Seminar talk

Title: How did our Galaxy form?

Speaker: James Binney (University of Oxford)
Time & Place: Tuesday, 3:00pm, April 11th, Lecture Hall, 3rd floor  

Abstract: Galaxies like are have continued through most of cosmic time to form stars in a thin disc that initially orbited in the gravitational field of a dark halo that formed  earlier. A huge amount can be learnt by modelling the accumulation of stars in the plane. Spiral instabilities and giant molecular clouds (GMCs) move stars to less circular orbits. The GMCs are most effective early on when the disc is low in mass and rich in gas. They establish an almost perfectly exponential vertical density profile. At some time a bar usually forms, but the timing of the bar depends on several factors, including the effectiveness of heating by GMCs. If the bar becomes strong, it buckles to form a peanut-bulge. Increasing the density of the dark halo delays/suppresses bar formation. For a halo density near that now measured locally & expected cosmologically, the model has components that closely resemble the thin disc and the bar/bulge. But there is no thick disc. A thick disc like the Galaxy's can be obtained by (i) starting the simulation with a thick stellar component, possibly formed in a major merger at z > 2, or (ii) allowing early stars to form with large random velocities. These models allow us to predict the distribution in phase-space of stars of every age, and the extent to which stars have migrated from their birth locations, which is key information for models of chemical evolution. Early indications are that generic models provide remarkably good fits to observations.