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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 29: When Galaxies Collide:
Interacting Galaxies

Readings: Ch 26, section 26-7

Key Ideas

Tidal Interactions between Galaxies:
Close Tidal Encounters
Galaxy-Galaxy Collisions
Splash encounters

Starbursts induced by interactions

Mergers & Galactic Cannibalism

Fate of the Milky Way & Andromeda?

Elbow Room

Galaxies are large compared to the distances between them:

Galaxies are likely to encounter other galaxies a few times over their histories.

Tidal Interactions

Galaxies interact via Gravitation.

Because of their large sizes, two galaxies passing near each other raise mutual tides.

Raising Tides

Tides in galaxy encounters

Tidal stretching along the encounter line.

Color Photo of the Whirlpool Galaxy (17k), an example of a tidally interacting pair.

Computer Simulations

Galaxy interactions are very slow

Much of what we know comes from computer simulations:

Requires the fastest supercomputers available.

Extra: Galaxy Collision Movies

Some of the simulations shown in lecture were done by Dr. John Dubinski, a senior research associate at the Canadian Institute for Theoretical Astrophysics (CITA). Visit his home page for more details.

A more "artistic" rendering of these simulations can be found at the website of GRAVITAS, a collaboration between John Dubinski and composer John Kameel Farah. [Beware: the QuickTime format movies are OK, but I find that the AVI format versions won't play without having some special software].

Another source of movies is Joshua Barnes' Galaxy Transformations Page at the University of Hawaii's Institute for Astronomy.

Galaxy Collisions

Rare direct collisions have more dramatic effects: [Image of "The Antennae", a directly colliding galaxy pair. Blue stars indicate sites of recent massive star formation, and dark dust clouds can be seen in silhouette against the inner parts of the two galaxies. OSU Galaxy Survey Image.]

Splash Encounters

A fast-moving smaller galaxy collides at (or near) the center of a larger disk galaxy (hits a "bulls-eye").

Results in a tidal "splash":

"Intruder" galaxy passes on through.

[Hubble images of the Cartwheel Galaxy, an example of an ring galaxy caused by a bull's eye "splash" encounter]


Case of intense star formation in a galaxy.

The most intense starbursts occur in violently interacting galaxy pairs.

[Hubble images of starburst in the colliding galaxy pair known as the Antennae]


If two colliding galaxies can dissipate enough orbital energy: Mergers may play a pivotal role in the formation ("assembly") of galaxies.

Galactic Cannibalism

Slow encounter between a large & a small galaxy: May be the way giant Ellipticals grow.

The Milky Way & Andromeda

The Milky Way (us) and Andromeda are perhaps on a collision course. Eventually, only 1 galaxy would remain behind, most likely an Elliptical galaxy.

This idea is controversial: it depends on knowing the tangential motion of the Milky Way relative to Andromeda, which cannot be measured reliably until the next generation of astrometric satellites.

[Coincidentally, the collision timescale quoted above is also the timescale on which the Sun will be evolving away from the Main Sequence if the scenario described above unfolds on the timescales indicated. See John Dubinski's The Metamorphosis of the Local Group page for more details.]

Return to [ Unit 4 Index | Astronomy 162 Main Page ]
Updated: 2006 February 11
Copyright Richard W. Pogge, All Rights Reserved.