The theory of cold dark matter (CDM) has been successful on large scales in explaining, among other things, structure formation in the early Universe. The scale-free numerical simulations describing the predicted behavior of CDM halos produce an 'NFW' halo (Navarro, Frenk & White 1996, 1997). The NFW halo is characterized by a mass density distribution that rises steeply toward the center. This is referred to as a "cuspy" halo. There is some debate among independent groups of simulators over the value of the inner slope; it may rise as r -1 or even more sharply (eg. Moore et al 1999; Klypin et al 2001), but they all agree on the cuspy form.
Low surface brightness (LSB) galaxies have been identified as ideal test subjects for the cuspy halo prediction. LSB galaxies are thought to be dark matter-dominated at all radii (de Blok & McGaugh 1997), with the light simply providing a tracer for the dark matter. The stellar mass contribution is low, which reduces errors involving the uncertainty in the stellar mass-to-light ratio, and in turn, the isolation of the dark matter component.
A number of studies of LSB galaxy dark matter halos using HI 21cm- and Halpha long-slit- derived rotation curves have shown disagreement with the cuspy halo prediction. Many independent observers have found the observations to be more consistent with a halo having a constant density core (eg. de Blok, Bosma & McGaugh 2003; de Blok & Bosma 2002; Marchesini et al 2002). Others have found their data to be adequately described as a cuspy halo (eg. Swaters et al 2003; van den Bosch & Swaters 2001).
The cored halo, though providing a good fit to the data, has no theoretical basis. The challenge to the appropriateness of the cuspy NFW halo, therefore, has not been readily accepted. The CDM model has had many successes and the NFW halo is based on this cosmological theory. As a result, there have been many suggestions of systematic errors which may affect the observations such that cored halos are erroneously inferred.
Beam smearing, or low spatial resolution, in the HI data has been one suggestion. Subsequent long-slit Halpha observations have improved the resolution by a factor of ~10 down to sub-kpc scales. Another suggestion has been slit misplacement in the long-slit Halpha observations. It is possible to miss the signature of a cusp if the slit does not hit the dynamical center of the galaxy. A third commonly suggested systematic error has been non-circular motions. Here again the signature of a cusp may be missed because the presence of non-circular motions may cause the underestimation of the true circular velocity of the gravitational potential.
To address these issues, I am obtaining high resolution two-dimensional velocity fields of a large sample of LSB and low mass dwarf galaxies using the DENSEPAK Integrated Field Unit on the WIYN telescope at Kitt Peak. DENSEPAK provides the necessary sub-kpc resolution of the cores of the galaxies, while simultaneously providing the two-dimensional information which will reveal the dynamical center, the presence of non-circular motions, and a pinching of isovelocity contours characteristic of the cuspy NFW halo, if one is present. There is no concern about slit misplacement in a two-dimensional velocity field. In fact, the two-dimensional velocity fields can be used to reconstruct the slit placement of previous long-slit observations to determine the actual slit placement. The two-dimensional velocity fields will also be used to determine the importance of non-circular motions.
In addition to the WIYN telescope, I am using the Kitt Peak 2.1meter and 4meter telescopes. The 2.1meter telescope is being used to obtain BVRI photometry of the galaxies, which enables mass and stellar population modeling. The long-slit capabilities of the RC Spectrograph on the 4meter telescope are being used to obtain rotation curves for the low mass dwarf galaxies. (The dwarf galaxies have the added bonus of extending the baryonic Tully-Fisher relation to lower masses than currently known).
The goal of this program is to determine the mass distribution of the dark matter halos of a large sample of dark matter-dominated LSB galaxies and low mass dwarf galaxies. By combining the high resolution DENSEPAK two-dimensional velocity fields of the cores of the galaxies with more extended long-slit Halpha and HI 21cm observations, a direct measurement of rho(r) for the dark matter will be obtained. These results will be used to test the predictions of the simulations of CDM halos. If cuspy NFW halos are found, good constraints can be put on halo concentrations and cosmological parameters. The observed velocity fields can also be compared to simulated velocity fields that include highly non-circular motions in order to quantify concerns of systematic errors.
