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Intrinsic alignments are believed to be a major source of systematics for future generation of weak gravitational lensing surveys like Euclid or LSST. Direct measurements of the alignment of the projected light distribution of galaxies in wide field imaging data seem to agree on a contamination at a level of a few per cent of the shear correlation functions, although the amplitude of the effect depends on the population of galaxies considered. Given this dependency, it is difficult to use dark matter-only simulations as the sole resource to predict and control intrinsic alignments. We report here estimates on the level of intrinsic alignment in the cosmological hydrodynamical simulation Horizon-AGN that could be a major source of systematic errors in weak gravitational lensing measurements. In particular, assuming that the spin of galaxies is a good proxy for their ellipticity, we show how those spins are spatially correlated and how they couple to the tidal field in which they are embedded. We will also present theoretical calculations that illustrate and qualitatively explain the observed signals.

Analytical models of galaxy-halo connection such as the Halo Occupation Distribution (HOD) model have been widely used over the past decades as a means to intensively test perturbative models on quasi-linear scales. However, these models fail to reproduce the galaxy-galaxy lensing signal on non-linear scales, over-predicting the observed signal up to 40%. With ongoing Stage-IV galaxy surveys such as DESI and EUCLID, it is now crucial to accurately model the galaxy-halo connection up to intra-halo scales to accurately estimate theoretical uncertainties of perturbative models. This paper compares the standard HOD model to an extended HOD framework that incorporates as additional features galaxy assembly bias and local environmental dependencies on halo occupation. These models have been calibrated against the observed clustering and galaxy-galaxy lensing signal of eBOSS Luminous Red Galaxies (LRG) and Emission Lines Galaxies (ELG) in the range 0.6 < z < 1.1. A combined clustering-lensing cosmological analysis is then performed on the simulated galaxy samples of both standard and extended HOD frameworks to quantify the systematic budget of perturbative models. The extended HOD model offers a more comprehensive understanding of the connection between galaxies and their surroundings. In particular, we found that the LRGs preferentially occupy denser and more anisotropic environments. Our results highlight the importance of considering environmental factors in galaxy formation models, with an extended HOD framework that reproduces the observed signal within 20% on scales below 10 Mpc/h. Our cosmological analysis reveals that our perturbative model yields similar constraints regardless of the galaxy population, with a better goodness of fit for the extended HOD. These results suggest that the extended HOD framework should be used to quantify modeling systematics.

Intrinsic alignments of galaxy shapes are a major systematic for weak gravitational lensing and provide insight into how galaxies acquire orientations within the cosmic web. Most studies rely on large statistical samples; here, we probe this signal in a single nearby superstructure, extending to outer regions where secondary infall should dominate. We measure intrinsic alignments in the Perseus-Pisces supercluster as a function of galaxy morphology and radial extent into the low surface brightness regime. Using deep CFHT r-band imaging covering 367 deg2 (mainly from the UNIONS survey) and reaching 28 mag/arcsec2, we compute correlation functions at three isophotal radii for 2004 galaxies with log M*/Msun > 8.6, stratified by morphology and stellar mass in comoving coordinates. We detect positive intrinsic alignment signals for both early- and late-type galaxies out to 1 Mpc/h, including a clear signal for spirals. Shape-shape correlations are stronger than position-shape correlations, while comoving measurements increase the fraction of strongly correlated systems. Correlation profiles show little radial dependence across the three isophotes despite ellipticity variations in 10-20% of galaxies. We find strong morphology dependence: late-type galaxies dominate the shape-shape signal, preferentially inhabiting filaments and displaying higher ellipticities consistent with edge-on orientations. Early-type galaxies instead cluster near group and cluster centers, with no comparable excess in position-shape correlations. This segregation suggests distinct alignment mechanisms: tidal stretching for early-types in dense environments and tidal torquing for late-types in filaments. These local-Universe constraints inform intrinsic alignment modeling for Euclid, DESI, and LSST, highlighting the contribution of blue spiral galaxies to low-redshift cosmic shear contamination.

