# On-the-fly Friends-Of-Friends and Black Hole seeding

The main purpose of the on-the-fly FOF is to identify haloes during a cosmological simulation in order to seed some of them with black holes based on physical considerations.

In this mode, no group catalogue is written to the disk. The resulting list of haloes is only used internally by SWIFT.

Note that a catalogue can nevertheless be written after every seeding call by setting the optional parameter dump_catalogue_when_seeding.

Once the haloes have been identified by the FOF code, SWIFT will iterate over the list of groups and will check whether each halo obeys the following criteria:

• Is above a user-specified mass threshold (typically $$10^{10}~\rm{M}_\odot$$ or more).

• Contains at least one gas particle.

• Does not contain any already existing black hole particle.

If a group satisfies all these requirements then the densest gas particle (based on their SPH density) in the group will be converted to a black hole. In practice this means that the gas particle is removed and a black hole particle with the same dynamical mass is inserted at the same position in the domain and with the same velocity. Additional properties are copied from the gas particle to the black hole particle depending on the specifics of the sub-grid model in use (see Galaxy Formation Subgrid Models).

Given that the halo mass considered for seeding black holes is usually many hundred times the mass of a single particle, it can be advantageous to raise the minimal group length required for a halo to appear in the catalogue of objects. This reduces the number of groups that are kept in memory and speeds up the seeding procedure by removing haloes that are obviously too small. For instance, in the case of EAGLE-like runs, the dark matter particle mass is around $$10^7\rm~{M}_\odot$$ and the minimal halo mass considered for seeding a black hole is of order $$10^{10}~\rm{M}_\odot$$. Groups will hence need to have at least 1000 particles to pass the first criterion outlined above. In this case, using a minimal group length of order 500 is beneficial over the more traditional value of 32 as it will reduce the number of haloes to consider by about a factor 10 (assuming a normal cosmology) whilst still being far enough from the actual user-defined mass limit to be safe.

# On-the-fly Friends-Of-Friends for snapshot output

It may be desirable to include FOF group membership information for each particle in the output snapshots even when black hole seeding is not in use. This can be achieved by setting the invoke_fof parameter in the Snapshots section of the parameter file.

FOF will be run just before each snapshot is written and the snapshot will include a dataset which specifies which FOF group each particle belongs to.