Problems for Mond in Clusters and the Lyα Forest

The observed dynamics of gas and stars on galactic and larger scales cannot be accounted for by self-gravity, indicating that there are large quantities of unseen matter, or that gravity is non-Newtonian in these regimes. Milgrom’s MOdified Newtonian Dynamics (MOND) postulates that Newton’s laws are modified at very low acceleration, and can account for the rotation curves of galaxies and some other astrophysical observations, without dark matter. Here we apply MOND to two independent physical systems: Lyα absorbers and galaxy clusters. While physically distinct, both are simple hydrodynamical systems with characteristic accelerations in the MOND regime. We find that Lyα absorbers are more dense and about ten times smaller than in Newtonian gravity with dark matter, in disagreement with sizes inferred from quasar pair studies. The mass density of gas in the absorption systems inferred using number counts of the absorption lines is, however, reasonable. In clusters MOND appears to explain the observed (baryonic) mass-temperature relation. However, given observed gas density and enclosed mass profiles and the assumption of hydrostatic equilibrium, MOND predicts radial temperature profiles which disagree badly with observations. We show this explicitly for the Virgo and Abell 2199 clusters, but the results are general, and seem very difficult to avoid. This result strongly disfavors MOND as an alternative to dark matter. Subject headings: cosmology: theory – gravitation – dark matter – galaxies: clusters: general – intergalactic medium – hydrodynamics