1. Decide which type of particles are to be grouped. This could be either dark matter particles, gas particles, star particles or gas and star particles depending on what is in the input tipsy binary file.
2. Calculate the densities (same method as used by smooth) of these particles (those considered for grouping only) and keep those which satisfy a certain cut criterion. Call these particles the moving particles. The cut criterion depends on the type of particle. For dark matter particles, keep only those with densities greater than or equal to a minimum density specified by the user. For gas particles, keep only those satisfying the minimum density criterion and in addition those satisfying a maximum temperature criterion (again specified by the user). In effect we keep only the cold dense gas. For star particles there is no extra criterion, they are all considered as moving particles.
3. Now we slide these moving particles along the initial density gradient toward higher density (hence moving). If particles move less than a user specified distance (eps) over several iterations they are no longer moved as they are now oscillating in a high density well.
4. This process of moving particles continues until all the particles stop moving and are localized in eps sized, high density regions.
5. Next we group together these localized particles using the friends- of-friends method with a linking-length of eps. Note that two high density regions seperated by less than about 2*eps will be grouped together by this method. This forms the initial groups of particles.
6. We reject groups with less than a user specified minimum members number of particles.
7. Now particles which are not bound to their group are removed from the group, by the unbinding procedure (to be discussed in more detail below).
8. One more rejection of groups with less than minimum members number of particles is performed and various group information (to be discussed below) is written to three different output files.

• Dark matter only: density and density gradients are calculated from all the particles, and the moving particles are those meeting the minimum density criterion.
• Gas only: density and density gradients are calculated from all the particles, and the moving particles are those meeting both minimum density and maximum temperature criteria.
• Star only: density and density gradients are calculated from all the particles, and all particles are moved.
• Gas and dark matter: density and density gradients are calculated from the gas particles only, and the moving particles are the gas particles which meet the density and temperature criteria.
• Stars and gas: density and density gradients are calculated from the star particles only, and the moving particles are all the star particles.
• Stars and dark matter: Same as the stars and gas case.
• Stars and gas and dark matter: density and density gradients are calculated from all the star particles and all the gas particles. The moving particles are then the gas particles meeting the density and temperature criteria and all the star particles.

• Case 1 unbinding:
  1. Calculate the potential energies of all the particles in the group taking into account the redshift of the simulation to get physical distances (if units of the input file are physical then the redshift is just set to zero).
  2. The center of mass and center of mass velocity of all particles in the group are found. The velocity relative to the center of mass velocity is found for all particles in the group. This relative velocity is converted to a physical velocity and corrected to include the Hubble flow (again if the simulation is in physical coordinates this is satisfied by setting redshift to zero and Hubble's constant to zero). Then the kinetic energy with respect to the center of mass is found for each particle.
  3. The least bound particle is found. If this particle is in fact bound then we have completed unbinding this group. If the particle is not bound then we remove it from the group and go back to step 1.

  Note that as particles are removed from the group the center of mass changes, the center of mass velocity changes (hence also the kinetic energies) and the potential energies of the particles is reduced.

• Case 2 unbinding:

  For gas the approach to unbinding has to be slightly different. For example, a galaxy depends on its dark matter environment for it to be bound. In these cases we include some of the environment of each group in the calculation of the potential energies.

  1. Calculate the potential energies of all the particles in the group in the same way as in case 1 unbinding.
  2. Include the potential contribution of all mass within a 2*eps ball about the center of the localized (or moved) positions of the particles in the group. In otherwords we want to include some of the environment about the density maximum.
  3. The center of mass and center of mass velocity is calculated as in case 1 unbinding and from this the kinetic energies of the particles are determined (again in the same way as case 1 unbinding).
  4. The least bound particle is found. If this particle is in fact bound then we have completed unbinding this group. If the particle is not bound then we remove it from the group and go back to step 3.

  Once again the center of mass, center of mass velocity and kinetic energies of the particles changes each time a particle is removed. However, in this case the potential energies remain fixed since any particle removed is considered to also be a part of the environment of the group.

The Output Files

• skid.grp: This file is in tipsy array format and contains the group number to which each particle in the input file belongs. Group number zero means that the particle does not belong to a group. This file can be read in by tipsy to color each particle by its group number (see picture ), or to look at only the group particles, etc. This output could also be used by a subsequent analysis tool.
• skid.ray: This file is in tipsy vector format and contains a vector for each particle pointing from the initial position to the final localized (or moved) position. This file can also be read in by tipsy and can be used to create a hedgehog plot. This is very useful for understanding how SKID finds the groups.
• skid.gtp: This file is in tipsy binary format (same format as the input to SKID) and contains one star particle to represent each group. Each particle in this file has as its position the center of the density maximum, as its velocity the center of mass velocity of the group (all in comoving coordinates if applicable), and as its mass the total mass of the group. The radius of the group is also recorded in the star_particle.eps field (also input file's time in the star_particle.tform field) for each star (group) particle. See the file tipsydefs.h for the structure of the tipsy binary format.

Release 1.3 New Features

Obtaining and Building Skid

To build skid,

zcat skid-1.4.1.tar.gz | tar xfv -

make

This should build skid on most systems.

There is also a demo script, which will use a sample file included in the tar file. The sample tipsy binary file is in the XDR standard binary format and is converted to a native binary with totipnat, also included in the tar file.

Use of SKID

Support for skid is now being hosted on SourceForge. For problems, questions and comments concerning this software, please go to the SourceForge Astrophysical N-body Tools site.