### Celine Gouin

#### PhD student on Cosmology and Astrophysics

I am a third year PhD student at the *Institut d’Astrophysique de Paris* (IAP). I am working on the prediction of the gravitational lensing signal through numerical simulations, under the supervisions of Christophe Pichon and Raphaël Gavazzi.

### Science interests

*Key words:*Observational Cosmology, Gravitational Lensing (Strong and Weak), Numerical methods, Statistics, Ray-tracing

*Projet 1:*Probing the angular shape of galaxy clusters through N-body simulation

Upcoming weak lensing surveys such as Euclid will provide an unprecedented opportunity to quantify the **geometry and topology of the cosmic web**, in particular in the **vicinity of lensing clusters**. Understanding the connectivity of the cosmic web with unbiased mass tracers, such as weak lensing, is of prime importance to probe the underlying cosmology, seek **dynamical signatures of dark matter**, and quantify **environmental effects on galaxy formation**.
In the first part of my phd, I am focused on a statistical way to characterised the **filamentary structure at the outskirts of galaxy clusters** by using weak lensing (Gouin et al. 2017). We choose to use the **multipolar moments of the density**, as defined by Schneider and Bartelmann (1997), which is a decomposition of the mass distribution in the harmonic space. Quadratic aperture moment of WL signal induced by cluster pairs, have already been studied to identify cosmic filaments (see Mead et al. 2010; Dietrich et al. 2005).

Mock catalogues of galaxy clusters are extracted from the N-body PLUS simulation. For each cluster, the aperture multipolar moments of the convergence are calculated in two annuli (inside and outside the virial radius). By stacking their modulus, a statistical estimator is built to characterise the angular mass distribution around clusters. The moments are compared to predictions from perturbation theory and spherical collapse.
First, we observe an excess of multipolar power, around galaxy clusters compared to the background density field, at all angular scales. This boost of spectral amplitude is understood as arising from **the contraction of the primordial cosmic web driven by the growing potential well of the cluster**. Besides this first signature, the quadrupole prevails in the cluster, as **ellipsoidal core**, while at the outskirts, harmonic distortions are spread on small angular modes, and trace the **non-linear sharpening of the filamentary structures**.

*Projet 2:*The prediction of strong and weak lensing signal via hydrodynamical simulation

With the advent of high performance computing, it has now become possible to address the problem of **the complex interplay of baryons and dark matter on all relevant scales**. In this context and for future cosmological experiments, such as the Euclid mission, the gravitational lensing signal (either weak or strong) needs to be calibrated and characterized with **state-of-the-art hydrodynamical cosmological simulations**. Two key issues, in which gas physics may play a crucial role for cosmology, are the so-called **missing satellite problem** and **halo profile problem** on the one hand, and the **intrinsic alignment of galaxies** and the **effect of galactic feedback** on the estimation of the **equation of state of the Dark Energy** on the other hand.
In this line, my second PhD project is to predict the gravitational lensing signal (either strong or weak) via hydrodynamical cosmological simulations. In particular, I am focused on propagate light-rays through the light-cone of the Horizon-AGN Simulation (see Dubois et al. 2014). Taking care of both dark and baryonic matter, I predict lensing signals by distinguish ing their individual impact, in turn, studying the physics of the bias.

#### Publications

**Gouin C.**, Gavazzi R., Codis S. et al., "Multipolar moments of weak lensing signal around clusters: Weighing filaments in harmonic space", 2017, A&A, 605, 27;- Codis S., Gavazzi R. and Pichon C., and
**Gouin C.**, "On the projected mass distribution around galaxy clusters : a Lagrangian theory of harmonic power spectra", 2017, A&A, 605, 80;

A complete up-to-date list of my publications can be found in the SAO/ADS abstract service.