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ELLIPTICAL FLYBY

Reference axes are 80 kpc long, equal to the diameter of G1, the disk galaxy. The colours are: X,Y,Z = green, blue, orange.

Time step and video durations are 0.5 million years and 5 billion years, respectively. Visible stars move under the influence of the gravity of a central bulge and concentric DM halo in G1, as well as a galaxy-size "bulge" and DM halo in G2, the elliptical flyby. The bulge and halo of each galaxy are concentric spheres having radially-variable mass densities. Stars do not feel the gravity of other stars.

G1 (disk galaxy) specifications: Bulge mass and radius are 1.0x10¹⁰ solar masses and 8 kpc, respectively. A kinematically distinct bulge population is not present. Disk stars orbiting within the bulge are coloured yellow; stars beyond appear blue. The disk initially extends to 40 kpc and contains 3000 stars total.

The DM halo of G1 masses 5.4x10¹⁰ solar masses and terminates at 50 kpc radius. The interaction spreads disk and - potentially - bulge stars around but does not change the sizes of either bulge or halo.

G2 (elliptical) specifications: The galaxy-size bulge masses 1.0x10⁹ solar masses and extends to a radius of 3.0 kpc. 316 red stars delineate this galaxy.The accompanying DM halo masses 5.4x10⁹ solar masses and extends to 5.0 kpc.

COMMENTS: The two-galaxy system is marginally unbound (i.e. near zero total energy). G2 passes through the G1 disk moving "down", i.e towards negative Z. A diffuse, extended tidal feature emerges from G1. Its disk is largely preserved, although a variety of ephemeral arc-like structures develop. In real life such features might spawn a long-lived spiral pattern.

NOTE: I've spent a lot of time trying to reproduce the structure of M51, perhaps the best known example of a spiral interacting with a smaller passing galaxy. No luck! One possible reason is the daunting size of the initial parameter space together with limited computing power. The following is an example of a more successful emulation of reality.

RING GALAXY

This is a special case of an elliptical flyby. Reference axes are 20 kpc long; the X axis is now yellow.

Time step and video duration are 0.5 million years and 1.5 billion years, respectively. As above, the gravity of each galaxy originates in a bulge and concentric DM halo. Visible stars respond to these four components, but not to the gravity of other stars.

G1 specifications: A kinematically distinct population of bulge stars is included - 264 yellow stars. Bulge mass is 1.0x10¹⁰ solar masses. The disk is delineated by 4000 blue stars and extends to 10 kpc.

The DM halo of G1 masses 5.4x10¹⁰ solar masses and extends to a radius of 15 kpc. As above, the interaction does not change the sizes of either bulge or halo.

G2 specifications: The galaxy-size bulge mass and size taken to be 3.0x10⁹ solar masses and radius 2 kpc. 1056 red stars delineate its size. The DM halo masses 1.6x10¹⁰ solar masses and extends to 4 kpc.

COMMENTS: The video records a single-pass flyby; the system is effectively unbound (positive total energy). G2 starts at Z = 35 kpc (out of sight above the field of view) and is moving at 100 km/sec towards G1. It passes through the G1 disk near it centre. The results are shown from the perspective of an observer outside G1 but moving along with its centre. G2 pulls more forcefully on G1 stars near where it passes through the disk, causing a pronounced bowl-shaped distortion. From above, a remarkable series of oscillating ring-like structures are generated in the disk. The slightly skewed path of G2 induces asymmetries. Perturbed disk stars continue their radial oscillations to the end of the video. the rather compact G2, on the other hand, survives the encounter relatively intact. Few of its stars are captured by G1, although the galaxy as a whole is somewhat "puffed up".

NOTE: Hoag's Object is a good example of a ring galaxy. Unfortunately we have only a face-on view of this system until sometime after Starship Enterprise is commissioned. To my knowledge the interloper has not been identified.