  We interpret the correlation between local star-forming galaxy positions and
ultra-high-energy cosmic ray (UHECR) directions, recently detected by the
Pierre Auger Observatory (PAO), in terms of physical parameters: the local
density of sources and the magnetic fields governing the UHECR propagation. We
include a Galactic magnetic field model on top of a random extragalactic
magnetic field description to determine the level of UHECR deflections expected
from an ensemble of source positions. Besides deflections in magnetic fields,
we also take into account energy losses with background photon fields as well
as spectrum and composition measurements by the PAO. We find consistency
between the PAO anisotropy measurement and the local star-forming galaxy
density for large extragalactic magnetic field strengths with $B > 0.2 \ \rm
nG$ (for a coherence length of $1 \ \rm Mpc$) at the $5\sigma$ confidence
level. Larger source densities lead to more isotropic background and
consequently allow for weaker extragalactic magnetic fields. However, the
acceleration of UHECR by such abundant sources is more challenging to motivate.
Too large source densities and extragalactic magnetic field strengths, on the
other hand, are also disfavored as that decreases the expected level of
anisotropy. This leads to upper limits of $B < 22 \ \rm nG$ and $\rho_0 < 8.4
\cdot 10^{-2} \ \rm Mpc^{-3}$ at the 90\% confidence level.
