Pavel Kroupa: Satellite Galaxies
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Dark matter, cosmology and progress
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 cosmological models becasue baryonic physics such as stellar feedback or AGN activity have no influence on these scales (10s-100skpc). Of the five nearest major galaxies (Milky Way, Andromeda, M81, CenA, M83) more than three have such disks of satellites. For a review see The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open Questions (Pawlowski, 2018, MPLA, in press). The discprepancy with the LCDM model is quantified in The Dark Matter Crisis . It is well above 5sigma.
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. We also know that all the galaxies in the Local Group of galaxies are arranged in a very unexpected highly symmetrical form Dwarf galaxy planes: the discovery of symmetric structures in the Local Group (Pawlowski, Kroupa & Jerjen, 2013, MNRAS). This symmetrical structure of the Local Group appears in major conflict with any dark-matter-based structure formation model.
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, Famaey, Lueghausen & Kroupa (2013). This line-of-thought is now being actively researched: MOND simulation suggests the origin of some peculiarities in the Local Group (Bilek, Thies, Kroupa & Famaey, 2018, A&A, in press) and Origin of the Local Group satellite planes in Modified Newtonian Dynamics (Banik, O'Ryan & Zhao, 2018, MNRAS, submitted).
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 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
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 cosmology.
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 dark-matter community.
The Disk of Satellites of the Milky Way
A movie of the whole vast polar structure (VPOS), of which the here shown satellites are only a part of the population, is available above.