Figure 1 summarizes the mass discrepancy in various galactic systems. It shows, approximately, the ratio of the dynamical mass, as determine with standard dynamics, to the mass so far accounted for by direct observations. The discrepancy is plotted against some "typical" system radius. (Masses in galactic systems show no sign of saturation with radius, and the value within that "typical" radius is used.) I note, in passing, that there is no correlation of the discrepancy with system size. Remark, in particular, that the small dwarf spheroidals and LSB discs show large discrepancies, while the large galaxy clusters evince only moderate discrepancies. This flies in the face of attempts to explain away the mass discrepancy by modifying gravity at large distances, predicting increase in the "discrepancy" with size. (Contrary to some lingering misconception, MOND is not a modification at large distances, but at low accelerations-which for a given mass are attained at large distances.)
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Figure 1. The mass discrepancy (dynamical mass over detected mass) in various galactic systems plotted against the typical system size. |
The use of MOND dynamics should eliminate the mass discrepancy in all systems. Put differently, MOND predicts the mass discrepancy expected when using Newtonian dynamics. Figure 2 shows the discrepancy plotted now against the typical inverse acceleration-as prescribed by MOND. It also shows the MOND prediction of the discrepancy as a solid line interpolating the value 1 at low a-1 and the predicted discrepancy, a0/a, at a << a0. The positions of the blobs describing the different galactic systems roughly represent detailed work on individual systems: dwarf spheroidals [6] - [9], disc-galaxy rotation curves [10] - [13], galaxy groups [14], x-ray clusters (e.g. [15] - [18]), and large-scale filaments [19]. I expand here on two types of systems.
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Figure 2. The mass discrepancy plotted against the typical system acceleration. |