Fifth force constraints from galaxy warps
  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 $\Delta G/G_N$ and range $\lambda_C$, 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 $\Delta G/G_N$ and $\lambda_C$ by
Markov Chain Monte Carlo, we find the overall likelihood to be significant
increased ($\Delta\log(\mathcal{L}) \simeq 20$) by adding a screened fifth
force with $\lambda_C \simeq 2$ Mpc, $\Delta G/G_N \simeq 0.01$. The variation
of $\Delta\log(\mathcal{L})$ with $\lambda_C$ 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 $\Delta G=0$. 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.
