Scaling Laws for Dark Matter Halos in Late-Type and Dwarf Spheroidal
  Galaxies
  Dark matter (DM) halos of Sc-Im galaxies satisfy scaling laws analogous to
the fundamental plane relations for elliptical galaxies. Halos in less luminous
galaxies have smaller core radii, higher central densities, and smaller central
velocity dispersions. If dwarf spheroidal (dSph) and dwarf Magellanic irregular
(dIm) galaxies lie on the extrapolations of these correlations, then we can
estimate their baryon loss relative to that of brighter Sc-Im galaxies. We find
that, if there had been no such enhanced baryon loss, then typical dSph and dIm
galaxies would be brighter in absolute magnitude by 4 and 3.5 mag,
respectively. Instead, these galaxies lost or retained as gas (in dIm galaxies)
baryons that could have formed stars. Also, typical dSph and dIm galaxies have
DM halos that are more massive than we thought, with velocity dispersions of
about 30 km/s or circular-orbit rotation velocities of V_circ ~ 42 km/s.
Comparison of DM and visible matter correlations confirms that, at V-band
absolute magnitudes fainter than -18, dSph and dIm galaxies form a sequence of
decreasing baryon-to-DM mass ratios in smaller dwarfs. We show explicitly that
galaxy baryon content goes to (almost) zero at halo V_circ = 42 +- 4 km/s, in
agreement with what we found from our estimate of baryon depletion. Our results
suggest that there may be a large population of DM halos that are essentially
dark and undiscovered. This helps to solve the problem that the fluctuation
spectrum of cold DM predicts more dwarfs than we observe.
