John Kormendy: Dark Matter Scaling Laws

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

John Kormendy (Institute for Astronomy, University of Hawaii) and

K. C. Freeman (Mount Stromlo Observatory)

Published mass models fitted to galaxy rotation curves are used to study the systematic properties of dark matter (DM) halos in late-type and dwarf spheroidal (dSph) galaxies. Halo parameters are derived by fitting non-singular isothermals to (V**2 - Vvis**2)**(1/2), where V(r) is the observed rotation curve and Vvis(r) is the rotation curve of the visible matter. The latter is calculated from the observed H I and optical brightness distributions assuming that the mass-to-light ratio M/L is constant with radius. Values of M/L are adjusted to fit as much of the inner rotation curve as possible without making the halo have a hollow core. That is, "maximum disk" decompositions are used (the results are not sensitive to this assumption). To increase the luminosity range, we include dSph galaxies, which are physically related to late-type galaxies. Combining the above data, we find that DM halos satisfy well defined scaling laws analogous to the fundamental plane correlations for elliptical galaxies. Halos in less luminous galaxies have smaller core radii r_c, higher central densities rho_0, and higher central velocity dispersions sigma. Scaling laws provide new and detailed constraints on the nature of DM and on galaxy formation and evolution. Most of these remain to be explored. Some simple implications include:

1. - The surprisingly high DM densities in the dSph galaxies Draco and UMi are normal for galaxies of such low luminosity. This implies that dwarf spheroidals are real galaxies and not just tidal fragments. The reason is that tides stretch, so that bound fragments pulled out of more luminous galaxies would have lower DM densities than their progenitors. But rho_0 is already lower in giant galaxies than in dwarfs even before tidal stretching.

2. - The fact that, as luminosity decreases, dwarf galaxies become more numerous and more nearly dominated by DM raises the possibility that there exists a large population of objects that are completely dark. If so, these empty halos are likely to be small and dense and to have small total masses.

This work was supported by NSF grant AST--9219221.

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