Merger trees

Merger tree description

The figure below shows a schematic of the structure of the merger trees. Walking back in time following along the main (most massive) progenitor branch consists of following the first progenitor links until they end (value equals -1). Similarly, walking forward in time along the descendants branch consists of following the descendant links until they end (value equals -1), which typically occurs at the last snapshot of the simulation (\(z = 5.5\)). The full progenitor history, and not just the main branch, requires following both the first and next progenitor links. In this way the user can identify all subhalos at a previous snapshot which have a common descendant.

The number inside each circle from the figure is the unique ID (within the whole simulation) of the corresponding subhalo, which is assigned in a depth-first fashion. The ordering within a single tree in the LHaloTree is not guaranteed to follow this scheme. Each subhalo spans a "subtree" consisting of the subhalo itself and all its progenitors. As an example, the subhalos belonging to the subtree of subhalo 5 are shown in darker green in the figure. Other subhalos not belonging to this subtree are shown in lighter green, and their links are indicated with dashed arrows. The trees contain the links 'first subhalo in FoF group' (light brown dotted arrow) and 'next subhalo in FoF group' (dark brown dotted arrow), which connect subhalos that belong to the same FoF group. The FoF groups do not play a direct role in the construction of the merger tree.

The LHaloTree algorithm is virtually identical to that used for the Millennium simulation, constructing trees based on subhalos instead of main halos, described fully in the supplementary information of Springel et al. 2005. In short, to determine the appropriate descendant, the unique IDs that label each particle are tracked between outputs. For a given halo, the algorithm finds all halos in the subsequent output that contain some of its particles. These are then counted in a weighted fashion, giving higher weight to particles that are more tightly bound in the halo under consideration, and the one with the highest count is selected as the descendant. In this way, preference is given to tracking the fate of the inner parts of a structure, which may survive for a long time upon infall into a bigger halo, even though much of the mass in the outer parts can be quickly stripped. To allow for the possibility that halos may temporarily disappear for one snapshot, the process is repeated for Snapshot \(n\) to Snapshot \(n+2\). If either there is a descendant found in Snapshot \(n+2\) but none found in Snapshot \(n+1\), or, if the descendant in Snapshot \(n+1\) has several direct progenitors and the descendant in Snapshot \(n+2\) has only one, then a link is made that skips the intervening snapshot.

The LHaloTree merger tree is one large data structure split across several HDF5 files named trees_sf1_[NumLastSnapshot].[chunkNum].hdf5, where [chunkNum] goes from e.g. 0 to 1200 for Thesan-1. Within each file there are a number of groups named TreeX, where X is an integer which simply increases from zero to the number of tree groups in that file chunk.

Note: Note that a given TreeX group may contain subhalos spanning different FoF groups as well as snapshots, and to efficiently locate a specific subhalo at a specific snapshot (e.g \(z=0\)) the offsets can be used.

The pair (SubhaloNumber,SnapNum) provides the indexing into the Subfind group catalog. The five other indices for each entry in a TreeX group (e.g. Descendant) index into that same group in the tree file.