Extended rotation curves from 21 cm observations show that low surface brightness galaxies are highly dark matter dominated. The most extreme examples give very hard upper limits to the ratio of disk to halo mass. These result in baryon fractions of < 10% at the most, and more likely < 3%. This contrasts sharply with measurements of the baryon fractions in clusters of galaxies. These are typically in the range 10% -- 20%, and are thought to be representative of the cosmic mean. This leads to a "baryon catastrophe" because primordial nucleosynthesis does not allow such high baryon fractions if Omega = 1. Interpreting both observations is problematic. Cluster measurements could be wrong, or unrepresentative. Some fraction of dark matter halos could themselves be baryonic. There could be real variation in the baryon fraction from halo to halo, perhaps dependent on scale. Or this could be an indication that an alteration of gravity is the solution of the mass discrepancy problem. None of these possibilities are satisfactory, but one is now required by observation.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1996