Identikit (Mk0): An N-body/Data Visualization Tool

John Hibbard, NRAO

Josh Barnes, IfA

!!!WARNING!!! Work in progress!!

The Identikit program is a visualization tool to help match N-body numerical simulations of interacting galaxies to observational data of such systems (primarily HI or CO spectral line mapping datacubes). It is composed of two primary functions:

  1. A display for multi-dimensional datacubes. Plots data in each of 4 orthogonal projections: (x,y), (x,Vz), (y,Vz) and (x,z). (See figure on the right and also this screen snapshot). The first three of these projections are measurable with spectral line mapping observations (yellow points in the figure), and the idea is to project numerical N-body data points onto observational data displayed in these three planes (blue and red points in the figure and also this screen snapshot). Menu items or keyboard commands then allow the N-body data to be rotated or scaled until they best match the observed data in all three planes (See this mpeg video).
  2. A database of N-body "snapshots" run offline and stored on disk or CD. These datafiles describe the dynamical encounter of two disk galaxies. The novel approach taken here is that in the simulations, each galaxy is modeled as having a live halo (N=4096), within which is embedded 9x18 massless disks. These disks sample inclinations (measured w.r.t. the orbital plane) ranging from -90 to +90 at increments of 20 deg, and arguments of periapse (measured from the direction of orbital periapse) from 0 to 340 deg in increments of 20deg. There is a disk of 512 massless particles for each combination of angles for each galaxy. Each snapshot therefore contains 2*(4096+512*9*18) = 174080 points. The chosen inclinations only span one sense of rotation, prograde or retrograde. This is b/c generally the appearance of tidal features allows the user to guess whether the progenitors experienced prograde or retrograde encounters. The models have been run using the treecode implemented in the Zeno software system developed by Josh Barnes at the Institute for Astronomy

Model matching proceeds in the following manner: The first step in any model-matching exercise is to survey a range of inclinations and arguments of periapse for each disk to get into the right "ballpark" for the encounter geometry. This program greatly facilities this process. Still, it is just a starting point - fully consistent N-body models must still be run. The ultimate goal to to automate, as much as possible, the determination of physically unimportant parameters such as the disk orientation angles in order to get at the physically interesting structural parameters, such as the galaxy mass models. The "Identikit" name came from the fact that this process is similar (at least in my mind) to the police-blotter program for matching noses, eyes, hairstyle etc to arrive at a composite drawing of a suspect.

Examples of matches to data:

Wish list: (superceded by Josh's Identikit 1 program)

  1. Much better control panel GUI, including slide bars for rotations, scales, and hopefully time, Rperi, ellipticity.
  2. Enable Click and drag rotations and rubber-banding directly in the x-y, x-Vz, y-Vz display planes
  3. More intelligent memory usage and/or navigation to allow more particles to be read in, or failing this more intelligent packaging of data on disk to allow quick input. The goal is cycle through different timesteps and encounter geometries as quickly as possible.
  4. Develop algorithms to optimize the match between the data and simulations in the observed planes and to quantify the "goodness of fit"
  5. Based on best matched model, set up scripts to run fully self-consistent N-body realizations of the chosen parameters.
Related links:
John Hibbard
Last modified: Sat Feb 7 16:39:57 EST 2004