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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:
Input file, in standard snapshot(5NEMO) format. No default.
Ringsize for binning radii. [Default 0.1]
If set, this will be the maximum radius used for binning. All particles outside this (cylindrical) radius will be discarded. Default: not used.
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 to select snapshot. Default: all.


Cannot handle external potentials properly, since the external potential contributions is only counted half.

See Also

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

Update History

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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