The Tight Empirical Relation between Dark Matter Halo Mass and Flat
  Rotation Velocity for Late-Type Galaxies
  We present a new empirical relation between galaxy dark matter halo mass
(${\rm M_{halo}}$) and the velocity along the flat portion of the rotation
curve (${\rm V_{flat}}$), derived from 120 late-type galaxies from the SPARC
database. The orthogonal scatter in this relation is comparable to the observed
scatter in the baryonic Tully-Fisher relation (BTFR), indicating a tight
coupling between total halo mass and galaxy kinematics at $r\ll R_{\rm vir}$.
The small vertical scatter in the relation makes it an extremely competitive
estimator of total halo mass. We demonstrate that this conclusion holds true
for different priors on $M_*/L_{[3.6\mu]}$ that give a tight BTFR, but requires
that the halo density profile follows DC14 rather than NFW. We provide
additional relations between ${\rm M_{halo}}$ and other velocity definitions at
smaller galactic radii (i.e. ${\rm V_{2.2}}$, ${\rm V_{eff}}$, and ${\rm
V_{max}}$) which can be useful for estimating halo masses from kinematic
surveys, providing an alternative to abundance matching. Furthermore, we
constrain the dark matter analog of the Radial Acceleration Relation and also
find its scatter to be small, demonstrating the fine balance between baryons
and dark matter in their contribution to galaxy kinematics.
