Galaxies as simple dynamical systems: observational data disfavor dark   matter and stochastic star formation
  (Abridged) Galaxies are observed to be simple systems but the standard model of cosmology (SMoC) implies a haphazard merger history driven by dynamical friction on dark matter halos. The SMoC is tested here with the results that the dual dwarf galaxy theorem is in contradiction to the data if the SMoC is true, and the action of dynamical friction is not evident in the galaxy population. A consistency test for this conclusion comes from the significantly anisotropic distributions of satellite galaxies. Independently, the long history of failures of the SMoC have the likelihood that it describes the observed Universe to less than 10^-4 per cent. The implication for fundamental physics is that exotic dark matter particles do not exist and that consequently effective gravitational physics on the scales of galaxies and beyond ought to be non-Newtonian/non-Einsteinian. The data imply that scale-invariant dynamics, as discovered by Milgrom, is an excellent description of galaxies in the weak-field regime. Observations of stellar populations in galaxies suggest the galaxy-wide IMF, the IGIMF, to vary with star formation rate and that stochastic descriptions of star formation are inconsistent with the data. A consequence of this understanding of galactic astrophysics is that most dwarf satellite galaxies are formed as tidal dwarf galaxies in galaxy-galaxy encounters, that they follow the mass-metallicity relation, that galactic mergers are rare, that galaxies immersed in external potentials are physically larger than isolated galaxies and that star-forming galaxies form a main sequence. Eight predictions are offered which will allow the concepts raised here to be tested. A very conservative, cold- and warm-dark-matter-free cosmological model may be emerging from these considerations. 
Pavel Kroupa (Bonn)
astro-ph.GA astro-ph.CO