Stellar streams in chameleon gravity

Abstract

Theories of gravity that incorporate new scalar degrees of freedom typically require "screening mechanisms" to ensure consistency with Solar System tests. One widely studied mechanism-the chameleon mechanism-can lead to violations of the equivalence principle (EP), as screened and unscreened objects fall differently. If the stars are screened but the surrounding dark matter is not, EP violation can lead to asymmetry between leading and trailing streams from tidally disrupted dwarf galaxies in the Milky Way halo. We provide analytic estimates of the magnitude of this effect for realistic Galactic mass distributions, demonstrating that it is an even more sensitive probe than suggested previously. Using a restricted N-body code, we simulate four satellites with a range of masses and orbits, together with a variety of strengths of the fifth force and screening levels of the Milky Way and satellite. The ratio of the cumulative number function of stars in the leading and trailing stream as a function of longitude from the satellite is computable from simulations and measurable from the stellar data and can provide a direct test of chameleon gravity. We forecast constraints for streams at large Galactocentric distances, which probe deeper into chameleon parameter space, using the specific example case of Hu-Sawicki fðRÞ gravity. Streams in the outer reaches of the Milky Way halo (with apocenters between 100 and 200 kpc) provide easily attainable constraints at the level of jf R0 j ¼ 10 −7. Still more stringent constraints at the level of 10 −7.5 or even 10 −8 are plausible provided the environmental screening of the satellite is accounted for, and screening of the Milky Way's outer halo by the Local Group is not yet triggered in this range. These would be among the tightest astrophysical constraints to date. We note three further signatures of chameleon gravity: (i) the trailing stellar stream may become detached from the dark matter progenitor if all the stars are lost, (ii) in the extreme fifth force regime, striations in the stellar trailing tail may develop from material liberated at successive pericentric passages, and (iii) if the satellite is fully screened, its orbital frequency is lower than that of the associated dark matter, which is preferentially liberated into the leading tidal tail.

Direct detection of dark energy: the XENON1T excess and future prospects

“…Refs. [26,27,[208][209][210][211][212][213]. These refer to searches for the effects of fifth forces rather than chameleon particle production.…”

Accurate computation of the screening of scalar fifth forces in galaxies

“…Much of the recent literature has studied the behaviour of these theories on galactic scales, where the screening phenomenon leads to a rich phenomenology, such as equivalence JCAP04(2024)004 principle violations and other kinematic signatures [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. The recent interest in galactic scales is a result of two circumstances: the fact that galaxies inhabit an unconstrained "desert" in modified gravity parameter space [39], and the recent advent of vast datasets from various galaxy surveys [40][41][42].…”

The e-TidalGCs project

“…Some works have presented models and simulations for specific streams (Dehnen et al 2004;Mastrobuono-Battisti et al 2012;Banik & Bovy 2019;Bonaca et al 2019;Banik et al 2021a;Mirabal & Bonaca 2021), contributing to understanding their morphology, density variations and extent. From these works, it is clear that the tidal loss of stars from globular clusters and the formation of related structures are important for several aspects: (1) quantifying to what extent globular clusters have contributed to the field stellar populations-from the halo to the disk to the bulge-and to what extent they still do, (2) reconstructing the properties (in terms of numbers and masses) of the early Galactic globular clusters, through their current mass loss, and, last but not least, (3) using globular cluster streams as a probe of the Galactic potential and-more generally-of the physical laws governing gravity (see for example Thomas et al 2018;Bianchini et al 2019;Naik et al 2020;Banik et al 2021a,b).…”