The discovery of the large-scale structure has transformed our view of galaxy formation and evolution. Filaments of the cosmic web provide key environments that channel the growth of structures. Guided by predictions from cosmological simulations, we study the morphological distribution of galaxies in the Perseus-Pisces Supercluster, a prominent filamentary complex at 70 Mpc. We focus on how galaxy morphology and structural disturbances relate to position within the filament network and to proximity to dense nodes. Our sample is built from a spectroscopic catalogue cross-matched with deep r-band CFHT/MegaCam imaging from UNIONS and additional time, enabling the detection of low-surface-brightness features and extended outer structures. Morphologies are determined both visually and through structural parameters extracted from surface-brightness profiles using AutoProf and AstroPhot. The 3D filamentary skeleton of Perseus-Pisces is reconstructed with the DisPerSE algorithm, providing distances from each galaxy to the nearest filament and to group or cluster centres. The 3D mapping reveals a network of interconnected sub-filaments converging around the Pisces cluster, forming a complex, multi-branched structure that likely shapes environmental effects on galaxy evolution. We observe clear morphological and stellar-mass segregation: massive early-type galaxies (E/S0) concentrate along filament spines and near dense nodes, while late-type and irregular systems are more broadly dispersed. About 10-13% of galaxies show strong signs of gravitational interaction, with stellar-halo asymmetries particularly common in filaments and groups. These findings underline the dual influence of filamentary environments, which both host evolved early-type systems and foster local tidal interactions and pre-processing that modify galaxy morphology.

The recently published analytic probability density function for the mildly non-linear cosmic density field within spherical cells is used to build a simple but accurate maximum likelihood estimate for the redshift evolution of the variance of the density, which, as expected, is shown to have smaller relative error than the sample variance. This estimator provides a competitive probe for the equation of state of dark energy, reaching a few percent accuracy on wp and wa for a Euclid-like survey. The corresponding likelihood function can take into account the configuration of the cells via their relative separations. A code to compute one-cell density probability density functions for arbitrary initial power spectrum, top-hat smoothing and various spherical collapse dynamics is made available online so as to provide straightforward means of testing the effect of alternative dark energy models and initial power-spectra on the low-redshift matter distribution.

This work investigates the alignment of galactic spins with the cosmic web across cosmic time using the cosmological hydrodynamical simulation Horizon-AGN. The cosmic web structure is extracted via the persistent skeleton as implemented in the DISPERSE algorithm. It is found that the spin of low-mass galaxies is more likely to be aligned with the filaments of the cosmic web and to lie within the plane of the walls while more massive galaxies tend to have a spin perpendicular to the axis of the filaments and to the walls. The mass transition is detected with a significance of 9 sigmas. This galactic alignment is consistent with the alignment of the spin of dark haloes found in pure dark matter simulations and with predictions from (anisotropic) tidal torque theory. However, unlike haloes, the alignment of low-mass galaxies is weak and disappears at low redshifts while the orthogonal spin orientation of massive galaxies is strong and increases with time, probably as a result of mergers. At fixed mass, alignments are correlated with galaxy morphology: the high-redshift alignment is dominated by spiral galaxies while elliptical centrals are mainly responsible for the perpendicular signal. These predictions for spin alignments with respect to cosmic filaments and unprecendently walls are successfully compared with existing observations. The alignment of the shape of galaxies with the different components of the cosmic web is also investigated. A coherent and stronger signal is found in terms of shape at high mass. The two regimes probed in this work induce competing galactic alignment signals for weak lensing, with opposite redshift and luminosity evolution. Understanding the details of these intrinsic alignments will be key to exploit future major cosmic shear surveys like Euclid or LSST.

