A radically new view of dwarf galaxies surrounding the Milky Way

It has long been considered that the dwarf galaxies of the Milky Way halo were ancient satellites orbiting our Galaxy for nearly 10 billion years.

This required them to contain huge amounts of dark matter to protect them from the enormous tidal effects due to the gravitational pull of our Galaxy. The large spreads in observed velocities observed in almost all dwarf galaxies were thought to require the presence of this dark and invisible matter.

The latest Gaia data has now revealed a completely different view of dwarf galaxy properties.

___________

See also the news
on the website of the Leibniz Institute for Astrophysics Potsdam (AIP) :
"A radically new view on dwarf galaxies surrounding the Milky Way"

___________

The international team was able to date the history of the Milky Way interactions with other galaxies, thanks to the relationship that connects the orbital energy of an object to its epoch of entry into the halo [1]. The orbital energies of dwarf galaxies are three times larger than that of the Sagittarius dwarf galaxy that entered the halo 5 to 6 billion years ago. This implies that most dwarf galaxies arrived much more recently, less than three billion years ago.

Such a recent arrival implies that the nearby dwarfs come from outside the halo, where all dwarf galaxies are filled with neutral gas. These gas-rich galaxies lost their gas when it collided with the hot gas of the Galactic halo (see video).

The violence of shocks and turbulence in the process completely changes the dwarf galaxy. While the gas-rich dwarf-galaxy-kinematics were dominated by the rotation of gas and stars, as they are transformed into gas-free systems their gravity is balanced by the random motions of their remaining stars. Dwarf galaxies lose their gas in a process so violent that it puts them out of equilibrium. The combined effects of gas loss and gravitational shocks due to the Galaxy nicely explains the wide spread of their observed velocities.

What role for dark matter ?

One of the curiosities of this study is the role of dark matter. First, the absence of an equilibrium prevents any estimation of the dynamical mass of the Milky Way and its dark matter content. Second, while in the previous scenario dark matter protected the supposed stability of dwarf galaxies, it becomes rather awkward for objects out of balance. In fact, if too much dark mass were added, it would stabilize the initial rotating disk of stars in the dwarf galaxy preventing its transformation into a galaxy supported by random stellar motions as observed.

The recent arrival of dwarf galaxies and their transformations in the halo explains well many observed properties of these objects, in particular why they have stars at large distances from their center. Their properties seem compatible with an absence of dark matter, contrary to our previous understanding of them as the most dark-matter dominated objects.

References

The article ‘The accretion history of the Milky Way. II. Internal kinematics of globular clusters and of dwarf galaxies’ par François Hammer, Jianling Wang Gary A. Mamon, Marcel S. Pawlowski, Yanbin Yang, Yongjun Jiao, Hefan Li, Piercarlo Bonifacio, Elisabetta Caffau, and Haifeng Wang is published in MNRAS, 20 November 2023.
DOI : doi.org/10.1093/mnras/stad2922
It can be also found at : https://arxiv.org/abs/2311.05677

Numerical simulations are described in :
Jianling Wang, François Hammer, Yanbin Yang, Marcel S. Pawlowski, Gary A. Mamon, and Haifeng Wang, ‘III. Hydrodynamical Simulations of Galactic Dwarf Galaxies at First Infall’, 2023, accepted at MNRAS, https://arxiv.org/abs/2311.05687