Publications by James Binney

Age velocity dispersion relations and heating histories in disc galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press (2016)

M Aumer, JJ Binney, RA Schönrich

We analyse the heating of stellar discs by non axisymmetric structures and giant molecular clouds (GMCs) in N-body simulations of growing disc galaxies. The analysis resolves long-standing discrepancies between models and data by demonstrating the importance of distinguishing between measured age-velocity dispersion relations (AVRs) and the heating histories of the stars that make up the AVR. We fit both AVRs and heating histories with formulae ∝ tβ and determine the exponents β bar R and β bar z derived from in-plane and vertical AVRs and βR and βz from heating histories. Values of βz are in almost all simulations larger than values of βbarz , wheras values of βR are similar to or mildly larger than values of β bar R. Moreover, values of βz (β bar z) are generally larger than values of βR (β bar R). The dominant cause of these relations is the decline over the life of the disc in importance of GMCs as heating agents relative to spiral structure and the bar. We examine how age errors and biases in solar neighbourhood surveys in uence the measured AVR: they tend to decrease β values by smearing out ages and thus measured dispersions. We compare AVRs and velocity ellipsoid shapes σz/σR from simulations to Solar-neighbourhood data. We conclude that for the expected disc mass and dark halo structure, combined GMC and spiral/bar heating can explain the AVR of the Galactic thin disc. Strong departures of the disc mass or the dark halo structure from expectation spoil fits to the data.

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