International Astronomical Union Symposium no. 220, held
21 - 25 July, 2003 in Sydney, Australia. Eds: S. D. Ryder, D. J. Pisano,
M. A. Walker, and K. C. Freeman. San Francisco: Astronomical Society of
the Pacific., p.377
astro-ph/0407321
For a PDF version of the article, click
here.
SCALING LAWS FOR DARK MATTER HALOS IN LATE-TYPE AND DWARF
SPHEROIDAL GALAXIES
John Kormendy
Department of
Astronomy,
The University of Texas
at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA
K. C. Freeman
Research
School of Astronomy and Astrophysics,
Mount
Stromlo Observatory,
The Australian National
University, Cotter Road, Weston Creek, Canberra, ACT 72611, Australia
Abstract.
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 (V2 -
Vvis2)1/2, where
V(r) is the observed rotation curve and
Vvis is the rotation curve of the visible matter.
The latter is calculated from the surface brightness assuming
that the mass-to-light ratio M / L is constant with
radius. "Maximum disk" 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. Rotation curve
decomposition becomes impossible fainter than absolute magnitude
MB 
-
14, where V becomes comparable to the velocity dispersion of
the gas. To increase the luminosity range further, we include
dSph galaxies, which are physically related to spiral and irregular
galaxies. Combining the data, we find that DM halos satisfy well
defined scaling laws analogous to the "fundamental plane" relations for
elliptical galaxies. Halos in less luminous galaxies have smaller core radii
rc, higher central densities
0,
and smaller central velocity dispersions
. Scaling laws provide
new and detailed constraints on the nature of DM and on galaxy formation
and evolution. Some simple implications include:
1 - A single, continuous physical sequence of increasing mass extends
from dSph galaxies with
MB -
7.6 to ScI galaxies with
MB -
22.4.
2 - The high DM densities in dSph galaxies are normal for
such tiny galaxies. Since virialized density depends on collapse redshift
zcoll,
0
(1 +
zcoll)3, the smallest dwarfs formed at least
zcoll
7 earlier than the biggest spirals.
3 - The high DM densities of dSphs implies that they are real galaxies
formed from primordial density fluctuations. They are not tidal fragments.
Tidal dwarfs cannot retain even the low DM densities of their giant-galaxy
progenitors. In contrast, dSphs have higher DM densities than do
giant-galaxy progenitors.
4 - The fact that, as luminosity decreases, dwarf galaxies become much
more numerous and also more nearly dominated by DM raises the
possibility that
there exists a large population of objects that are completely dark. Such
objects are a canonical prediction of cold DM theory. If they exist, "empty
halos" are likely to be small and dense - that is, darker versions of Draco
and UMi.
5 - The slopes of the DM parameter correlations provide a measure on
galactic mass scales of the slope n of the power spectrum
|
k|2
kn
of primordial density fluctuations.
Our preliminary results not yet corrected for baryonic compression of DM
give n -1.9 ±
0.2. This is consistent with cold DM theory.
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