Constraints on Galileons from the positions of supermassive black holes

Galileons are scalar field theories which obey the Galileon symmetry φ→φ+b+c_μx^μ and are capable of self-acceleration if they have an inverted sign for the kinetic term. These theories violate the strong equivalence principle, such that black holes (BHs) do not couple to the Galileon field, whereas nonrelativistic objects experience a fifth force with strength ΔG/G_N relative to gravity. For galaxies falling down a gradient in the Galileon field, this results in an offset between the center of the galaxy and its host supermassive BH. We reconstruct the local gravitational and Galileon fields through a suite of constrained N-body simulations (which we dub CSiBORG) and develop a Monte Carlo-based forward model for these offsets on a galaxy-by-galaxy basis. Using the measured offset between the optical center and active galactic nucleus of 1916 galaxies from the literature, propagating uncertainties in the input quantities and marginalizing over an empirical noise model describing astrophysical and observational noise, we constrain the Galileon coupling to be ΔG/G_N<0.16 at 1σ confidence for Galileons with crossover scale r_C ≳ H₀^-1.