Local-Group tests of dark-matter concordance cosmology

Abstract

Predictions of the concordance cosmological model (CCM) of the structures in the environment of large spiral galaxies are compared with observed properties of Local Group galaxies. Five new, most probably irreconcilable problems are uncovered: 1) A wide variety of published CCM models consistently predict some form of relation between dark-matter-mass and luminosity for the Milky Way (MW) satellite galaxies, but none is observed.2) The mass function of luminous sub-haloes predicted by the CCM contains too few satellites with dark matter (DM) mass ≈10 7 M within their innermost 300 pc than in the case of the MW satellites.3) The Local Group galaxies and data from extragalactic surveys indicate there is a correlation between bulge-mass and the number of luminous satellites that is not predicted by the CCM. 4) The 13 new ultra-faint MW satellites define a disc-of-satellites (DoS) that is virtually identical to the DoS previously found for the 11 classical MW satellites, implying that most of the 24 MW satellites are correlated in phase-space. 5) The occurrence of two MW-type DM halo masses hosting MW-like galaxies is unlikely in the CCM. However, the properties of the Local Group galaxies provide information leading to a solution of the above problems. The DoS and bulge-satellite correlation suggest that dissipational events forming bulges are related to the processes forming phase-space correlated satellite populations. These events are well known to occur since in galaxy encounters energy and angular momentum are expelled in the form of tidal tails, which can fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star clusters. If Local Group satellite galaxies are to be interpreted as TDGs then the substructure predictions of the CCM are internally in conflict. All findings thus suggest that the CCM does not account for the Local Group observations and that therefore existing as well as new viable alternatives have to be further explored. These are discussed and natural solutions for the above problems emerge.

Making counter-orbiting tidal debris

“…Still, not only the observed number of Milky Way satellite galaxies is in conflict with the expectation, they also do not match the dark-matter mass -luminosity relation nor the form of the mass function of luminous dark matter halos (Kroupa et al 2010). The distribution is exceedingly anisotropic, too (e.g., Lynden-Bell 1976;Majewski 1994;Hartwick 2000).…”

“…The details of this origin, in contrast, are still disputed and not even the nature of the dominant dark matter component in dSphs is well defined (Gilmore et al 2007). However, Zwicky's (1956) proposition that new dwarf galaxies form when galaxies interact and his suggestion (Zwicky 1937) that galaxies contain dark matter today lead to the Fritz Zwicky Paradox (Kroupa et al 2010) since the implied standard cosmological model leads to a large population of tidal dwarf galaxies (TDGs) that ought to have the properties of dE and dSph galaxies (Okazaki & Taniguchi 2000). This would leave little room for the existence of colddark-matter dominated satellite dwarf galaxies, clashing with the expected existence of large numbers of such objects.…”

“…As Kroupa et al (2010) show, the standard CDM model (e.g. Libeskind et al 2009) can at best reproduce 0.4 percent of all MW-type dark matter halos having a host galaxy of similar luminosity as the MW with 11 satellites in a DoS.…”

Star formation triggered by galaxy interactions in modified gravity

“…Gentile et al 2007;Lelli et al 2015;Flores et al 2016) and regarding the potential origin of satellite galaxies sitting on satellite planes in the context of MOND, which could actually be old TDGs instead of primordially formed dwarf galaxies (e.g. Kroupa et al 2010;Kroupa 2015).…”

Distant star clusters of the Milky Way in MOND

“…Even though the dark matter hypothesis has successfully explained the internal dynamics of galaxy clusters, gravitational lensing, and the standard model of cosmology within the framework of general relativity (GR) (Spergel 2003), much experimental effort has failed to yield a detection of dark matter particles. Moreover the results of high-resolution simulations of structure formation do not reproduce some observations on galactic scales, such as the central structures of rotation curves, the prevalence of low bulgeto-disc ratios, and the numbers and spatial distribution of the subhalos (Klypin et al 1999;Moore et al 1999;Metz et al 2008;Kroupa et al 2010). Even the ability of the dark matter theory to account for the Tully-Fisher and Freeman relations is controversial (Bosch & Dalcanton 2000;Governato et al 2010).…”