Orbit count results

The orbit counting result counts pericenters. This information can be used to separate particles into a 1-halo term (particles that have undergone at least one pericenter) and material on a first infall trajectory.

Algorithm

As a tracer is created, we set its orbit counter to zero. If the tracer is born outside the halo, we know that this count is reliable. If it is born inside or under other dubious circumstances, we set the n_is_lower_limit flag to indicate that there could have been pericenters before the count was started.

Similar to the Splashback results, we look for a change in radial velocity from negative to positive. Each time such a switch occurs, we increase the orbit counter. Unlike other results, the orbit count result would never end, i.e., a tracer can always accumulate more pericenters. This means that tracers are being kept alive even if all their other results have been recorded. To avoid unnecessary tracers that cost memory and computation, the user can set a maximum number of pericenters, res_oct_max_norbit.

The orbit count result allows us to tell which tracers are on a first infall into the halo (zero pericenters), which are on their first orbit, or have had multiple orbits. This information can then be used for example in the Density profile analysis.

Compile-time parameters

Parameter	Explanation
OUTPUT_RESULT_ORBITCOUNT	Write orbit count results to output file
OUTPUT_RESULT_ORBITCOUNT_OCT	Save the orbit count to the output file (the result makes little sense without this number)
OUTPUT_RESULT_ORBITCOUNT_LOWER_LIMIT	Save a flag indicating whether the orbit count is a lower limit (recommended)
OUTPUT_RESULT_ORBITCOUNT_LAST_SNAP	Save the snapshot of infall/where lower limit was set/where last pericenter was detected

Run-time parameters

Parameter	Type	Default	Explanation
res_oct_max_norbit	int	3	When a tracer has completed this many orbits, stop counting

Output fields

Field	Type	Dimensions	Exists if	Explanation
halo_first	int64	n_halos	Always	The index of the first result for each halo (or -1 if none exists for a halo).
halo_n	int32	n_halos	Always	The number of results of this type for each halo (can be 0).
tracer_id	int64	n_rs_orbitcount	Always	The ID of the tracer to which this result refers.
n_pericenter	int16	n_rs_orbitcount	OUTPUT_RESULT_ORBITCOUNT_OCT	The number of orbits (completed pericenters).
n_is_lower_limit	int8	n_rs_orbitcount	OUTPUT_RESULT_ORBITCOUNT_LOWER_LIMIT	Whether n_pericenter is a lower limit (a tracer’s origin was not tracked).
last_pericenter_snap	int16	n_rs_orbitcount	OUTPUT_RESULT_ORBITCOUNT_LAST_SNAP	If no orbit counts recorded, the snapshot of infall; if no orbit counts but a lower limit, the snapshot where the lower limit was set; if the orbit count is greater than zero, the last snapshot where a pericenter was detected.