Fifth force constraints from the separation of galaxy mass components

One of the most common consequences of extensions to the standard models of particle physics or cosmology is the emergence of a fifth force. While generic fifth forces are tightly constrained at Solar System scales and below, they may escape detection by means of a screening mechanism which effectively removes them in dense environments. We constrain the strength Delta G/G(N) and range lambda(C) of a fifth force with Yukawa coupling arising from a chameleon-or symmetron-screened scalar field-as well as an unscreened fifth force with differential coupling to galactic mass components-by searching for the displacement it predicts between galaxies' stellar and gas mass centroids. Taking data from the Alfalfa survey of neutral atomic hydrogen (HI), identifying galaxies' gravitational environments with the maps of [H. Desmond, P.G. Ferreira, G. Lavaux, and J. Jasche, Mon. Not. R. Astron. Soc. 474, 3152 (2018)] and forward modeling with a Bayesian likelihood framework, we find, with screening included, 6.6 sigma evidence for Delta G > 0 at lambda(C) greater than or similar to 2 Mpc. The maximum-likelihood Delta G/G(N) is 0.025. A similar fifth force model without screening gives no increase in likelihood over the case Delta G = 0 for any lambda(C). Although we validate this result by several methods, we do not claim screened modified gravity to provide the only possible explanation for the data: this conclusion would require knowing that the signal could not be produced by galaxy formation physics. We show also the results of a more conservative-though less well-motivated-noise model which yields only upper limits on Delta G/G(N), ranging from similar to 10(-1) for lambda(C) similar or equal to 0.5 Mpc to similar to few x 10(-4) at lambda(C) similar or equal to 50 Mpc. Corresponding models without screening receive the somewhat stronger bounds similar to few x 10(-3) and similar to few x 10(-4) respectively. We show how these constraints may be improved by future galaxy surveys and identify the key features of an observational program for directly constraining fifth forces on scales beyond the Solar System. This paper provides a complete description of the analysis summarized in [H. Desmond, P.G. Ferreira, G. Lavaux, and J. Jasche, arXiv:1802.07206].