The tidal stability of Fornax cluster dwarfs in Newtonian gravity and MOND (lecture for St Andrews)
Channel: Indranil Banik
Date: 24/10/2021

Lecture by Elena Asencio, slides are available here:
https://www.dropbox.com/s/9j2o3eg6lyiyzyb/standrews_presentation.pdf?dl=0

Dwarf galaxies are small, faint galaxies that are usually found in galaxy clusters or near larger galaxies. This makes them potentially susceptible to the gravitational effect of these structures, which can disrupt their morphological appearance. The Fornax Deep Survey dwarf galaxy catalog provides us with information about several properties of the dwarfs in the Fornax cluster, including the observed level of morphological disturbance of the dwarfs. This can be used to compare if their theoretically calculated tidal susceptibility matches observations. Dwarfs are not expected to be very susceptible to tides in the standard model (ΛCDM) where galaxies follow Newtonian gravity and have dark matter halos as these halos enhance their self-gravity, which makes the dwarf more resilient to the effect of gravitational tides. However, Milgromian dynamics (MOND) predicts that dwarfs are more easily disturbed by tides despite the boost to gravity provided by MOND. The reason is that this enhancement mimics the effect of a dark matter halo for an isolated dwarf, but the enhancement to gravity gets significantly damped once the gravitational field of a larger object dominates over that of the dwarf (this is known as the external field effect). In this project, we compare the observed morphological disturbance of the dwarfs with their tidal susceptibility in ΛCDM and MOND using different types of analyses. Our results show that the MOND prediction for the level of disturbance of the Fornax dwarfs matches the observations much better than the ΛCDM prediction, which is in significant tension with the observational data.

The results are briefly described in this blog (first link) and press release (second link):
https://darkmattercrisis.wordpress.com/2021/08/02/
https://www.uni-bonn.de/en/news/176-2022

Thanks to everyone who helped us get this paper published in MNRAS:
https://doi.org/10.1093/mnras/stac1765
https://arxiv.org/abs/2208.02265

For a recent review of MOND, see here (first link to a related news article):
https://theconversation.com/dark-matter-our-review-suggests-its-time-to-ditch-it-in-favour-of-a-new-theory-of-gravity-186344
https://arxiv.org/abs/2110.06936