1.1. Towards a Model of the Antennae
The Antennae galaxies (NGC 4038 / 9) are fast becoming the ``Rosetta stone'' of interacting systems; detailed observations in almost every waveband from 21 cm to X-rays provide a remarkably complete picture of the behavior of interstellar material and star formation in the earlier stages of a galactic merger. These galaxies have also long been a favorite of N-body experimenters. But until recently, the available line-of-sight velocity data were not good enough to support detailed simulations. New VLA observations (Hibbard, van der Hulst, & Barnes, in preparation) offer the chance to refine existing models. Goals for an improved model of the Antennae include:
Matching the observed velocity field. The radial velocities of the two galaxies differ by only ~ 40 km sec-1. To produce this, the galaxies must either be near apocenter, or falling together almost perpendicular to our line-of-sight.
Reconciling the adopted orbit with cosmological expectations. Simulations by Toomre & Toomre (1972) and Barnes (1988) adopted elliptical (e 0.6) orbits; parabolic orbits seem more plausible.
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).
Explaining the ``overlap region''. Recent ISO maps show this dusty region is brighter than either disk in mid-IR wavebands (eg., Mirabel et al. 1998).
Goals one and two involve adjusting the orbit, the viewing angle, and the orientations of the two disks. To rapidly explore this vast parameter space, I run ``semi-consistent'' simulations in which each galaxy is represented by a self-gravitating spheroid with a number of embedded test particle disks; the two disks best matching the observations are selected interactively after the calculation has run. Starting with orbits as eccentric as e = 0.8, this technique yields models which roughly reproduce the velocity field as well as the crossed-tail morphology of NGC 4038 / 9. But still less than satisfactory are the shapes of the gently curving tails and the orientations of their parenting disks; experiments are under way to study these problems and make models with parabolic initial orbits.
Goals three and four depend on gas dynamics. In high-resolution HI maps, gas in the southern tail seems to join continuously onto the ring in NGC 4038. Rings of similar size and morphology may arise as a result of gas falling back along tidal tails; the formation of such a ring is illustrated in Figure 1. Simulations of the Antennae reproducing this feature might shed some light on the conditions of star-formation in this system. Perhaps more challenging is to account for the IR-luminous overlap region. This seems to be more than just the superposition of two disks; it is probably some sort of bridge, perhaps extended along the line of sight.
Figure 1. Formation of a gas ring by accretion of material from a tidal tail (see Barnes & Hernquist 1998, video segment 5, section 2).