Other
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt.........2s&link_type=abstract
PhD dissertation. Proquest Dissertations And Theses 2005. Section 0028, Part 0606 225 pages; [Ph.D. dissertation].United State
Other
1
Dark Matter, Dwarf Galaxies
Scientific paper
Over the last decade, several crucial shortcomings of the Cold Dark Matter (CDM) model have been recognized on galaxy-size scales. In this thesis I present new observations addressing two of these problems: the substructure problem and the central density problem. I describe results from our search for a connection between high-velocity clouds (HVCs) and the low-mass dark matter halos seen in CDM simulations that appear to be missing from the real universe. This survey demonstrates that HVCs do not have stellar counterparts similar to other Local Group dwarf galaxies; if HVCs contain any stars at all, their surface brightnesses must be substantially lower than those of any known dwarfs. I also discuss observations of two candidate dark galaxies in the Local Group: Complex H and HVC 127-41-330. Complex H is a massive, nearby HVC containing ˜2 × 107 Msun of HI and less than ˜3 × 105 Msun of stars, leaving it with an HI mass to light ratio several times higher than any known galaxy. HVC 127-41-330 is a smaller HVC that appears to be interacting with the dwarf galaxy LGS 3, placing it at a distance of 700 kpc. The HVC is rotating, with a dynamical mass that makes it dark matter-dominated for any plausible distance. The existence of these two starless objects suggests that the substructure problem is related to the difficulty of forming stars in low-mass dark matter halos. However, many more dark galaxies must be located in order to resolve the substructure problem.
In addition, I present a rotation curve analysis of five nearby dwarf galaxies, based on high-resolution two-dimensional velocity fields in Halpha and CO. In contrast to previous studies, most of the galaxies in this sample do not seem to have a strong preference for pseudo-isothermal density profiles. Instead, these objects exhibit the full range of dark matter density profiles between constant density and NFW halos. One galaxy, NGC 2976, definitively contains a large constant-density core that presents a significant challenge for CDM models. However, for another galaxy, NGC 5963, the derived density profile has a very steep central slope and is well fit by an NFW profile, confirming that some galaxies do contain cuspy profiles like the ones predicted by CDM. This observing program was designed to overcome some of the limitations of the traditional techniques of long-slit Halpha spectroscopy and low-resolution HI observations. The results demonstrate that there is a distribution of central density slopes rather than the single universal density profile seen in CDM simulations. Four out of the five observed galaxies show evidence for noncircular motions, which are attributed to the triaxiality of their dark matter halos. These motions constrain the ellipticities of the halos in the planes of the disks to be larger than a few percent.
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