K-mouflage effects on clusters of galaxies

TL;DR

This paper studies how K-mouflage modified gravity affects galaxy cluster dynamics, revealing significant deviations from standard cosmology in cluster abundance and properties, with potential observational signatures in X-ray and Sunyaev-Zel'dovich measurements.

Contribution

It extends K-mouflage theory to include pressure-coupled fluids, analyzes effects on cluster properties, and computes observable deviations from Λ-CDM in a comprehensive framework.

Findings

Cluster halo mass function deviates by order one for M > 10^{14} h^{-1} M_sun.

Cluster temperature function and counts are significantly affected by K-mouflage.

Scaling relations like mass-temperature are only slightly modified due to Solar System constraints.

Abstract

We investigate the effects of a K-mouflage modification of gravity on the dynamics of clusters of galaxies. We extend the description of K-mouflage to situations where the scalar field responsible for the modification of gravity is coupled to a perfect fluid with pressure. We describe the coupled system at both the background cosmology and cosmological perturbations levels, focusing on cases where the pressure emanates from small-scale nonlinear physics. We derive these properties in both the Einstein and Jordan frames, as these two frames already differ by a few percents at the background level for K-mouflage scenarios, and next compute cluster properties in the Jordan frame that is better suited to these observations. Galaxy clusters are not screened by the K-mouflage mechanism and therefore feel the modification of gravity in a maximal way. This implies that the halo mass function deviates from $\Lambda$-CDM by a factor of order one for masses $M\gtrsim 10^{14} \ h^{-1} M_\odot$. We then consider the hydrostatic equilibrium of gases embedded in galaxy clusters and the consequences of K-mouflage on the X-ray cluster luminosity, the gas temperature, and the Sunyaev-Zel'dovich effect. We find that the cluster temperature function, and more generally number counts, are largely affected by K-mouflage, mainly due to the increased cluster abundance in these models. Other scaling relations such as the mass-temperature and the temperature-luminosity relations are only modified at the percent level due to the constraints on K-mouflage from local Solar System tests.