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We analyse the heating of stellar discs by non axisymmetric structures andgiant molecular clouds (GMCs) in N-body simulations of growing disc galaxies.The analysis resolves long-standing discrepancies between models and data bydemonstrating the importance of distinguishing between measured age-velocitydispersion relations (AVRs) and the heating histories of the stars that make upthe AVR. We fit both AVRs and heating histories with formulae proportional tot^beta and determine the exponents beta_R and beta_z derived from in-plane andvertical AVRs and ~beta_R and ~beta_z from heating histories. Values of beta_zare in almost all simulations larger than values of ~beta_z, whereas values ofbeta_R are similar to or mildly larger than values of ~beta_R. Moreover, valuesof beta_z (~beta_z) are generally larger than values of beta_R (~beta_R). Thedominant cause of these relations is the decline over the life of the disc inimportance of GMCs as heating agents relative to spiral structure and the bar.We examine how age errors and biases in solar neighbourhood surveys influencethe measured AVR: they tend to decrease beta values by smearing out ages andthus measured dispersions. We compare AVRs and velocity ellipsoid shapessigma_z/sigma_R from simulations to Solar neighbourhood data. We conclude thatfor the expected disc mass and dark halo structure, combined GMC and spiral/barheating can explain the AVR of the Galactic thin disc. Strong departures of thedisc mass or the dark halo structure from expectation spoil fits to the data.

Age velocity dispersion relations and heating histories in disc galaxies