# 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 R200m 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 R200m (as determined by the tracer creation radius). The code then follows the particle’s trajectory and determines the exact infall time tifl by linear interpolation. There are, however, a number of reasons why the moment of crossing R200m 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 R200m 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 R200m. 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 R200m. As the halo merger trees are based on R200m as computed only from bound host halo particles, some subhalos may already lie within the R200m 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)

Results

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