# The $f(R)$ halo mass function in the cosmic web

## TL;DR

This paper investigates how modified gravity, specifically $f(R)$ models, influences halo properties and abundance within the cosmic web, using an extended formalism that aligns well with simulations on large scales.

## Contribution

It generalizes the excursion set formalism to produce a halo mass function conditioned on large-scale structure, revealing environment-independent effects of $f(R)$ gravity on halo properties.

## Key findings

- $f(R)$ affects halo abundances, spin, and concentration-mass relation.
- No significant deviation from $ ext{Lambda}$CDM in fixed environment density.
- Greater deviations occur in underdense environments and for high-mass haloes.

## Abstract

An important indicator of modified gravity is the effect of the local environment on halo properties. This paper examines the influence of the local tidal structure on the halo mass function, the halo orientation, spin and the concentration-mass relation. We generalise the excursion set formalism to produce a halo mass function conditional on large-scale structure. Our model agrees well with simulations on large scales at which the density field is linear or weakly non-linear. Beyond this, our principal result is that $f(R)$ does affect halo abundances, the halo spin parameter and the concentration-mass relationship in an environment-independent way, whereas we find no appreciable deviation from $\Lambda$CDM for the mass function with fixed environment density, nor the alignment of the orientation and spin vectors of the halo to the eigenvectors of the local cosmic web. There is a general trend for greater deviation from $\Lambda$CDM in underdense environments and for high-mass haloes, as expected from chameleon screening.