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snapopt - Special Ostriker-Peebles ’t’ calculator
snapopt in=snapshot
[parameter=value ...]
snapopt computes, or aids in computing, t
(little-t) from Ostriker & Peebles 1973 (OP73) paper. t is defined as the
ratio of bulk kinetic energy to the potential energy (cf. eq.(3)
and eq. (11)
in OP73).
For this the snapshot is divided in concentric cylinders (i.e.
the angular momentum axis is assumed to be along the Z-axis) of specified
width. In each cylinder the mean velocity in the angular direction is computed
(hence backward-moving particles are counted as negative), and with the
total mass in a ring this gives the total bulk motion in the galaxy. If
potentials are present in the snapshot, the total potential energy is
also computed. Output contains time, bulk kinetic and potential energy.
The
following parameters are recognized in any order if the keyword is also
given:
- in=snap_file
- Input file, in standard snapshot(5NEMO)
format. No default.
- dr=width
- Ringsize for binning radii. [Default 0.1]
- rmax=global_rmax
- If set,
this will be the maximum radius used for binning. All particles outside
this (cylindrical) radius will be discarded. Default: not used.
- w=
- Total
Potential energy of the external halo. Note this has to be of the same
sign as your potential values in the snapshot, in general NEMO uses negative
potentials [Default: 0].
- times=t1,t2,...
- Times to select snapshot. Default:
all.
Cannot handle external potentials properly, since the external
potential contributions is only counted half.
OP73 = J.P. Ostriker
& P.J.E. Peebles - ApJ 186, 467-480, 1973. A numerical study of the stability
of flattened galaxies: or, can cold galaxies survive?
snapkinem(1NEMO)
,
snapprint(1NEMO)
Peter Teuben
29-Oct-93 V1.0 Created Peter
12-nov-93 V1.1 added rmax= and allow non-equal size snapshots PJT
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