  Kinematically cold tidal streams of globular clusters (GC) are excellent
tracers of the Galactic gravitational potential at moderate Galactocentric
distances, and can also be used as probes of the law of gravity on Galactic
scales. Here, we compare for the first time the generation of such streams in
Newtonian and Milgromian gravity (MOND). We first compute analytical results to
investigate the expected shape of the GC gravitational potential in both
frameworks, and we then run N-body simulations with the Phantom of Ramses code.
We find that the GCs tend to become lopsided in MOND. This is a consequence of
the external field effect which breaks the strong equivalence principle. When
the GC is filling its tidal radius the lopsidedness generates a strongly
asymmetric tidal stream. In Newtonian dynamics, such markedly asymmetric
streams can in general only be the consequence of interactions with dark matter
subhalos, giant molecular clouds, or interaction with the Galactic bar. In
these Newtonian cases, the asymmetry is the consequence of a very large gap in
the stream, whilst in MOND it is a true asymmetry. This should thus allow us in
the future to distinguish these different scenarios by making deep observations
of the environment of the asymmetric stellar stream of Palomar 5. Moreover, our
simulations indicate that the high internal velocity dispersion of Palomar 5
for its small stellar mass would be natural in MOND.

Astrophysics of Galaxies