### Galaxy Clustering

**Principal Investigator: ** Emiliano Sefusatti

**Area: **

- Astroparticle Physics
- Early Universe
- Structures in the Universe and Emergent Phenomena
- Theory and Phenomenology of Gravity

**Abstract: **This line of research is devoted to the statistical analysis of the large-scale distribution of galaxies as observed in spectroscopic and photometric redshift surveys. It entails the theoretical modelling of matter and halo/galaxy correlation functions with analytical predictions from cosmological perturbation theory and their comparison against results from numerical N-body simulations. The goal is the optimal estimation of the cosmological parameters describing the Standard Model, including neutrino masses, as well as extensions due to modified theories of gravity, dark energy, general relativistic effects and non-Gaussian features in the early Universe.

**Status of project and perspectives: **The group is currently involved in several projects ranging from addressing observational and data analysis problems (also with direct applications to the Euclid mission) to more speculative investigations on the modelling of the large-scale structure in galaxy and weak lensing surveys and intensity mapping measurements.
One project of the group consists in developing a pipeline for the joint analysis of the power spectrum and higher-order correlation functions, such as the bispectrum, of the galaxy distribution. A likelihood code capable of analysing simulated catalogs in redshift space is about to be made public and has been used for testing models in perturbation theory in several papers.
In the near future we will be working on more technical aspects related to the analysis of actual data, such as the estimation of galaxy correlation functions, the effects due to the window function of the survey and the covariance properties of large-scale structure observations. The problem of a robust estimate of the covariance matrix for Fourier-space correlation function, in particular, is being explored both in terms of numerical measurements based on a large-set of mocks as well as in terms of analytical predictions based on perturbation theory.
In particular, the measurement of galaxy clustering is subject to several systematics that must be controlled to percent level, while the residual uncertainty must be propagated to the covariance matrix used for parameter inference. This involves the definition of the spectroscopic sample, redshift measurements, the estimation of the clustering statistics and the models and simulation themselves that induce a so-called theory error. We are developing a pipeline to predict the effect of specific systematics on the measurement of the power spectrum of a set of mock galaxy catalogs, with the aim of quantifying the impact of each systematics and propagate the uncertainty of its mitigation to parameter estimation.
Specific projects investigate the modelling of nonlinearities at medium and small scales in massive neutrino cosmologies, including the development of massive neutrino halo models, and of baryonic corrections to the matter power spectrum by means of hydrodynamical simulations of structure formation.
At the same time, we explore departures from the standard cosmology, testing alternatives to the CDM paradigm such as mixed Warm+Cold & Warm Dark Matter or Fuzzy Dark Matter, using dedicated N-body simulations and tailored HOD to determine effects on the galaxy distribution.
The large-scale structure can also provide important tests on the early Universe, discriminating among alternative models of inflation with constraints on a possible non-Gaussianity of primordial origin. We explore different techniques to achieve this goal from the traditional analysis of summary statistics to innovative topological tools.
Almost all members of the group are actively involved, to various degrees, in the Euclid Mission, bringing their expertise to different tasks from the correlation function estimation pipeline, observational systematic effects, galaxy clustering likelihood analysis and beyond standard model constraints.
The group has a regular meeting at IFPU every Thursday serving in the first place as a progress report and occasionally as a Journal Club or hosting talks by external visitors.
We sponsored a few IFPU activities, including a meeting of the Euclid Galaxy Clustering WP in October 2019, a meeting on recent developments in Large-Scale Structure modelling in Perturbation Theory in July 2022 and a meeting of the Euclid Observational Systematics WP in September 2022. Additional informal workshops on more technical aspects of the preparatory work for Euclid also have taken place at IFPU.

**Co-PIs: **

**Members: **

**Publications: **

- On the degeneracy between baryon feedback and massive neutrinos as probed by matter clustering and weak lensing
- A robust measurement of the first higher-derivative bias of dark matter halos
- Estimating the galaxy two-point correlation function using a split random catalog
- Beyond two-point statistics: using the minimum spanning tree as a tool for cosmology
- Weighing neutrinos with the halo environment
- Toward a robust inference method for the galaxy bispectrum: likelihood function and model selection
- The Quijote Simulations
- Fast numerical method to generate halo catalogs in modified gravity (part I): second-order Lagrangian Perturbation Theory
- The Accuracy of Weak Lensing Simulations
- Non-linear damping of superimposed primordial oscillations on the matter power spectrum in galaxy surveys
- Testing the Reliability of Fast Methods for Weak Lensing Simulations: WL-MOKA on PINOCCHIO
- The effects of massive neutrinos on the linear point of the correlation function
- The impact of massive neutrinos on halo assembly bias
- On the impact of baryons on the halo mass function, bias, and cluster cosmology
- The Persistence of Large Scale Structures I: Primordial non-Gaussianity
- Euclid preparation. X. The Euclid photometric-redshift challenge
- Towards cosmological constraints from the compressed modal bispectrum: a robust comparison of real-space bispectrum estimators
- Primordial Non-Gaussianity from Biased Tracers: Likelihood Analysis of Real-Space Power Spectrum and Bispectrum
- Quantifying the impact of baryon-CDM perturbations on halo clustering and baryon fraction
- Detecting Neutrino Mass by Combining Matter Clustering, Halos, and Voids
- Euclid: Effect of sample covariance on the number counts of galaxy clusters
- Euclid preparation. XII. Optimizing the photometric sample of the Euclid survey for galaxy clustering and galaxy-galaxy lensing analyses
- The Formation Probability of Primordial Black Holes
- Galaxy bias from forward models: linear and second-order bias of IllustrisTNG galaxies
- Euclid preparation: XIII. Forecasts for galaxy morphology with the Euclid Survey using Deep Generative Models
- Mixed dark matter: matter power spectrum and halo mass function
- Euclid preparation. XV. Forecasting cosmological constraints for the Euclid and CMB joint analysis
- Gravitational waves × HI intensity mapping: cosmological and astrophysical applications
- Euclid Preparation. XIV. The Complete Calibration of the Color-Redshift Relation (C3R2) Survey: Data Release 3
- Assembly bias in quadratic bias parameters of dark matter halos from forward modeling
- The reach of next-to-leading-order perturbation theory for the matter bispectrum
- Cosmological parameters from the likelihood analysis of the galaxy power spectrum and bispectrum in real space
- Euclid preparation. XVI. Exploring the ultra-low surface brightness Universe with Euclid/VIS
- Cosmic voids and BAO with relative baryon-CDM perturbations
- Constraining beyond ΛCDM models with 21cm intensity mapping forecast observations combined with latest CMB data
- Euclid preparation: XIX. Impact of magnification on photometric galaxy clustering
- Euclid preparation: XVIII. Cosmic Dawn Survey. Spitzer observations of the Euclid deep fields and calibration fields
- The Covariance of Squeezed Bispectrum Configurations
- Euclid preparation. XVIII. The NISP photometric system
- Bispectrum-window convolution via Hankel transform
- Fisher Forecasts for Primordial non-Gaussianity from Persistent Homology