Sneaking Up On The Mice (NGC 4676)

Joshua E. Barnes & John E. Hibbard

February 2004

NGC 4676, also known as `The Mice', is one of the best nearby examples of a collision between two spiral galaxies. The numerical models presented here show how two ordinary galaxies can be transformed into a dramatic interacting system.

Optical image of `The Mice', NGC 4676. North is right, east is up. Image by John Hibbard.

On account of their rounded bodies and long tails, the two galaxies making up NGC 4676 were dubbed the `Playing Mice' by Vorontsov-Vel'yaminov (1958). The individual galaxies are NGC 4676a, to the north, and NGC 4676b, to the southeast. The length and straightness of the bright northern tail imply that we are viewing the aftermath of a nearly-direct passage roughly edge-on to the orbit and disk planes, while the curve of the fainter southern tail is consistent with a more face-on view of an inclined disk. The fairly equal lengths of the two tails suggests that the galaxies involved had roughly equal masses. Along with long-slit data indicating that both `hulks' rotate with north receding (Burbidge & Burbidge 1961; Theys, Spiegel, & Toomre 1972), these considerations led Toomre & Toomre (1972) to a simple model for this system.

Modeling The Mice

John Hibbard and I began working on a self-consistent model of the Mice in 1995. Our goal was to find a plausible model matching the morphology and VLA observations; the model shown here is a good match. For more data, see John Hibbard's web page on NGC 4676.

Simulation of 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, shown at left, occurs about one time unit after pericenter.
Rotation about NS axis. The three-dimensional structure of this model, at the time best matching the observations, is revealed by rotation about the north-south axis. As the view above shows, the northern tail appears straight because we view it almost edge-on.
Simulation of The Mice. This version shows the material from each disk in a different color.
Very Large Array observations and computer simulation. Besides matching the appearance of The Mice, a good simulation should also reproduce the motions of both galaxies and their tails. Here, red contour lines show velocities of interstellar hydrogen gas in The Mice measured using the VLA, and blue dots show the velocities predicted by the computer. The agreement is quite good.

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, which was used for the animations seen below.

Advanced Camera for Surveys image of The Mice. For more images, see the Space Telescope ACS press release.
Computer simulation of The Mice (2.4 Mbyte MPEG or Selected Frames). This view matches the angle and field of the ACS image (which unfortunately cuts off the ends of the tails).
Morphing The Mice (3.4 Mbyte MPEG). In this version, produced by STScI, the computer simulation morphs into the ACS image and back.
Another rendition of the same simulation (6.6 Mbyte MPEG). This animation shows a somewhat larger field of view. In addition, the dark halos of the galaxies, accounting for 80% of the mass, are shown in red.
Image of The Mice, taken with the University of Hawaii 2.2 meter telescope on Mauna Kea. Although not nearly as detailed as the ACS image, this picture shows the full extent of the tidal `tails'. For more images, see John Hibbard's web page on NGC 4676.

Models With Star Formation

These models were run to test a new treatment of star formation. To date, most simulations in the literature set the rate of star formation proportional to a power n of the local gas density; this concentrates star formation in nuclear regions where the gas density is highest. The new approach taken here makes the star formation rate proportional to a power m of the local dissipation rate; this yields star formation in regions where strong shocks exist. For more details, see Shock-induced star formation in a model of the Mice, to appear in MNRAS.

The animations below present three simulations of NGC 4676: one with density-dependent star formation and two with shock-induced star formation. Two views of each simulation are available: one is projected on the plane of the sky, and the other is projected onto the orbital plane; our view of NGC 4676 is roughly edge-on to the orbital plane. In each animation, disk stars (blue) and bulge stars (red) are on the left, gas density (red) and shocks (blue) are in the middle, and new star particles, with color indicating time since formation, are on the right.

Density-dependent Star Formation (n = 1.5). Left: Sky-plane view (0.40 Mbyte MPEG). Right: Orbit-plane view (1.04 Mbyte MPEG). This simulation used the older rule, which links star formation to gas density. There is no obvious starburst as the galaxies first collide, and most star formation is concentrated in the nuclei.
Shock-induced Star Formation (m = 0.5). Left: Sky-plane view (0.44 Mbyte MPEG). Right: Orbit-plane view (1.20 Mbyte MPEG). This simulation used the new rule linking star formation to shocks. Note the widespread starburst as the galaxies first interpenetrate, and the continued star formation outside the nuclei at later times.
Shock-induced Star Formation (m = 1.0). Left: Sky-plane view (0.42 Mbyte MPEG). Right: Orbit-plane view (1.14 Mbyte MPEG). This is a variant of the model above; here, the star formation rate depends more steeply on dissipation. The bursts at first and second passage are more pronounced, with less extended star formation elsewhere.

This animation compares the distributions of new stars in all three simulations.

New star particles (1.34 Mbyte MPEG). Left: Density-dependent star formation (n = 1.5). Middle: Shock-induced star formation (m = 0.5). Right: Shock-induced star formation (m = 1).

Photorealistic Images

Models including star formation can be combined with the output of a stellar population synthesis code to produce realistic images in standard photometric wavebands. Graduate student Lisa Chien and I are using Starburst 99 for this purpose. The images included here take one additional step: the gas along the line of sight to each luminous particle is assigned some opacity to simulate dust extinction. The calibration of the opacity in these images is still somewhat preliminary. These are, however, perhaps the most realistic renditions of N-body simulations now in existence.

The disk galaxies shortly before 1st passage. Note the effects of dust extinction on the inclined disk at left.
The galaxies shortly after 1st passage. Note the dust lane!
This frame and the next two are plausible matches to the morphology of the Mice. Again, note the dust features, as well as the blue colors.
The future of the Mice?

Joshua E. Barnes (

Last modified: September 24, 2004