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The angular momentum of galactic discs in semi-analytic models of galaxyformation is usually updated in time as material is accreted to the disc byadopting a constant dimensionless spin parameter and little attention is paidto the effects of accretion with misaligned angular momenta. These effects arethe subject of this paper, where we adopt a Monte-Carlo simulation for thechanges in the direction of the angular momentum of a galaxy disc as itaccretes matter based on accurate measurements from dark-matter haloes in theMillennium II simulation. In our semi-analytic model implementation, the flipsseen the dark matter haloes are assumed to be the same for the cold baryons;however, we also assume that in the latter the flip also entails a difficultyfor the disc to increase its angular momentum which causes the disc to becomesmaller relative to a no-flip case. This makes star formation to occur faster,specially in low mass galaxies at all redshifts allowing galaxies to reachhigher stellar masses faster. We adopt a new condition for the triggering ofstarbursts during mergers. As these produce the largest flips it is natural toadopt the disc instability criterion to evaluate the triggering of bursts inmergers instead of one based on mass ratios as in the original model. The newimplementation reduces the average lifetimes of discs by a factor of 2, whilestill allowing old ages for the present-day discs of large spiral galaxies. Italso provides a faster decline of star formation in massive galaxies and abetter fit to the bright end of the luminosity function at z = 0.

Stochastic angular momentum slews and flips and their effect on discs in galaxy formation models