The Milky Way Halo

Dark matter haloes and their subhaloes constitute the fundamental building blocks of the Lambda Cold Dark Matter (CDM) paradigm. Structures assemble hierarchically through the accretion of lower mass constituents; thus, the halo of the Milky Way is littered with lower mass subhaloes and stripped subhalo material due to interactions with the Galactic tidal field. Subhaloes hosting luminous galaxies are likened to the dwarf satellites that orbit the Milky Way, whereas the stellar remains from destroyed dwarfs comprise the bulk of the stellar halo. By studying tracer populations in the Milky Way halo, such as the satellite galaxies, halo stars and globular clusters, we are able to address the following fundamental questions:

• How did the Milky Way halo form and evolve?
• How massive is the Milky Way? How is the dark matter distributed?
• Do the properties of the Milky Way's satellite galaxies agree with the predictions of the CDM model?
• How did the first galaxies form? Where are they now in the Milky Way?
• How does reionization shape the properties of the dwarf galaxies and the lowest metallicity halo stars?

At the ICC we use state-of-the-art cosmological simulations and observational survey data to answer these questions. Below we briefly describe the main simulations and survey data that we use.

Group Members:

Staff - Dr Alis Deason, Prof. Carlos Frenk

Postdocs - Dr Azadeh Fattahi, Dr Kyle Oman, Dr Shi Shao

Students - Tom Callingham, Tilly Evans, Anna Genina

Selected Recent publications:

• Deason, Alis J, Belokurov, Vasily & Sanders, Jason L (2019). The total stellar halo mass of the Milky Way. Monthly Notices of the Royal Astronomical Society 490(3): 3426-3439. abstract
• Genina, Anna, Frenk, Carlos S, Benitez-Llambay, Alejandro, Cole, Shaun, Navarro, Julio F, Oman, Kyle A & Fattahi, Azadeh (2019). The distinct stellar metallicity populations of simulated Local Group dwarfs. Monthly Notices of the Royal Astronomical Society 488(2): 2312-2331. abstract
• Shao, Shi, Cautun, Marius & Frenk, Carlos S (2019). Evolution of galactic planes of satellites in the eagle simulation. Monthly Notices of the Royal Astronomical Society 488(1): 1166-1179. abstract
• Deason, Alis J, Fattahi, Azadeh, Belokurov, Vasily, Evans, N Wyn, Grand, Robert J J, Marinacci, Federico & Pakmor, Rüdiger (2019). The local high-velocity tail and the Galactic escape speed. Monthly Notices of the Royal Astronomical Society 485(3): 3514-3526. abstract
• Callingham, Thomas M, Cautun, Marius, Deason, Alis J, Frenk, Carlos S, Wang, Wenting, Gómez, Facundo A, Grand, Robert J J, Marinacci, Federico & Pakmor, Ruediger (2019). The mass of the Milky Way from satellite dynamics. Monthly Notices of the Royal Astronomical Society 484(4): 5453-5467. abstract
• Fattahi, Azadeh, Belokurov, Vasily, Deason, Alis J, Frenk, Carlos S, Gómez, Facundo A, Grand, Robert J J, Marinacci, Federico, Pakmor, Rüdiger & Springel, Volker (2019). The origin of galactic metal-rich stellar halo components with highly eccentric orbits. Monthly Notices of the Royal Astronomical Society 484(4): 4471-4483. abstract
• Cautun, Marius, Deason, Alis J, Frenk, Carlos S & McAlpine, Stuart (2019). The aftermath of the Great Collision between our Galaxy and the Large Magellanic Cloud. Monthly Notices of the Royal Astronomical Society 483(2): 2185-2196. abstract
• Oman, Kyle A, Marasco, Antonino, Navarro, Julio F, Frenk, Carlos S, Schaye, Joop & Benitez-Llambay, Alejandro (2019). Non-circular motions and the diversity of dwarf galaxy rotation curves. Monthly Notices of the Royal Astronomical Society 482(1): 821-847. abstract

Recent press releases:

• Milky Way heading for catastrophic collision
• Physicists reveal oldest galaxies
• Major impact with 'Sausage' galaxy changed the Milky Way

Simulations of Milky Way mass galaxies

• EAGLE (Evolution and Assembly of GaLaxies and their Environments) is one of the largest hydrodynamical simulations ever produced. The ``reference'' box is 100 Mpc in size and has a dark matter particle resolution of 10⁷M_☉. This huge volume allows us to select a large sample (N ~ 1000) of Milky Way-mass haloes, and study their bulk properties.
• APOSTLE (A Project Of Simulating The Local Environment) is a suite of 12 zoom-in hydrodynamical simulations of Local Group pairs. The pairs were selected to be broadly consistent with the properties of the Milky Way-M31 system. The resulting suite has 24 Milky Way/M31 mass haloes, with roughly 100 times better mass resolution than the reference EAGLE volume. This allows us to resolve dwarf galaxies down to M* = 10⁵ M_☉, which is roughly the ultra-faint dwarf galaxy regime.

