Galaxies: Structure and Kinematics; Magnetic fields: Interstellar and Intergalactic >> Projects overview
Dark Matter in Galaxies
The existence of Dark Matter is still one of the big challenges in modern astrophysics. Since its early inference Jan Oort, who found the stars in the local neighbourhood to be moving somewhat fast as compared to the gravitational potential they should give rise to, and by Fritz Zwicky in 1933 who found that galaxies in clusters are apparently moving too fast, this phenomenon has accompanied astronomy and astrophysics ever since. The pioneering work by Vera Rubin and her collaborators on optical (Hα) galaxy rotation curves proved the prevailance of Dark Matter in galaxies, which was later on followed up by the radio (HI) observations, first systematically conducted by Albert Bosma. It was in the mid 90's when the study of Dark Matter experienced kind of a revival: with the advent of numerical simulations of structure formation with Cold Dark Matter (CDM) à la Navarro, Frenk & White (1996) it became possible to juxtapose observations and theory for a critical evaluation - and this is still going on. When fitting an analytical density profile to the resulting density distributions, these cosmological simulations predict a density profile (the so-called 'NFW profile') of the form
Here, ρc is the critical density, δs the characteristic radius, and ρc is a dimensionless constant. On large scales (clusters, superclusters, cosmological filaments), CDM simulations have proven very successful, while on smaller (galaxy) scales, they were faced with a number of problems, namely
1. The cusp problem: the density profiles of simulated galaxies possess a central cusp, according to the above law, while this is not observed in many galaxies, in particular in those with low masses (so-called dwarf galaxies).
2. The angular momentum problem: simulated galaxies come out too small, or have too little angular momentum.
3. The satellite problem: simulated DM halos possess a host of substructure. However, one observes but few satellite galaxies around larger ones (e.g our Milky Way).
Another density profile that appears to fit the observed rotation curves a lot better was worked out by Burkert (1995), who tried to find the best-fitting density law complying with the observed rotation curves of dwarf galaxies, which are known to be dominated by Dark-Matter throughout. The Burkert profile is an empirical law that resembles a pseudol-isothermal halo. In contrast to the CDM profiles, it has a central core and is characterized by the core radius r0 and by the central density ρ0.
The solution of these problems has been sought for in observations, which always suffer from artefacts. They could naturally also be due to the (unknown) properties of the Dark Matter (e.g. finite cross-sections to interactions with baryons, self-annihilation, etc.). It is at this point where the whole subject becomes (at last) exciting: if we are able to map the density profiles of the dark halos of galaxies with sufficient precision, we should be able to contribute significantly to disclose the nature of Dark Matter! This is one of the goals of our investigations. My working group on galaxy structure and kinematics at AIfA currently has the following cast of players: Nady Ben Behkti, Gyula Jozsa (AIfA & ASTRON, Dwingeloo), Philip Schmidt (undergraduate). We are collaborating with Paolo Salucci (SISSA/Trieste), Tom Oosterloo (ASTRON, Dwingeloo), Christian Struve (Univ. Groningen), and Gianfranco Gentile (University of Ghent). In an ongoing study of galaxy kinematics using HI and Halpha rotation curves, we aim at exploring the central density distributions of the dark halos. We need sensitive measurements of the 21-cm HI line, which we achieve using the WSRT and the VLA in the northern and the ATCA in the southern sky. From the observed HI data cubes, i.e. the measured brightness temperature as a function of position and velocity, Tb(ξ,η,v), the rotation curve is derived and may be combined with optical (Hα) or interferometric CO measurements to achieve higher spatial resolution in the central regions. The observed rotation curve delivers a mass decomposition via
In case of a spheroidal distribution of the Dark Matter, the density profile of the dark halo may be obtained via
where the Poisson equation takes its most simple form. Our results so far favour pseudo-isothermal dark halos, rather than distributions predicted by CDM simulations. The example below shows measured rotation curves (Ha and HI combined) of three low-luminosity galaxies. In the upper row, Burkert halos have been fitted, while the lower one exhibits fits top NFW density profiles. The individual lines indicate the total rotational velocity (solid), the dark halo (long-dashed), the stellar (dotted), and the gaseous (short-dashed) component. It is obvious that the former yield a better fit. In particular, the NFW profiles require unrealistic mass-to-light ratios (taken from Gentile et al. 2004).
In the course of our studies, we also aimed at studying the outer peripheries of galaxies. Whatever the amount and property of the Dark Matter, galaxies should eventually exhibit a decline in their rotation curves. Alas, such a decline has not been encountered hitherto, in spite of sporadic claims. In order to derive reliable rotation curves, it is indispensable to deproject the observed velocities. This is an easy thing to do in case of flat disks; all one needs is the position angle and inclination of the galaxy under study. However, nature is more complicated. When studying galaxies out to large distances using the 21-cm line of neutral hydrogen one almost invariably encounters deviations from flat disks, such as warps or lopsidedness. A prominent example of such a warp is visible in outer HI disk of the spiral galaxy NGC5055, as shown in the following picture (from Battaglia et al. 2006; see also Tom Osterloo;'s web pages for spectacular examples of the extent of gaseous disks of galaxies). Even without the illustrating sketch, the warp is readily seen.
Our newly developed code tirific (