Galaxy-galaxy lensing with Rubin Observatory

Abstract

This thesis project focuses on the study of galaxy-galaxy lensing and aims to address some of the effects contributing to the ‘lensing is low ’ problem which consists in the over-prediction of the lensing signal from mock catalogue estimates using standard models when compared to observations. The goal is thus to model such features and their effects on galaxy-galaxy lensing observables in a way that is compatible with current methods and pipelines and to assess their effect on the signal-to-noise ratio. This work is part of a bigger effort within the Legacy Survey of Space and Time (LSST) community to mitigate systematics and improve the modelling of weak lensing signal in order to make full use of the survey’s early data. The project has been declared to the Dark Energy Science Collaboration (DESC) and now consti- tutes project n°438: ‘Forecasts of galaxy-galaxy lensing for early science’. We explore the effects of baryons which smooth the dark matter distribution by redistributing matter from small to large scales i.e. from the center of halos to their outskirts, through events such as active galactic nuclei feedback and stellar winds. Additionally, we look at the magnification of lens galaxies by the large scale structure between the observed population and us which affects the signal by either increasing or decreasing the number counts of lensing objects as well as their apparent brightness. After computing the analytical covariance ma- trix for the lensing observable ∆Σ, called the excess surface density as well as a compatible data vector, we analyse the systematics contributions’ results on the signal-to-noise ratio, as expected from performing a galaxy-galaxy lensing analysis with lens sample from the Dark Energy Spectroscopic Survey DESI and year 1 and 10 of LSST for the source sample. We find that, within our modelling assumptions, baryons have a relatively low effect on the lensing signal-to-noise ratio, only inducing modest modifications and restricted to small scales. In contrast, lens magnification produces a larger effect, decreasing with distance but persisting to larger scales than baryons, the amplitude of which depends on the sample’s magnification bias value.