Title: Modified Newtonian Dynamics and gas expulsion in star clusters
Time: 2018.07.23（Monday） 14:30-16:30
Place: Physics Building 552, Haiyun Campus, Xiamen University
Abstract: The observations of the distant globular cluster, NGC 2419, imply an extremely radial anisotropy profile for its velocity dispersion in Newtonian dynamics, whose physical origin remains unknown. The gravitational potential of a young star cluster becomes shallower after undergoing sudden gas expulsion, and a large number of stars end up moving along elongated orbits. We study the kinematics of stars in clusters undergoing gas expulsion in standard Newtonian dynamics and also in Milgromian dynamics (MOND). Gas expulsion can successfully explain the velocity dispersion profile of NGC 2419 in Newtonian dynamics. The velocity dispersion of a star cluster in the outer halo of a galaxy can only have a strong radial anisotropy profile in Newtonian dynamics after gas expulsion. Comparing to the case of Newtonian gravity, MOND displays several different properties. In particular, the slope of the central velocity dispersion profile is less steep in MOND for the same SFE. Gas expulsion alone is unlikely to be the physical mechanism for the observed velocity dispersion profile of NGC 2419 in MOND. Moreover, to leave a bound star cluster, the star formation efficiency (SFE) of an embedded cluster dominated by deep MOND gravity can be reduced down to 2.5%. For a given SFE, the star clusters that survive in MOND can bind a larger fraction of mass compared to those of the Newtonian dynamics. The more diffuse the embedded cluster is, the less substantial the size expansion of the final star cluster is. Our results may help to understand the low concentration and extension of the distant low-density globular clusters and ultra-faint and diffuse satellite galaxies around the Milky Way.