The mass, accretion rate and formation time of dark matter haloes near proto-filaments (identified as saddle points of the potential) are analytically predicted using a conditional version of the excursion set approach in its so-called \"upcrossing\" approximation. The model predicts that at fixed mass, mass accretion rate and formation time vary with orientation and distance from the saddle, demonstrating that assembly bias is indeed influenced by the tides imposed by the cosmic web. Starved, early forming haloes of smaller mass lie preferentially along the main axis of filaments, while more massive and younger haloes are found closer to the nodes. Distinct gradients for distinct tracers such as typical mass and accretion rate occur because the saddle condition is anisotropic, and because the statistics of these observables depend on both the conditional means and their covariances. The theory is extended to other critical points of the potential field. The response of the mass function to variations of the matter density field (the so-called large scale bias) is computed, and its trend with accretion rate is shown to invert along the filament. The signature of this model should correspond at low redshift to an excess of reddened galactic hosts at fixed mass along preferred directions, as recently reported in spectroscopic and photometric surveys and in hydrodynamical simulations. The anisotropy of the cosmic web emerges therefore as a significant ingredient to describe jointly the dynamics and physics of galaxies, e.g. in the context of intrinsic alignments or morphological diversity.

Context. Accurate model predictions including the physics of baryons are required to make the most of the upcoming large cosmological surveys devoted to gravitational lensing. The advent of hydrodynamical cosmological simulations enables such predictions on sufficiently sizeable volumes. Aims. Lensing quantities (deflection, shear, convergence) and their statistics (convergence power spectrum, shear correlation functions, galaxy-galaxy lensing) are computed in the past lightcone built in the Horizon-AGN hydrodynamical cosmological simulation, which implements our best knowledge on baryonic physics at the galaxy scale in order to mimic galaxy populations over cosmic time. Methods. Lensing quantities are generated over a one square degree field of view by performing multiple-lens plane ray-tracing through the lightcone, taking full advantage of the 1 kpc resolution and splitting the line of sight over 500 planes all the way to redshift z~7. Two methods are explored (standard projection of particles with adaptive smoothing, and integration of the acceleration field) to assert a good implementation. The focus is on small scales where baryons matter most. Results. Standard cosmic shear statistics are impacted at the 10% level by the baryonic component for angular scales below a few arcmin. The galaxy-galaxy lensing signal, or galaxy-shear correlation function, is consistent with measurements for the redshift z~0.5 massive galaxy population. At higher redshift z>1, the impact of magnification bias on this correlation is relevant for separations greater than 1 Mpc. Conclusions. This work is pivotal for all current and upcoming weak lensing surveys and represents a first step towards building a full end-to-end generation of lensed mock images from large cosmological hydrodynamical simulations.

Context. 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. Aims. 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. Methods. 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. Results. The main weakly chromatic excess of multipolar power on large scales is understood as arising from the contraction of the primordial cosmic web driven by the growing potential well of the cluster. Besides this boost, the quadrupole prevails in the cluster (ellipsoidal) core, while at the outskirts, harmonic distortions are spread on small angular modes, and trace the non-linear sharpening of the filamentary structures. Predictions for the signal amplitude as a function of the cluster-centric distance, mass, and redshift are presented. The prospects of measuring this signal are estimated for current and future lensing data sets. Conclusions. The Euclid mission should provide all the necessary information for studying the cosmic evolution of the connectivity of the cosmic web around lensing clusters using multipolar moments and probing unique signatures of, for example, baryons and warm dark matter.

Simple parameter-free analytic bias functions for the two-point correlation of densities in spheres at large separation are presented. These bias functions generalize the so-called Kaiser bias to the mildly non-linear regime for arbitrary density contrasts. The derivation is carried out in the context of large deviation statistics while relying on the spherical collapse model. A logarithmic transformation provides a saddle approximation which is valid for the whole range of densities and shown to be accurate against the 30 Gpc cube state-of-the-art Horizon Run 4 simulation. Special configurations of two concentric spheres that allow to identify peaks are employed to obtain the conditional bias and a proxy to BBKS extrema correlation functions. These analytic bias functions should be used jointly with extended perturbation theory to predict two-point clustering statistics as they capture the non-linear regime of structure formation at the percent level down to scales of about 10 Mpc/h at redshift 0. Conversely, the joint statistics also provide us with optimal dark matter two-point correlation estimates which can be applied either universally to all spheres or to a restricted set of biased (over- or underdense) pairs. Based on a simple fiducial survey, this estimator is shown to perform five times better than usual two-point function estimators. Extracting more information from correlations of different types of objects should prove essential in the context of upcoming surveys like Euclid, DESI, PFS or LSST.