that paper. Data for the galaxy population at all redshifts are available at the Millennium web site for the updated model of @deluciab07, as well as for the independent galaxy formation model of @bower06. All these models are tuned to fit a wide variety of data on the nearby galaxy population, and in addition fit many (but not all!) available data at higher redshift [see, for example, @kw06]. The details of the galaxy formation modelling are not, however, important for the dynamical issues which are the focus of our own paper.

At $z=0$ there are $18.2 \times 10^{6}$ halos/subhalos identified in the simulation to its resolution limit of $20$ particles. The galaxy formation model populates these with $8,394,180$ galaxies brighter than an absolute magnitude limit of $M_{B} = -16.7$ above which the catalogue can be considered reasonably complete. These catalogues list a number of properties for the halos, subhalos and galaxies which will be important for us. Galaxies are categorised into three types according to the nature of their association with the dark matter. A Type 0 galaxy sits at the centre of the dominant or main subhalo and can be considered the central galaxy of the halo itself (formally, 