K-mouflage effects on clusters of galaxies

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.Comment: 34 page

f(R)-gravity model of the Sunyaev-Zeldovich profile of the Coma cluster compatible with Planck data

“…Cluster of galaxies have been widely used to constrain chameleon f (R) models ( [19,25,27]), the Galileon gravity model [26], the K-mouflage modification of gravity [85] and f (χ) gravity models [86]. We have constrained a particular class of ETGs using the TSZ temperature anisotropies due to the Coma cluster.…”

3D weak lensing: Modified theories of gravity

“…In this case, the fifth-force constraints will be negligible due to a suppression of the effective coupling to matter βðφÞ= ffiffiffiffiffiffiffiffiffiffi ZðφÞ p ≪ 1. This method is used by both the Vainshtein mechanism [6] as well as the K-mouflage mechanism [13,14,51,52]. More precisely, whereas the Vainshtein mechanism involves second-order derivative terms □φ in the factor Z, the K-mouflage mechanism only involves first-order derivative terms ð∂φÞ 2 .…”

“…[13,14], this is not true, and the background dynamics can deviate from that of GR. On the other hand, the perturbative regime often breaks degeneracies between MG and DE.…”

Lyman-α power spectrum as a probe of modified gravity

“…In practice,K ′ ≃ 1 for the background for z 6, so that the precise form of K(χ) does not play any role and our results are set by the value of the coupling β K . Indeed, in this model clusters of galaxies are still in the unscreened linear regime of the scalar field [13] and this is even more so for the Lyman-α forest clouds. Then, in the linear regime the growth of matter density perturbations can again be described by an effective Newton constant as in Eq.…”

“…We concentrate on models of the f (R) type for which the astrophysical constraints imply that deviations from the Λ-CDM template are limited [11]. Nevertheless, for models of the K-mouflage type [12], where large clusters are not screened [13] and see the effects of the scalar field with no suppression, large-scale observables such as the Lyman-α forest are of relevance.…”