Stellar kinematics from the symmetron fifth force in the Milky Way disk

TL;DR

This paper investigates how a symmetron fifth force could influence stellar motions in the Milky Way, offering a potential alternative to dark matter and providing new constraints on modified gravity models.

Contribution

It demonstrates how symmetron scalar fields can be modeled to explain stellar kinematics and updates constraints, revealing viable parameter space for this modified gravity scenario.

Findings

Symmetron field can account for stellar velocities up to 1 kpc height.

Parameter space exists where symmetron effects mimic dark matter.

Distinct force behavior at larger heights can differentiate from dark matter halos.

Abstract

It has been shown that the presence of non-minimally coupled scalar fields giving rise to a fifth force can noticeably alter dynamics on galactic scales. Such a fifth force must be screened in the Solar System but if unscreened it can have a similar observational effects as a component of non-baryonic matter. We consider this possibility in the context of the vertical motions of local stars in the Milky Way disk by reframing a methodology used to measure the local density of dark matter. By attempting to measure the properties of the symmetron field required to support vertical velocities we can test it as a theory of modified gravity and understand the behaviour of screened scalar fields in galaxies. In particular this relatively simple setup allows the symmetron field profile to be solved for model parameters where the equation of motion becomes highly nonlinear and difficult to solve in other contexts. We update the existing Solar System constraints for this scenario and find a region of parameter space not already excluded that can explain the vertical motions of local stars out to heights of 1 kpc. At larger heights the force due to the symmetron field profile exhibits a characteristic turn over which would allow the model to be distinguished from a dark matter halo.