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  Understanding “dynamical friction”

  “Dynamical friction” is a phenomenon first theorised by Chandrasekhar in the 1930's. It is a gravitational drag force that is likely responsible for galaxy-galaxy mergers, star cluster mergers, and even the merger of binary “super-massive” black holes. Yet despite its long history, it still throws us some surprises. A decade ago now, it was noticed that the frictional force appears to disappear in galaxies that have a constant density. Our PhD student James Petts has finally shown why this happens and developed a new model to explain the behaviour: http://arxiv.org/abs/1607.04284.

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  A new test of the standard cosmological model at the edge of galaxy formation

  Surrey astrophysicists Prof. Justin Read and Dr. Oscar Agertz, along with collaborators at the University of Bologne, have developed a new test of our standard cosmological model. By fitting the rotational velocities of nearby “dwarf” galaxies, they were able to weigh these tiny galaxies accurately, for the first time. They find that their masses are perfectly consistent with the standard cosmological model, suggesting that dark matter is a cold, collisionless particle. http://arxiv.org/abs/1607.03127v1.

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  New models for globular clusters

  Surrey astrophysicists Prof. Mark Gieles and Dr. Alice Zocchi developed a new family of models that can be used to infer detailed properties of globular clusters from observational data: http://arxiv.org/abs/1508.02120.

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  Gas around galaxies

  In a study by Cameron J. Liang (University of Chicago), Andrey V. Kravtsov (University of Chicago) and Oscar Agertz (University of Surrey), we use cosmological zoom-in simulations to investigate the origin of observed multi-phase gas around galaxies. http://arxiv.org/abs/1507.07002.

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  Understanding dwarf galaxy rotation curves

  Surrey astrophysicists Prof. Justin Read & Dr. Oscar Agertz, with collaborators from the University of Bologne have used high fidelity simulations of isolated dwarf galaxies to solve a now long-standing cosmological puzzle. The gas rotation velocity of isolated dwarf galaxies appears to rise far less steeply than predicted by current models. Here we show that this - and a related problem of “rotation curve diversity” - owes to previously unmodelled gas physics. Our latest simulations that carefully model such processes give a remarkable match to the data for isolated dwarf galaxy rotation curves. This lends further support to our current cosmological model and suggests that dark matter is cold and collisionless. http://arxiv.org/abs/1601.05821.

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  Light versus dark in strong lens galaxies

  Surrey astrophysicist Prof. Justin Read and ETH Zurich PhD student Claudio Bruderer compare light and dark matter in 11 strong lens galaxies with excellent data. They find that the dark halos are rounder than the visible stars and sometimes highly misaligned. This is expected in our standard cosmological model, but hard to understand if dark matter is a manifestation of some new theory of gravity. These results provide further evidence for particulate dark matter, and give new and exciting constraints on galaxy formation theory: http://arxiv.org/abs/1511.01097.

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  Weighing a galaxy backwards in time

  In a study published in Nature Communications, Dr Ugur Ural (Potsdam) and Prof. Justin Read (University of Surrey), with collaborators from Leicester and Carnegie Mellon, have developed a new technique for weighing tiny “dwarf” galaxies both today and, for the first time, backwards in time. We have applied the method to the `Carina' dwarf spheroidal, finding that it is surprisingly light and always was. Such a low mass suggests that there must be a large population of ghostly massive dwarf galaxies, as massive as their visible counterparts, orbiting the Milky Way. For the full article see: http://www.nature.com/ncomms/2015/150702/ncomms8599/full/ncomms8599.html.

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  Our Galaxy likely had no massive mergers since its disc formed with little or no accreted stellar/dark disc!

  In a new study using the Gaia-ESO survey data, Dr. Greg Ruchti (University of Lund); Prof. Justin Read (University of Surrey); and members of the Gaia-ESO survey team have found that our Galaxy had no massive mergers since its disc formed. These would have been detectable from the stellar debris that they deposit in the disc, but none was found. The result implies that our Galaxy has little or no accreted disc component and therefore no “dark matter disc”. For further details see: http://arxiv.org/abs/1504.02481.

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  The kinematic imprints of multiple populations in globular clusters

  In a new paper from Dr. Vincent Hénault-Brunet and co-authors from the University of Surrey, we highlighted the potential of using kinematics to probe the formation of multiple stellar populations in globular clusters: http://arxiv.org/abs/1503.07532. We identified the differential rotation of subpopulations as a key signature to distinguish between the proposed scenarios.

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  The effect of secular galactic growth on the evolution of star clusters

  Dr. Florent Renaud and Prof. Mark Gieles (both Surrey) studied the evolution of star clusters in the cosmological context by including the time-dependent potential of a Milky Way-like galaxy in a collisional N-body code. Somewhat surprising, perhaps, they find that the evolving galaxy has little effect on star clusters, and their debris (i.e. “tidal streams”). The results are accepted for MNRAS and details can be found here: http://arxiv.org/abs/1503.04815.

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  New dwarf galaxy Hydra II discovered by the SMASH survey

  The SMASH survey, including our very own Dr. Noelia Noel, have recently uncovered a new dwarf galaxy orbiting the Milky Way: Hydra II. The dwarf was found orbiting close to the Large and Small Magellanic Cloud galaxies in the southern hemisphere, suggesting a possible association.

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  Eliminating 'spurious halos' in Warm Dark Matter simulations

  In a new paper led by Dr. Alex Hobbs (ETH) and Prof. Justin Read (University of Surrey), we develop a new method for modelling the growth of structure in the Universe. This promises increased accuracy, in particular for “Warm Dark Matter” simulations. For further details see: http://arxiv.org/abs/1503.02689.

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  Milking the spherical cow

  In a new paper led by Dr. Andrew Pontzen (UCL) and Prof. Justin Read (University of Surrey), we find an apparent attractor solution for spherical systems: http://arxiv.org/abs/1502.07356. This could explain why simulated dark matter halos appear the way they do, and shed new light on galactic structure!

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  Stability of galactic discs

  In a new paper led by Dr. Oscar Agertz (University of Surrey), we investigate how gas turbulence affects the gravitational stability of Milky Way-like galaxies. For further details see: http://arxiv.org/abs/1502.05583.

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  No evidence for variations in the stellar mass function in globular clusters

  A recent study by R. Shanahan (University of Edinburgh) and Prof. M. Gieles (University of Surrey) shows that the recently found variations in the mass-to-light ratio of globular clusters as a function of metallicity can be fully accounted for by dynamical effects: http://arxiv.org/abs/1501.04971. The results argue against the need for variations in the stellar mass function.

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