Galactic Rotation Curves vs. Ultra-Light Dark Matter: Implications of
  the Soliton -- Host Halo Relation
  Bosonic ultra-light dark matter (ULDM) would form cored density distributions
at the center of galaxies. These cores, seen in numerical simulations, admit
analytic description as the lowest energy bound state solution ("soliton") of
the Schroedinger-Poisson equations. Numerical simulations of ULDM galactic
halos found empirical scaling relations between the mass of the large-scale
host halo and the mass of the central soliton. We discuss how the simulation
results of different groups can be understood in terms of the basic properties
of the soliton. Importantly, simulations imply that the energy per unit mass in
the soliton and in the virialised host halo should be approximately equal. This
relation lends itself to observational tests, because it predicts that the peak
circular velocity, measured for the host halo in the outskirts of the galaxy,
should approximately repeat itself in the central region. Contrasting this
prediction to the measured rotation curves of well-resolved near-by galaxies,
we show that ULDM in the mass range $m\sim (10^{-22}\div 10^{-21})$ eV, which
has been invoked as a possible solution to the small-scale puzzles of
$\Lambda$CDM, is in tension with the data. We suggest that a dedicated analysis
of the Milky Way inner gravitational potential could probe ULDM up to
$m\lesssim 10^{-19}$ eV.
