Fifth force constraints from galaxy warps

Abstract

Intra-galaxy signals contain a wealth of information on fundamental physics, both the dark sector and the nature of gravity. While so far largely unexplored, such probes are set to rise dramatically in importance as upcoming surveys provide data of unprecedented quantity and quality on galaxy structure and dynamics. In this paper, we use warping of stellar disks to test the chameleon- or symmetron-screened fifth forces which generically arise when new fields couple to matter. We take r-band images of mostly late-type galaxies from the Nasa Sloan Atlas and develop an automated algorithm to quantify the degree of U-shaped warping they exhibit. We then forward-model the warp signal as a function of fifth-force strength $ΔG/G_N$ and range $λ_C$, and the gravitational environments and internal properties of the galaxies, including full propagation of the non-Gaussian uncertainties. Convolving this fifth-force likelihood function with a Gaussian describing astrophysical and observational noise and then constraining $ΔG/G_N$ and $λ_C$ by Markov Chain Monte Carlo, we find the overall likelihood to be significant increased ($Δ\log(\mathcal{L}) \simeq 20$) by adding a screened fifth force with $λ_C \simeq 2$ Mpc, $ΔG/G_N \simeq 0.01$. The variation of $Δ\log(\mathcal{L})$ with $λ_C$ is quantitatively as expected from the correlation of the magnitude of the fifth-force field with the force's range, and a similar model without screening achieves no increase in likelihood over the General Relativistic case $ΔG=0$. Although these results are in good agreement with a previous analysis of the same model using offsets between galaxies' stellar and gas mass centroids (Desmond et al. 2018), we caution that the effects of confounding baryonic and dark matter physics must be thoroughly investigated for the results of the inference to be unambiguous.