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 inthe gravitational field of a dark halo that formed earlier. A huge amount can be learnt by modelling theaccumulation of stars in the plane. Spiral instabilities and giant molecular clouds (GMCs) move stars to lesscircular orbits. The GMCs are most effective early on when the disc is low in mass and rich in gas. Theyestablish an almost perfectly exponential vertical density profile. At some time a bar usually forms, but thetiming of the bar depends on several factors, including the effectiveness of heating by GMCs. If the barbecomes strong, it buckles to form a peanut-bulge. Increasing the density of the dark halo delays/suppressesbar formation. For a halo density near that now measured locally & expected cosmologically, the model hascomponents that closely resemble the thin disc and the bar/bulge. But there is no thick disc. A thick disc likethe Galaxy's can be obtained by (i) starting the simulation with a thick stellar component, possibly formed in amajor merger at z > 2, or (ii) allowing early stars to form with large random velocities. These models allow us topredict the distribution in phase-space of stars of every age, and the extent to which stars have migrated fromtheir birth locations, which is key information for models of chemical evolution. Early indications are thatgeneric models provide remarkably good fits to observations.