A constant characteristic volume density of dark matter haloes from
  SPARC rotation curve fits
  We study the scaling relations between dark matter (DM) haloes and galaxy
discs using 175 galaxies from the SPARC database. We explore two cosmologically
motivated DM halo profiles: the Einasto profile from DM-only simulations and
the DC14 profile from hydrodynamic simulations. We fit the observed rotation
curves using a Markov Chain Monte Carlo method and break the disc-halo
degeneracy using near-infrared photometry and $\Lambda$CDM-motivated priors. We
find that the characteristic volume density $\rho_{\rm s}$ of DM haloes is
nearly constant over $\sim$5 decades in galaxy luminosity. The scale radius
$r_s$ and the characteristic surface density $\rho_s\cdot r_s$, instead,
correlate with galaxy luminosity. These scaling relations provide an empirical
benchmark to cosmological simulations of galaxy formation.
