Modified Newtonian Dynamics in the Milky Way

Abstract

Both microlensing surveys and radio-frequency observations of gas flow imply that the inner Milky Way is completely dominated by baryons, contrary to the predictions of standard cold dark matter (CDM) cosmology. We investigate the predictions of the Modified Newtonian Dynamics (MOND) formula for the Galaxy given the measured baryon distribution. Satisfactory fits to the observationally determined terminal-velocity curve are obtained for different choices of the MOND's interpolating function mu(x). However, with simple analytical forms of mu(x), the local circular speed v_c(R_0) can be as large as 220 km/s only for values of the parameter a_0 that are excluded by observations of NGC 3198. Only a numerically specified interpolating function can produce v_c(R_0)=220 km/s, which is therefore an upper limit in MOND, while the asymptotic velocity is predicted to be v_c(infty)=170+-5 km/s. The data are probably not consistent with the functional form of mu(x) that has been explored as a toy model in the framework of Bekenstein's covariant theory of gravity.