Models of Interacting Galaxies

Joshua E. Barnes & John E. Hibbard

February 2004

Numerical modeling is an effective way to test the tidal origins of peculiar galaxies. In addition, detailed modeling of individual systems complements and helps interpret the observational data.

Since the first `reconstructions of orbits and outer shapes' of galaxies such as NGC 4676 and NGC 4038/9 (Toomre & Toomre 1972), modeling galactic collisions has attracted much attention -- and consumed much computer time! The rules of this game are to build models of isolated galaxies, place them on approaching orbits, and evolve the system until it matches the observations; if the model fails to match the observations, one adjusts the initial conditions and tries again. Such experiments helped show that tidal interactions could explain the specific features of interacting systems.

Sneaking Up On The Mice (NGC 4676)

Modeling the Mice. A close encounter between two identical disk galaxies produces a configuration resembling the Mice (Hibbard & Barnes, in preparation). Here the encounter is seen from our viewpoint, almost edge-on to the orbital plane. The numbers at upper right show elapsed time in units of about 160 million years. The best match to NGC 4676 is about one time unit after pericenter.
HST/ACS Images & High-Resolution Models. The Mice were selected as one of the initial targets of the Hubble Space Telescope's Advanced Camera for Surveys. Holland Ford and Garth Illingworth asked us for an animation; we ran a model with N = 1048576 particles.
Models With Star Formation. Most simulations in the literature set the rate of star formation proportional to a power of the local gas density. The new approach taken here makes the star formation rate proportional to a power of the local dissipation rate; this yields star formation in regions where strong shocks exist.

Getting A Feel For The Antennae (NGC 4038/9)

Simulation of `The Antennae', NGC 4038/9. This is a self-consistent adaptation of the Toomres' test-particle model (Barnes 1988). The best match to the present morphology occurs about half-way through the animation; the galaxies merge shortly thereafter.

Constructing models by trial and error is a rather slow process. To rapidly explore the vast parameter space of initial conditions, I devised a semi-consistent approach in which each galaxy is represented by a self-gravitating halo with a number of embedded test particle disks; the two disks best matching the observations are interactively selected after the calculation has run. J. Hibbard (NRAO) and I are using this `identikit' technique to produce models of The Antennae (NGC 4038/9). We are using VLA data of this system (Hibbard et al. 2001) to determine the HI velocity field. Models matching the optical appearance are fairly easy to create (e.g. Barnes 1988), but the velocity field data provides a significant additional constraint. At present we have a very preliminary model matching the overall morphology and velocity data. Goals for improved models of the Antennae include:

  1. Reconciling the adopted orbit with cosmological expectations. Earlier simulations used elliptical orbits (e = 0.6), but parabolic orbits are more plausible.
  2. Upgrading from semi-consistent to fully self-consistent models. Massive disks will accelerate orbit decay. Moreover, self-gravitating disks can respond to external torques by precessing something like solid bodies.
  3. Reproducing the gas-rich ring in NGC 4038. This ring, clearly seen in maps of HI as well as in mid-IR (Mirabel et al. 1998), contains many luminous young star clusters (Whitmore & Schweizer 1995).
  4. Exploring the nature of the `overlap region'. ISO maps show this dusty region is brighter than either disk in mid-IR wave-bands (e.g. Mirabel et al. 1998).

Smashing The `Atoms For Peace' Galaxy

Another galaxy we plan to model self-consistently -- including gas dynamics -- is NGC 7252. Earlier work produced a model which reproduces the overall morphology and HI velocity field of this system (Hibbard & Mihos 1995), but the calculations did not include a dissipative component which could form a central disk like the one detected in HST images (Whitmore et al. 1993). It's worth repeating these experiments with a combined N-body/SPH code; a single model reproducing both the tails and the central disk would aid in interpreting the observations of this system, and may constrain the gas content of NGC 7252's progenitors.

Joshua E. Barnes (

Last modified: February 12, 2004