Towards Accurate Modelling of Galaxy Clustering on Small Scales: Testing the Standard $\Lambda\mathrm{CDM}$ + Halo Model

TL;DR

This paper advances galaxy clustering modeling on small scales using a numerical, mock-based approach to test the standard mbda CDM + halo model against SDSS data, revealing tensions that suggest the model needs extensions.

Contribution

It introduces a fully numerical, mock-based methodology for more accurate small-scale galaxy clustering modeling and tests the standard mbda CDM + halo model against multiple clustering statistics.

Findings

The model fits individual statistics but struggles with simultaneous fitting.

Adding group statistics tightens constraints and tests the model more stringently.

A mbda CDM model with Mvir halos matches low luminosity galaxy clustering but shows tension with luminous galaxies.

Abstract

Interpreting the small-scale clustering of galaxies with halo models can elucidate the connection between galaxies and dark matter halos. Unfortunately, the modelling is typically not sufficiently accurate for ruling out models statistically. It is thus difficult to use the information encoded in small scales to test cosmological models or probe subtle features of the galaxy-halo connection. In this paper, we attempt to push halo modelling into the "accurate" regime with a fully numerical mock-based methodology and careful treatment of statistical and systematic errors. With our forward-modelling approach, we can incorporate clustering statistics beyond the traditional two-point statistics. We use this modelling methodology to test the standard $\Lambda\mathrm{CDM}$ + halo model against the clustering of SDSS DR7 galaxies. Specifically, we use the projected correlation function, group multiplicity function and galaxy number density as constraints. We find that while the model fits each statistic separately, it struggles to fit them simultaneously. Adding group statistics leads to a more stringent test of the model and significantly tighter constraints on model parameters. We explore the impact of varying the adopted halo definition and cosmological model and find that changing the cosmology makes a significant difference. The most successful model we tried (Planck cosmology with Mvir halos) matches the clustering of low luminosity galaxies, but exhibits a 2.3$\sigma$ tension with the clustering of luminous galaxies, thus providing evidence that the "standard" halo model needs to be extended. This work opens the door to adding interesting freedom to the halo model and including additional clustering statistics as constraints.