CASSA Colloquium 5: Lower Dark Matter in the Local Group Galaxies and the Origin of the MW Dwarfs

Speaker:

Istiak Akib, PhD candidate, Laboratoire d’Instrumentation et de Recherche en Astrophysique (LIRA), Observatoire de Paris, Paris Sciences et Lettres University (PSL).

Abstract:

Fitting the flat rotation curves of the Galaxies indicate vast majority of the matter content in the galaxies is dark matter, even higher than the cosmological estimate of 85% and giving rise to the missing baryon problem. However, these rotation curve fits assume equilibrium conditions at the disk outskirts. This is not necessarily always fulfilled since on average galaxies have gone through a major merger 6 Gyr ago. For the MW, the last major merger was 9-10 Gyr ago and the disk outskirts had enough time to reasonably fulfill the equilibrium conditions. Gaia DR3 rotation curve for the MW differs significantly from a flat fit and is consistent with a Keplerian decline. This gives a mass of $2.06\times {10}^{11}$ M${}_{\odot }$, resulting in a dark matter fraction of 70%. Another estimate for the galaxy mass comes from comparing the time frame of the bulge formation and the accretion of the satellites with their binding energies. For the MW, this is in agreement with the mass from the rotation curve. Andromeda Galaxy (M31) had a recent 2-3 Gyr major merger and hence the disk outskirts are likely not in equilibrium. Hydrodynamical modeling of this merger resulting in an M31 of mass $4.5\times {10}^{11}$ M${}_{\odot }$ and 68% dark matter reproduces the observational features of the M31 and especially its rotation curve. These M31 merger models indicate tidal tails coming towards the MW with significant stellar and gas particles. Part of these tails is found in agreement in the 6D space with most of the VPOS (Vast POlar Structure) satellites including the LMC for the case of such low mass MW models. This may explain the origin of the MW’s plane of satellites and is a first indication of matter exchange between these two galaxies.

Relevant papers:

1. Jiao et al 2023: Detection of the Keplerian decline in the Milky Way rotation curve.
2. Hammer et al 2024a: The Milky Way accretion history compared to cosmological simulations.
3. Hammer et al 2024b: Dark matter fraction derived from the M31 rotation curve.
4. Akib et al 2025: An intriguing coincidence between the majority of the VPOS dwarfs and a recent major merger at the M31 position (in revision at A&A).
5. Jiao et al 2025: Dark matter mass range in the Milky Way derived from the rotation curve (in preparation).