  An example APOSTLE volume. The top left shows star light emitted from the galaxies that have formed, with the two large spiral galaxies similar to the Milky Way and Andromeda. The bottom right fades into the distribution of dark matter in the same simulation region, and shows the vast number of dark matter halos predicted to exist within the Local Group. Credit T. Sawala

• Auriga:
  The Auriga suite consists of 40 isolated Milky Way mass haloes. The haloes were selected from the EAGLE reference box and re-simulated using the Arepo MHD code. These simulations have similar mass resolution to the APOSTLE suite, but the hosts are in a different environment (Isolated vs. Paired) and were run using a different code.
• COCO/COLOR and Galform:
  The Copernicus Complexio (COCO) suite of simulations is a set of cosmological zoom-in simulations that follow about 12 billion high-resolution dark matter particles, each of mass 10⁵M_☉. COCO is roughly 24 Mpc in radius and was extracted from the 100 kpc parent volume, called COLOR (Copernicus Complexio Low Resolution). These simulations are dark matter only, but our team have used the Durham semi-analytic Galform model to populate the dark matter subhaloes with galaxies.

Observational Data

• Gaia:
  Gaia will chart an unprecedented 3 dimensional map of the Milky Way by measuring the positions, parallaxes, and proper motions of about one billion stars in the Galaxy. This ambitious space astrometry mission will provide a crucial tool to study the formation and evolution of the Milky Way, and marks a new era in Galactic astronomy.

  One of the most striking discoveries from Gaia: Evidence for a massive accretion event around 10 Gyr ago. A population of halo stars on highly radial orbits was found in the Gaia data, resulting from the head-on collision between a massive dwarf galaxy and the Milky Way. Left panel: The stars of the Sausage galaxy form a characteristic "Sausage-like" shape in velocity space. This is caused by the strong radial motions of the stars. The vertical axis represents the star's circular motion. Credit V. Belokurov. Right panel: An analog of the sausage merger in the Auriga simulations. Credit A. Fattahi

  Gaia launched in 2013 and will monitor every star down to G~20 about 70 times over a 5 year period. The first data release in September 2016 mainly comprised of the positions and magnitudes of the billion star survey. However, it is only since the second data release in April 2018 that the true power of Gaia has been unveiled. In DR2 the first (5 parameter) astrometric measurements were given. Our group at Durham has exploited this game-changing data set to further understand the assembly history of the Milky Way halo. Some examples of these works are given above.
• HSTPROMO:
  The High-resolution Space Telescope Proper Motion Collaboration (HSTPROMO) uses proper motions derived from the high resolution Hubble Space Telescope to improve our understanding of stars, clusters, and galaxies in the nearby Universe. Our group has used several of the derived data products from this collaboration in our work.
• DESI, 4MOST, WEAVE:
  Durham is an institutional member of both DESI (Dark Energy Spectroscopic Instrument) and 4MOST (4-metre Multi-Object Spectroscopic Telescope), and several staff at Durham are members of WEAVE (WHT Enhanced Area Velocity Explorer). These are all current/upcoming wide-field, multi-object spectroscopic surveys in the 2020s that will provide radial velocities and chemistry for hundreds of thousands of halo stars down to V~20. Members of our group are involved in preparatory work for these surveys. The combination of these radial velocities with proper motion measurements from Gaia will provide the ultimate 6D phase-space map of the Galactic halo.
• LSST:
  Looking further into the future, the Legacy Survey of Space and Time (LSST) will conduct a 10 year image of the sky, building up the deepest, widest, image of the Universe. For Milky Way studies, this survey will probe the stellar halo out to its furthest reaches, and will discover the faintest dwarf galaxies known. The predictions from our simulations will be essential for this exciting future mission, which has the potential to rule-out or validate the cold dark matter paradigm.