Pavel Kroupa: Satellite Galaxies
Originally pointed out by
Kroupa, Theis & Boily (2005, MNRAS, The great disk of Milky-Way
satellites and cosmological sub-strutures), the distribution
of satellite galaxies around the Milky Way in a huge
disk-of-satellites (DoS), stands as
perhaps the greatest challenge for dark-matter based
Today we know that the DoS is part of the vast polar structure (VPOS) which includes all
satellite galaxies, young halo globular clusters and a large fraction
of stellar and gas streams.
The only physically viable interpretation of this structure is for
the satellite galaxies to be ancient tidal dwarf galaxies (TDGs),
formed together with star clusters in a tidal tail when the 2-4 Gyr
old Milky Way had an encounter with the Andromeda galaxy, as
calculated by Zhao et al. (2013). An alternative scenario may
be that a merger occurred in Andromeda about 5 Gyr ago and the tidal
tail thrown out then together with the TDGs and star clusters that
formed in it has swept past the Milky Way ( Hammer et al. 2013 ). Andromeda must be involved,
because the DoS around Andromeda and the DoS of the Milky Way are
strongly correlated ( Pawlowski et al. 2013, Dwarf galaxy planes: the
discovery of symmetric structures in the Local Group ).
An example of this on-going in the present-day Universe is seen
nicely in the the Tadpole galaxy and has been described by Pawlowski et al. (2011, A & A) for the first time.
But if this is the case, then the satellite galaxies cannot contain
dark matter. However, they appear to be dominated by dark
matter. The only way to reconcile this contradicting evidence is to
accept non-Newtonian gravitation such that the classical generalised
gravitational law equals Newton's, but effectively becomes stronger
below a critical gradient in the effective potential. MilgrOmiaN Dynamics (MOND) stands as the currently
best available such theory.
The Vast Polar Structure (VPOS) of the Milky Way
VPOS: Pawlowski et al. (2012)
Video of the VPOS including all currently (April 2012) known satellite
galaxies plus all young halo globular clusters plus all known stellar
and gaseous streams (from
Pawlowski, Pflamm-Altenburg & Kroupa, 2012,
MNRAS). The centrtal blue line in the video is the Milky
Way seen edge-on. The grey equatorial regions indicate the zones which
are obscured by the disk of the Milky Way.
The VPOS extends from at least about 10 kpc from the
Galactic centre to probably beyond 250 kpc. It has a
thickness/diameter ratio of about 1:10.
The existence of this vast phase-space correlated structure
constitutes the observational falsification of the satellite galaxies
being individual dark-matter dominated primordial satellites that
individually fell-in to the Milky Way dark matter halo (see e.g.
Pawlowski et al. 2014, MNRAS and
Pawlowski et al. 2015, ApJ). But because every major galaxy such
as the Milky Way must have a substantial number such
dark-matter-dominated satellite galaxies in the standard
dark-matter-based cosmological model, the DoS falsifies this model of
It is because the DoSs or the VPOS or the planes of satellites have
such very deep implications for theoretical physics that the mere
existence of these structures is being challenged heavily by the
The Disk of Satellites of the Milky Way
The Disk of Satellites including all currently (July 2010) known
satellites. See Fig.4 in
Kroupa et al. (2010, A & A). Here, the upper panel shows the
fitted edge-on DoS with dashed lines indicating 1xDelta_min (rather
than 1.5xDelta_min in Fig.4).
A movie of the whole vast polar structure (VPOS), of which the here
shown satellites are only a part of the population, is available
Helmut Jerjen: The Stromlo Milky Way Satellite Survey
SMS program is a critical endeavor to investigate whether the
predictions of standard cold dark matter cosmology are consistent with
the observed matter distribution in the Milky Way halo.
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