, the uncertainty on δz has to be 0.005 and the uncertainty on σz should be 0.1, with no significant dependence on the IA model. This required high precision will be a challenge for the redshift calibration of these future surveys. Finally, we investigate whether the interplay between redshift systematics and IA modelling can explain the S8-tension between cosmic shear results and CMB measurements. We find that this is unlikely to explain the current S8-tension.


Comment: 36 pages including 15 figures and 9 tables

Subject: Astrophysics - Cosmology and Nongalactic Astrophysics