Dark Matter Halos

Constraining dark matter (DM) on galactic scales is crucial for understanding its fundamental physics beyond the standard model of particle physics. ΛCDM has proved a very successful model of the universe on large scales where it can be tested at high precision. However, the properties of DM halos on galactic scales are less known; for example, the debate concerning their central densities remains lively. The presence of dense cusps in DM halos is such a robust prediction of ΛCDM that failure to find them in real galaxies would constitute a setback in our understanding of cosmology.

Bars, which are present in more than 2/3 of all disk galaxies, are ideal for probing inner DM densities because they represent the strongest, most robust and long-lived equilibrium departure from axisymmetry. A consequence of this is that the angular momentum of stars is not conserved, making bars efficient drivers of angular momentum transport by well-understood, purely gravitational forces.

In all cases in which the bar pattern speed has been measured, fast bars, (i.e., ones with corotation radii slightly past the end of the bar) have been found. However, if the bar is surrounded by a dense halo, then dynamical friction will slow the bar down considerably on quite short time-scales. In collaboration with J.A. Sellwood, I have used N-body simulations to study the evolution of bar pattern speeds for various halo masses. We start out with initially axisymmetric disks which are unstable to bar formation; an example of such an N-body bar is shown in figure 1. We find that, in the presence of a dense halo, an initially fast bar slows down very rapidly, as predicted by theory. On the other hand, when the disk is dominant a bar takes a very long time to slow down. Since real bars are fast, we concluded that real barred galaxies, and by extension unbarred disk galaxies, dominate their rotation curves.

References: Debattista & Sellwood (1998, 2000), Sellwood & Debattista (2006).

In collaboration with J.A. Sellwood, I proposed a model for warp formation in which a misalignment of disk and dark matter halo angular momenta drives transient warps by dynamical friction, even when the initial dark matter halo is spherical. An example of such a warp, produced in an N-body simulation, is shown in figure 2. This model accounts for the behavior of warp lines-of-nodes as obtained from observations (Briggs 1990) and for the fact that it is usually the HI disk that warps. This model can also account for the high fraction of warped disk galaxies since such misalignments are almost inevitable if galaxies form through a merging hierarchy.

Another interesting result of this study, which is more general than the model we proposed, is that hotter disks (as measured by the Toomre-Q parameter, or velocity dispersion) tend to have stiffer disks, so that, all other things being equal, the resulting warp will be smaller.

References: Debattista & Sellwood (1999)