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J. D. Simon (UC Berkeley)
Over the last several years, three crucial shortcomings of the Cold Dark Matter (CDM) model have been discovered on galaxy-size scales. We present new observations addressing one of these problems: the central density problem. We derive rotation curves for four nearby, low-mass spiral galaxies and use them to constrain the shapes of their dark matter density profiles. This analysis is based on high-resolution two-dimensional H\alpha and CO velocity fields, which we use in order to overcome some of the limitations of other rotation curve studies that rely mostly on long-slit spectra or low-resolution HI observations. In combination with our previous study of NGC 2976, the full sample of five galaxies contains dark matter density profiles that span the range from constant-density to steeper than predicted by CDM simulations. The scatter in density profile slopes from galaxy to galaxy is substantially larger than in the simulations, and the mean slope we derive is shallower than predicted. These results call into question the hypothesis that all galaxies share a universal dark matter density profile. In spite of these differences between observations and simulations, the relatively cuspy density profiles we find do not suggest that this problem represents a crisis for CDM. Improving the resolution of the simulations and incorporating additional physics may resolve the remaining discrepancies. We also find that four of the galaxies contain detectable radial motions in the plane of the galaxy. We investigate the hypothesis that these motions are caused by a triaxial dark matter halo, and place lower limits on the ellipticity of the orbits in the plane of the disk of a few percent.
This research was partially supported by the National Science Foundation.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.