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Dancing with giants: dynamics of dwarf satellite galaxies

Periodic Reporting for period 1 - DancingGalaxies (Dancing with giants: dynamics of dwarf satellite galaxies)

Reporting period: 2019-05-01 to 2021-04-30

The goal of the project was to determine what the population of satellite galaxies of the Milky Way tells us about the assembly history of our galaxy and also about the nature of dark matter. This was motivated by previous observations that the Milky Way satellites are in tension with the predictions of the standard cosmological model. The Milky Way system is the one we have the largest amount of data for and likely encodes critical information for answering two fundamental problems in physics: what is the nature of dark matter and dark energy?, the two major components of the Universe.

The project led to the most up-to-date measurement of the dark matter distribution in our galaxy, from the inner few kiloparsecs up to the edge of the halo, at roughly 220 kiloparsecs. This has been possible by using state-of-the-art hydrodynamical simulations to design models for the distribution of dark matter when accounting for galaxy formation physics. It also showed that the Milky Way dark matter halo is twisted, that is it experiences a sudden 90 degrees reorientation at the edge of the Milky Way stellar disc. Such twists are typically produced due to a reorientation in the past filaments that feed the Milky Way and only a small fraction of haloes in the standard cosmological model experience such a process. It highlights that our galaxy is a special place with its own unique formation history.
Work was carried on two fronts: (i) what is the Milky Way assembly history?, and (ii) what is the dark matter distribution within our galaxy.

Firstly, I have shown that the plane of satellites of the Milky Way indicates that its satellites were accreted in a very narrow plane, much narrower than expected for an average system of the Milky Way’s mass. The same highly anisotropic accretion is also expected for the dark matter halo, which is more flattened than the average dark halo (Shao, Cautun, et al 2019, 2021).

Secondly, I have developed a model for describing the dark matter distribution in galaxies such as our own. This is based on the results of several state-of-the-art hydrodynamical simulations, such as the EAGLE, AURIGA, and APOSTLE projects. The model can predict with high accuracy the dark matter profile of many halos (Cautun et al 2020; Callingham, Cautun, et al 2020). I then used this model to constraint the dark matter profile of our galaxy from its inner regions up to the edges of the Milky System (Cautun et al 2020). I showed how inaccuracies in previous models led to biased results and an inability to match rotation curve measurements with those based on satellite galaxies and globular clusters.

I used the population of Milky Way satellites to rule out dark matter models in which the dark matter particle is too light (Newton, Cautun, et al 2020). Such models do not produce enough substructure to host all the observed Milky Way satellites and can be ruled out as unphysical. The study provides the tightest constraints on the dark matter particle mass from all available astrophysical probes (Enzi, …, Cautun, et al 2020).

I also supervised five MSc students on various projects related to the Milky Way satellites, such as what are the orbits of the satellites in the presence of a massive Large Magellanic Cloud, using deep learning to determine the accretion times of the Milky Way satellites and implications for star-formation quenching, and what are the filaments and their dwarf galaxy populations around Milky Way analogues.

I presented these results at the following international conferences and meetings:
The Cosmic Web: From Galaxies to Cosmology (Edinburgh, UK, June 17-19 2019)
Small Galaxies, Cosmic Questions (Durham, UK, July 29 - August 2 2019)
Gaia Treasure Hunt (Cambridge, UK, September 3 - 5 2019)
Galaxy Angular Momentum Alignment (Shanghai, China, October 21 - 25 2019)
The Local Dark Matter Distribution (Durham, UK, December 3 2019)
Virgo Consortium for Computational Cosmology (Durham, UK, January 7 - 10 2020)
The Cosmic Web in the Local Universe (Leiden, the Netherlands, January 27 - 31 2020)
European Astronomical Society Annual Meeting (virtual meeting, June 29 - July 3 2020)
Streams 2021: Constraints on Dark MatterWorkshop (virtual meeting, February 22 - 26 2021)
My work has shown that the satellite distribution encodes plenty of information about the mass distribution and the assembly of its host halos. In particular, have revealed for the first time that the our galaxy has experience a larger degree of anisotropic accretion than expected for a typical galaxy of its mass and that the dark matter halo is twisted. This finding has been corroborated by recent studies and is a crucial ingredient for modelling the stellar halo, globular clusters, and satellite galaxies of the Milky Way.

I have also provided a physically motivated dark matter profile of the Milky Way that, for ease of use within the community, has been publicly released and implemented in several popular codes used in the community. This has been widely adopted and resulted in a highly cited paper.

My work has also provided the tightest to date constraints on the dark matter particle mass, ruling out a large range of possible dark matter models. This is especially useful for particle physicists by providing which models pass an ever more stringent set of astrophysical tests.
The total mass profile of the Milky Way as found in Cautun et al (2020).
The shape of the twisted dark matter halo in one of the Milky Way-analogues.