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Host haloes

The first step that HBT-HERONS does when analysing a simulation output is to assign to every pre-existing subhalo a host halo. This is the first step in the algorithm because:

  • Haloes that contain no subhaloes are used as seeds for new subhaloes in the simulation.
  • The subhalo hierarchy, which determines mass transfer between subhaloes, is based on subhaloes within the same host halo.

Assignment of host haloes

For a subhalo that was resolved in the previous simulation output, HBT-HERONS uses its NumTracerHostFinding most bound tracer particles as identified from that output. It then checks which FoF group each of those particles belongs to in the current simulation output, assigning a particle weight that reflects the bound ranking it had in the last output (\(r_{i}\)): $$ w_{i} = \dfrac{1}{1 + \sqrt{r_{i}}}. $$ The weighting gives greater importance to particles that had larger binding energies when determining the host halo of the subhalo.

HBT-HERONS scores all potential host halo candidates using the total sum of the weights of tracer particles associated to each halo candidate. The assigned host halo of a subgroup is the highest-scoring halo. Note that HBT-HERONS also allows subhaloes to have no assigned host halo, in which case subhaloes are termed hostless subhaloes.

Orphan subhaloes

The method is the same as for resolved subhaloes, but the difference is that it only uses the most bound tracer particle when the subhalo was last resolved (SnapshotOfDeath - 1).

Hostless subhaloes

There is a population of subhaloes that do not have an assigned host halo in HBT-HERONS. These cases occur when a significant fraction of the most bound tracer particles used to identify host haloes does not belong to a FoF group. This is often caused by the fragmentation of poorly-resolved FoF groups, because a missing particle link can make the number of particles drop down below the minimum number required to identify a FoF group.

However, the lack of a FoF group does not mean that there is no self-bound subhalo in that region. Whereas other subhalo finders do not consider this possibility (their starting point is often a 3D FoF group), HBT-HERONS uses past subhalo memberships to check if there is still a (hostless) subhalo.