What is the difference between a halo and a subhalo?
There are two types of dark matter structures in the Millennium / Millennium-II databases: friends-of-friends (FOF)
halos and subhalos. The term "halo" in the documentation almost always refers to
subhalos.
While FOF halos better correspond to the standard picture of a virialized dark matter halo, subhalos are more easily
connected to galaxies. Each FOF halo may contain several subhalos
(see Springel et al. 2005 /
Boylan-Kolchin et al. 2009
for details on how FOF halos and subhalos were found in the Millennium / Millennium-II Simulations).
The merger trees in the Millennium-II database use subhalos as their fundamental unit;
as far as the merger trees are concerned, FOF halos only exist as a property of the subhalos (this is also true
for the MPAHaloTrees in the Millennium database but is not the case for the DHaloTrees).
While it is possible to construct FOF merger trees either based on the subhalo merger trees or directly from
the FOF halos, no such FOF merger trees currently exist in the Millennium-II database.
Why is the subhalo mass equal to zero even though it has over one million particles?
Only subhalos that are identified as FOF-dominant (those for which haloId=firstHaloInFOFgroupID) can have a
spherical overdensity mass associated with them. All non-FOF-dominant subhalos will have m_crit200, m_mean200,
and m_tophat identically equal to zero. The number of particles in the subhalo is still well-defined, however,
and this is the basis for the standard definition of a subhalo's mass.
There are almost no major mergers in the merger trees - what's wrong?
Mergers in the merger trees are mergers of subhalos. These subhalos may have been orbiting for several
dynamical times within a larger FOF halo and can lose substantial amounts of mass before merging. If you are
interested in the merger of FOF halos, there are several possibilities. One is to follow merging subhalos
backward in time until they reside in separate FOF halos; another is to find FOF-dominant subhalos with descendants
that reside in the same FOF halo.
Are there any structural differences between the Millennium and Millennium-II databases?
We have added a few new columns for the Millennium-II database that are currently not present for the Millennium database.
If these prove useful, we will update the Millennium database to include them.
The additional columns in the subhalo merger trees include the radius at which the peak circular velocity is reached
(vMaxRad), the mainLeafID (which allows easy extraction of the main progenitor branch of any subhalo),
a direct link to the host FOF halo for each subhalo via fofID, and a link to the rank of each subhalo within
its host FOF halo via subhaloId (as opposed to subhaloFileId; see below).
An important difference is the number of snapshots: 64 for Millennium (0-63), 68 for Millennium-II (0-67).
Note that for a given snapshot number, the corresponding redshift is different in the
Millennium Simulation than in the Millennium-II simulation. Please see the MField..Snapshots (Millennium)
and MIISnapshots (Millennium-II) tables for the relationship between snapshot number and redshift / expansion factor.
What is the nextProgenitorId?
The nextProgenitorId of subhalo S is a pointer to the next most massive subhalo that has the same descendant as subhalo S.
The nextProgenitorId pointer is NOT a pointer from a subhalo to one of its progenitors; it links subhalos that have
the same descendant. See the example
here
for a schematic view of several of the pointers in the merger trees,
including the nextProgenitorId.
Why are there so many different IDs? How do they help me?
The raw particle data for the Millennium-II Simulation was saved at 68
snapshots. Each snapshot is distributed over 512 approximately equal-sized
files. At each snapshot, a friends-of-friends (FOF) groupfinder with linking
length b=0.2 was run. During post-processing, each FOF group was searched for
bound substructure using the SUBFIND algorithm. For each snapshot,
there are 2048 files that contain the results of SUBFIND. Merger
trees were built based on these subhalos. The merger trees were split over 512
files, each of which contains approximately 14,000 trees.
In making the database, the structure of these files is retained in the subhalo
(and FOF halo) IDs as follows:
haloId = 1015 * treeFile + 109 * (rank of tree in file) + (depth-first
ordering of subhalo in tree)
Note: the structure for the Millennium Simulation is different!!!
Knowing this data structure can help in your queries. For example, suppose you are interested in finding all
subhalos beloning to the FOF group with a given fofID; this can be done trivially by selecting subhaloId between
106 * fofID and 106* (fofID + 1) - 1.
