The tidal stability of Fornax cluster dwarf galaxies in Newtonian and Milgromian gravity
Channel: Indranil Banik
Date: 2021-07-24

Dwarf galaxies are characterised by their small size, low mass, and low surface brightness. This makes them ideal for studying the effects of gravitational interactions on galaxies, as their structure is more susceptible to tidal forces than larger galaxies. In turn, this also makes them very useful for comparing and testing different gravity models, since the expected degree of disturbance in the dwarf structure is highly reliant on the gravity law assumed and on the model-dependent properties of the dwarf (e.g. dark matter halo). In this project we use the recent Fornax Deep Survey Dwarf galaxy Catalog (FDSDC) to compare the observed properties of the dwarf galaxies in the Fornax cluster with their predicted properties in the standard model of cosmology (ΛCDM) and Milgromian dynamics (MOND). Assuming that a dwarf will be disturbed if its half mass radius is of the same order as its tidal radius, we proceed by calculating the ratio of these two radii (tidal susceptibility) for the dwarfs in the FDSDC catalog – both in ΛCDM and MOND – and comparing it with their observed morphological classification (disturbed/undisturbed). From this quick test, we can already intuit that the required value in ΛCDM for the tidal susceptibility threshold for disturbance is significantly lower than expected. Next, we construct a forward model of the system – a test mass simulation of the orbits of the Fornax dwarfs – and, using the MCMC method, we fit it to the real Fornax system based on the observed parameters: projected position, distribution of tidal susceptibility, and fraction of disturbed dwarfs. This allows us to have a better constraint on the tidal susceptibility value at which a dwarf galaxy starts to look disturbed and the tidal susceptibility value at which it gets destroyed, according to ΛCDM and MOND. Finally, we perform N-body simulations of a Fornax dwarf in MOND using the Phantom of Ramses (PoR) code to check the actual tidal susceptibility threshold above which a dwarf galaxy should be destroyed in this model. Our results show that, in MOND, the dwarf tidal stability limit inferred from observations (by MCMC) is in agreement with that inferred from N-body simulations. While, for ΛCDM, the dwarf stability limit inferred from observations is significantly lower than that predicted by the ΛCDM N-body simulations of dwarf galaxies performed in the last years.

Slides of the talk (5MB pdf):
https://www.dropbox.com/s/eucyofeu70f1kvy/ESO_presentation.pdf?dl=0