Type I Shell Galaxies as a Test of Gravity Models

Abstract

Shell galaxies are understood to form through the collision of a dwarf galaxy with an elliptical galaxy. Shell structures and kinematics have been noted to be independent tools to measure the gravitational potential of the shell galaxies. We compare theoretically the formation of shells in Type I shell galaxies in different gravity theories in this work because this is so far missing in the literature. We include Newtonian plus dark halo gravity, and two non-Newtonian gravity models, MOG and MOND, in identical initial systems. We investigate the effect of dynamical friction, which by slowing down the dwarf galaxy in the dark halo models limits the range of shell radii to low values. Under the same initial conditions, shells appear on a shorter timescale and over a smaller range of distances in the presence of dark matter than in the corresponding non-Newtonian gravity models. If galaxies are embedded in a dark matter halo, then the merging time may be too rapid to allow multi-generation shell formation as required by observed systems because of the large dynamical friction effect. Starting from the same initial state, in the dark halo model the observation of small bright shells should be accompanied by large faint ones, while for the case of MOG, the next shell generation patterns iterate with a specific time delay. The first shell generation pattern shows a degeneracy with the age of the shells and in different theories, but the relative distance of the shells and the shell expansion velocity can break this degeneracy.

From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity

“…velocity would drop substantially between pericentres (Vakili et al 2017). A more uniform distribution of shell radii would be expected in the absence of CDM, which is broadly more in line with observations of NGC 3923 (Bílek et al 2016).…”

“…The use of shells to test MOND was discussed further in Bílek et al (2015a) and Vakili et al (2017), with the latter authors also considering MOG (subsequently falsified in Figure 11). In general, the strong dynamical friction expected in the ΛCDM scenario suggests that large faint shells should be accompanied by small bright shells because the launch Table 2.…”

MOND simulation suggests an origin for some peculiarities in the Local Group

“…When integrated backwards, the orbits of the Large and Small Magellanic Clouds were close to pericenter almost when the MW and M 31 were at the pericenter. Dynamical friction during encounters of comparable galaxies is known to be weaker in MOND than in ΛCDM from the simulations by ; Nipoti et al (2008); Combes (2016) (see also, Renaud et al 2016or Vakili et al 2017 because of the absence of the large and massive dark matter halos (Kroupa 2015). A close MW-M 31 encounter in MOND would therefore be more likely to avoid ending in a merger than in ΛCDM.…”

Detailed Solar System dynamics as a probe of the Dark Matter hypothesis

“…Dynamical friction considerations have often been used to constrain dark matter properties, such as dark halo density profiles through requiring the survival of observed globular clusters in dwarf spheroidal galaxies (e.g. Hernandez & Gilmore 1998, Goerdt et al 2006, the consistency of the observed morphology of shell galaxies in dark matter models (Vakili et al 2017), the merger dynamics of forming galaxies (Peñarrubia et al 2002, Kroupa 2015 or the survival of ultra-faint dwarf galaxies (Hernandez 2016). In terms of future tests, Pani (2015) showed that dynamical friction on binary pulsars detected close to the galactic centre could also yield interesting restrictions.…”