Research Activities

the formation of galactic discs

probing galaxy merger histories with galactic archaeology

secular evolution from bars and spiral arms

dark matter around the Milky Way

synthetic observations of galaxies

The Evolution of spiral arms

The blue line and white cross mark the loci of the stellar bar and spiral arm in this sequence of snapshots taken from a N-body/SPH simulation.

Since the 1960s, the most accepted and widely reported explanation of spiral arms has been the spiral density wave theory (Lin & Shu, 1964, ApJ 140 646), which spiral arms rotate rigidly with constant angular rotation. This allows stars to move independently and pass through spiral arms, allowing them to be long-lived features.

However, our idealised N-body simulations show that individual spiral arms seem to rotate at the same speed as the stars, and thus wind up with time (left). Spiral arms constantly disrupt and form anew, ensuring that a spiral morphology exists at all times, consistent with their ubiquitousness in the Universe.

Simulating spiral galaxy formation: Auriga

The Auriga project is a large suite of cosmological magnetohydrodynamic (MHD) simulations of Milky Way analogues simulated with the AREPO code (Springel 2010). One of the main results of these simulations is that they produce disc-dominateed rotationally supported, star-forming late-type systems (right) across the expected Milky Way mass range (0.5 – 2 x 1012 M⊙), thereby overcoming a historic difficulty for simulations in the Lambda Cold Dark Matter paradigm.

They have so far been used in more than 50 scientific studies covering a wide range of scientific topics, from the nature of dark matter to the formation of magnetic fields and more exotic physics such as cosmic rays. For more information, please visit the Auriga project website.

The simulation starts just after the Big Bang from cosmological initial conditions. Prescriptions for gas cooling, star formation, magnetic fields, stellar evolution, feedback, and black holes are included in the physics model.

Simulating the Local Group: HESTIA

This movie shows a snapshot of the gas density distribution around a Milky Way-Andromeda like pair of galaxies from different angles. Many smaller dwarf galaxies are visible around and in between these two dominant galaxies.

The HESTIA simulation suite: High-resolution Environmental Simulations of The Immediate Area, apply the Auriga galaxy formation model to the Local Group environment. Initial conditions constrained by the observed peculiar velocity of nearby galaxies are employed to accurately simulate the local cosmography. Within high-resolution regions of 3-5 Mpc lies simulated Local Groups consisting of a Milky Way and Andromeda like galaxy pair, whose description is in excellent agreement with observations. The simulated Local Group galaxies resemble the Milky Way and Andromeda in terms of their halo mass, mass ratio, stellar disc mass, morphology separation, relative velocity, rotation curves, bulge-disc morphology, satellite galaxy stellar mass function, satellite radial distribution, and in some cases, the presence of a Magellanic cloud like object. Because these simulations properly model the Local Group in their cosmographic context, they provide a testing ground for questions where environment is thought to play an important role. Please see the HESTIA project website for more information.

Mock observations for the Gaia mission

Galactic surveys such as Gaia are delivering an unprecedented amount of data for billions of stars in our Milky Way, highlighting out-of-equilibrium dynamics and a wealth of stellar streams and substructure in the stellar halo. Understanding the origin and evolutionary history of these fascinating features of the Galaxy requires the forward modelling of theoretical models to provide much needed interpretation.

To this end, I developed the first-ever synthetic Gaia catalogues for the second data release (Aurigaia, Grand et al. 2018, MNRAS, 481, 1726), which provides a “mock” view of billions of stars in 6 different simulated Milky Way-mass galaxies. These catalogues take into account dust extinction, the Gaia magnitude limit, and the astrometric, photometric and spectroscopic errors of the survey. These catalogues provide powerful assessments of the biases and limitations of Gaia. Mock catalogues for Gaia, and recently for the PAndAS survey of M31, have been publicly released at this website.

Left: Mock stellar light for all stars within 20 kpc (top) and between 5 and 20 kpc (bottom) of a solar-like position in one of the Auriga galaxies. The x- and y-axis indicate the Right Ascension and Declination coordinates. Dust extinction is heaviest in the mid-plane, and removing stars within 5 kpc of the Sun emphasises the older, yellow bulge in favour of the bright blue nearby disc. Right: Face-on (top) and edge-on (bottom) stellar surface density viewed in Cartesian coordinates. The Galactic Centre (GC) is marked as a black cross.

Deciphering the dark matter annihilation detection signal

Using six, high-resolution simulations, we studied the dark matter annihilation signal emanating from surrounding subhaloes compared to the smooth halo background predicted in the Λ-cold dark matter (ΛCDM) paradigm. We showed that baryonic effects enhance annihilation radiation from the dominant smooth component of the galactic halo by a factor of 3, and its central concentration increases substantially. In contrast, subhalo fluxes are reduced by almost an order of magnitude, partly because of changes in internal structure, partly because of increased tidal effects; they drop relative to the flux from the smooth halo by 1.5 orders of magnitude. The expected flux from the brightest Milky Way subhalo is four orders of magnitude below that from the smooth halo, making it very unlikely that any subhalo will be detected before robust detection of the inner Galaxy. We use recent simulations of halo structure across the full ΛCDM mass range to extrapolate to the smallest (Earth-mass) subhaloes, concluding, in contrast to earlier work, that the total annihilation flux from Milky Way subhaloes will be less than that from the smooth halo, as viewed both from the Sun and by a distant observer. Fermi-Large Area Telescope may marginally resolve annihilation radiation from the very brightest subhaloes, which, typically, will contain stars.