# Identikit Parameters

 Abstract representation of the parameter space for an encounter of two disk galaxies. The radial coordinate encompasses the parameters describing the relative orbit. The angular coordinate represents the angles which specify the orientations of the two disks. The vertical coordinate includes the time since pericenter, viewing angles, and scale factors. A conventional N-body simulation starts at a single point on the horizontal plane and explores the dotted line above it. In contrast, a single Identikit simulation can explore the entire surface of the cylinder.

### Orbit Parameters

Assuming that the two galaxies were originally well-separated, their initial orbit will approximate a Keplerian orbit of two point masses. The parameters used to specify this ideal orbit are:

• The pericentric separation (minimum distance), p. Note that p describes the idealized Keplerian orbit; the actual separation of the two galaxies at first pericenter will be different since the galaxies depart the idealized orbit once they begin to overlap.
• The galactic mass ratio, μ.
• The orbital eccentricity e. A parabolic orbit has e = 1, while a circular orbit has e = 0.

Since a separate Identikit simulation must be run for each combination of orbit parameters (p, μ, e), these parameters are chosen from a set of available values. At present, eight values of p and two values of μ are available, while e is fixed at e = 1 (parabolic orbit).

### Disk Orientations

The initial Keplerian orbit is confined to the (x, y) plane, with its angular momentum vector along the positive z axis. The orbit is oriented so that the point masses lie on the x axis at pericenter. The orientation of each disk with respect to this orbit is specified by two parameters:

• Inclination i is the angle between the disk's angular momentum vector and the z axis. Note that 0° ≤ i ≤ 180°.
• Azimuth φ is the angle between the disk's angular momentum vector, projected onto the (x, y) plane, and the y axis. Note that 0° ≤ φ ≤ 360°.

### Viewing Parameters

The next set of parameters specifies how a simulation is viewed once it has been run.

• The time, t, is defined such that t = 0 at the instant when the idealized Keplerian orbit reaches pericenter. Allowed values are −2 ≤ t ≤ 8; by the latter time, almost all encounters have merged.
• The viewing angles (θX, θY, θZ) map simulation coordinates (xyz) to sky coordinates (XYZ). The transformation is (XYZ) = RZ RX RY (xyz), where Rα rotates by θα around axis α.
• The field-of-view parameters Xf and Vf specify the range of coordinates shown in each panel. Position coordinates range from −Xf ⁄ 2 to Xf ⁄ 2, while velocity coordinatess range from −Vf ⁄ 2 to Vf ⁄ 2.

### Image Parameters

• The σ parameter governs how many particles are plotted. Excessively large values cause the magenta galaxy to disappear.
• The size parameter specifies the length of each of the four panels displayed.

 Joshua E. Barnes      (barnes at ifa.hawaii.edu) Updated: 31 March 2011 http://www.ifa.hawaii.edu/~barnes/research/identikit/parameters.html