199 research outputs found
The Tully-Fisher Zero Point Problem
A long standing problem for hierarchical disk galaxy formation models has
been the simultaneous matching of the zero point of the Tully-Fisher relation
and the galaxy luminosity function (LF). We illustrate this problem for a
typical disk galaxy and discuss three solutions: low stellar mass-to-light
ratios, low initial dark halo concentrations, and no halo contraction. We
speculate that halo contraction may be reversed through a combination of mass
ejection through feedback and angular momentum exchange brought about by
dynamical friction between baryons and dark matter during the disk formation
process.Comment: 4 pages, 1 figure, to appear in proceedings of "Formation and
Evolution of Galaxy Disks", Rome, October 2007, Eds. J.G. Funes, S.J. and
E.M. Corsin
Concentration, Spin and Shape of Dark Matter Haloes as a Function of the Cosmological Model: WMAP1, WMAP3 and WMAP5 results
We investigate the effects of changes in the cosmological parameters between
the WMAP 1st, 3rd, and 5th year results on the structure of dark matter haloes.
We use a set of simulations that cover 5 decades in halo mass ranging from the
scales of dwarf galaxies (V_c ~30 km/s) to clusters of galaxies (V_c ~ 1000
km/s). We find that the concentration mass relation is a power law in all three
cosmologies. However the slope is shallower and the zero point is lower moving
from WMAP1 to WMAP5 to WMAP3. For haloes of mass log(M_200/Msun) = 10, 12, and
14 the differences in the concentration parameter between WMAP1 and WMAP3 are a
factor of 1.55, 1.41, and 1.29, respectively. As we show, this brings the
central densities of dark matter haloes in good agreement with the central
densities of dwarf and low surface brightness galaxies inferred from their
rotation curves, for both the WMAP3 and WMAP5 cosmologies. We also show that
none of the existing toy models for the concentration-mass relation can
reproduce our simulation results over the entire range of masses probed. In
particular, the model of Bullock et al (B01) fails at the higher mass end (M >
1e13 Msun), while the NFW model of Navarro, Frenk & White (1997) fails
dramatically at the low mass end (M < 1e12 Msun). We present a new model, based
on a simple modification of that of B01, which reproduces the
concentration-mass relations in our simulations over the entire range of masses
probed (1e10 Msun < M < 1e15 Msun). Haloes in the WMAP3 cosmology (at a fixed
mass) are more flatted compared to the WMAP1 cosmology, with a medium to long
axis ration reduced by ~10 %. Finally, we show that the distribution of halo
spin parameters is the same for all three cosmologies.Comment: 16 pages, 16 figures, references updated, minor changes. Accepted for
publication on MNRAS. WMAP5 simulations available upon reques
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