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.
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.