Scaling Laws for Dark Matter Halos in Late-Type and Dwarf Spheroidal Galaxies

Abstract

Dark matter (DM) halos of Sc–Im galaxies satisfy structural scaling laws analogous to the fundamental plane relations for elliptical galaxies. Halos in less luminous galaxies have smaller core radii r c , higher central densities ρ ^ , and smaller central velocity dispersions σ. 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 Sc–Im galaxies. We find that, if there had been no enhanced baryon loss relative to Sc–Im galaxies, typical dSph and dIm galaxies would be brighter by Δ M B ≃ -4.0 mag and Δ M B ≃ -3.5 mag, respectively. Instead, the galaxies lost or retained as gas (in dIm galaxies) baryons that could have formed stars. Also, dSph and dIm galaxies have DM halos that are more massive than we thought, with σ ~ 30 km s −1 or circular-orbit rotation velocities V circ ~ 42 km s −1 . Comparison of DM and visible matter parameter correlations confirms that, at M V ≳ -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 V circ ≲ 42 ± 4 km s −1 , in agreement with V circ as found from our estimate of baryon depletion. Our results suggest that there may be a large population of DM halos that are dark and undiscovered. This helps to solve the problem that the initial fluctuation spectrum of cold dark matter predicts more dwarf galaxies than we observe.