2.2.2  :  Semi-analytic modelling at the ICC: Galform

Halos and Merger Trees

The halo merger trees for the Bower et al model are contained in the DHalo and DSubhalo tables. The merger trees are built using the Friends of Friends (b=0.2) groups output by L-Gadget2 and the subgroups identified by SubFind. In the description below 'subhalo' means a set of particles grouped together by SubFind, which looks for gravitationally bound local density maxima. A 'halo' is a collection of subhalos grouped together in the following way:

Initially subhalos are grouped into halos by looking at the FoF groups - a halo just consists of all the subhalos belonging to one FoF group. Particles in the FoF group which do not belong to any subhalo are discarded. Note that SubFind identifies the background mass distribution of the halo as a separate subhalo, so each halo will normally contain one large subhalo (with most of the mass of the halo) and a set of smaller "satellite" subhalos.

The Friends of Friends algorithm often links together objects which should probably be treated as separate halos as far as the merger trees are concerned. So under certain conditions a subgroup may be split from its parent halo and considered to be a halo in its own right. This is done if 1) the subhalo is outside twice the half mass radius of the parent halo or 2) the subhalo has retained 75% of the mass it had at the last output time where it was an independant halo. This second condition is based on the assumption that a halo which has fallen into a more massive halo will rapidly be stripped of its outer layers, whereas a halo which has been artificially linked to another halo will retain its mass. When a subhalo is split off, any less massive subhalos within twice its half mass radius are also split off and become part of the new halo.

The descendant of a subhalo is found by following the most bound 10% of its mass or the 10 most bound particles, whichever is the greater mass. The descendant is the subhalo which contains the largest number of these particles. The descendant of a halo is the halo which contains the descendant of the most massive subhalo in the halo.

We refer to the halos defined in this way as "DHalo"s to differentiate them from the halos used in the Munich version of the merger trees.

WARNING: there is rather conflicting terminology between the two versions of the merger trees. The bound groups of particles identified by SubFind are refered to as subhalos in the Durham version of the merger trees described here, but are referred to as halos in the Munich version.

Galaxies

The galaxy catalogues are stored in the DGalaxy table. They were produced by using the GALFORM semi-analytic code to populate the N-body halos from the Millennium simulation with galaxies. The galaxy formation model is decsribed in detail in the Bower et al (2006) paper. The properties available for each galaxy include position, velocity, stellar mass and magnitudes in various bands.

Within a single galaxy merger tree, the galaxies in the DGalaxy table are assigned ID numbers in depth first order so that all progenitors of a particular galaxy may be easily located. The ID of the descendant of each galaxy is also provided.