Infall results

As the name suggests, infall results record the time and circumstances of the first infall of a tracer into a halo. These events can contain more information such as the radial and tangential velocities at infall, as well as whether a tracer was part of a subhalo. Specifically, infall results are created at the time when the tracer crosses R_200m of a host halo. However, an infall result is also written in cases where the true infall cannot be determined.

Particle tracers

In the ideal (and most common) case, SPARTA begins tracking particles well outside of R_200m (as determined by the tracer creation radius). The code then follows the particle’s trajectory and determines the exact infall time t_ifl by linear interpolation. There are, however, a number of reasons why the moment of crossing R_200m cannot be determined:

• the particle is already in the halo when it is born, that is, when it first appears in the halo catalog. This inevitably happens to the first particles the halo finder detects as a new halo.

• when a halo becomes a subhalo, we determine a set of particles that truly belong to the subhalo (see Halos & Subhalos) and create tracers for those particles. In some cases, those particles might not have crossed R_200m and might thus not have an infall result associated with them.

In those cases, we still save an infall event, but with the flag born_in_halo, which indicates that the time of infall was not actually recorded and simply reflects the time when the tracer object was created. Thus, every particle tracer should either already have an infall result, or we should be looking for one because the tracer has not yet entered R_200m. As a consequence, infall events are numerous and can occupy a significant amount of memory or output space. They should only be written to file if necessary.

Subhalo tracers

Infall events for subhalos are recorded whenever they cross R_200m. As the halo merger trees are based on R_200m as computed only from bound host halo particles, some subhalos may already lie within the R_200m of all particles when they first become subhalos. However, infall events can be constructed from their saved trajectories in most cases. One exception occurs when halos are newly created as subhalos: in this case, the subhalo never technically crossed into its host, and no infall result is recorded.

Compile-time parameters

Parameter	Explanation
OUTPUT_RESULT_INFALL	Write infall results to output file
OUTPUT_RESULT_INFALL_TIME	Save the time of infall in output file
OUTPUT_RESULT_INFALL_BORNINHALO	Save the born-in-halo flag in output file
OUTPUT_RESULT_INFALL_SMR	Save the subhalo-to-host halo mass ratio in output file
OUTPUT_RESULT_INFALL_VRV200	Save vr/v200m to the output file
OUTPUT_RESULT_INFALL_VTV200	Save vt/v200m to the output file
OUTPUT_RESULT_INFALL_X	Save the 3D position at infall to the output file

Run-time parameters

This result does not add any config parameters.

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_infall	Always	The ID of the tracer to which this result refers.
t_infall	float	n_rs_infall	OUTPUT_RESULT_INFALL_TIME	The time of infall in Gyr since the Big Bang; not reliable when born_in_halo
born_in_halo	float	n_rs_infall	OUTPUT_RESULT_INFALL_BORNINHALO	If True, infall of tracer was not tracked, tracer was created inside (sub-)halo
sub_mass_ratio	float	n_rs_infall	OUTPUT_RESULT_INFALL_SMR	SubMassRatio, >0 for subhalos or particles that fell in with a subhalo
vrv200	float	n_rs_infall	OUTPUT_RESULT_INFALL_VRV200	The radial velocity at infall divided by v200m.
vtv200	float	n_rs_infall	OUTPUT_RESULT_INFALL_VTV200	The tangential velocity at infall divided by v200m.
x	float	n_rs_infall * 3	OUTPUT_RESULT_INFALL_X	The position at infall relative to the halo center (in physical kpc/h)