The kinematics of star clusters undergoing gas expulsion in Newtonian
  and Milgromian dynamics
  We study the kinematics of stars in clusters undergoing gas expulsion in
standard Newtonian dynamics and also in Milgromian dynamics (MOND). Gas
expulsion can explain the observed line-of-sight (LoS) velocity dispersion
profile of NGC 2419 in Newtonian dynamics. For a given star formation
efficiency (SFE), the shapes of the velocity dispersion profiles, which are
normalised by the velocity dispersion at the projected half-mass radius, are
almost indistinguishable for different SFE models in Newtonian dynamics. The
velocity dispersion of a star cluster in the outer halo of a galaxy can indeed
have a strong radial anisotropy in Newtonian dynamics after gas expulsion. MOND
displays several different properties from Newtonian dynamics. In particular,
the slope of the central velocity dispersion profile is less steep in MOND for
the same SFE. Moreover, for a given SFE, more massive embedded cluster models
result in more rapidly declining central velocity dispersion profiles for the
final star clusters, while less massive embedded cluster models lead to flatter
velocity dispersion profiles for the final products. The onset of the
radial-orbit instability in post-gas-expulsion MOND models is discussed. SFEs
as low as a few percent, typical of molecular clouds, lead to surviving
ultra-diffuse objects. Gas expulsion alone is unlikely the physical mechanism
for the observed velocity dispersion profile of NGC 2419 in MOND.
