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

Lecture 29: When Galaxies Collide:
Interacting Galaxies

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

• Most galaxies are separated by only ~20 times their diameters.
• By comparison, most stars are separated by ~10⁷ times their diameters.

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

Tidal Interactions

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

• These tides distort the shapes of the galaxies
• Gives rise to dramatic effects without direct collision.
• Many "peculiar galaxies" are interacting pairs.

Raising Tides

Tidal stretching along the encounter line.

• Near side feels stronger gravitational pull from the companion
• Far side feels weaker gravitational pull and lags behind the near side.

Computer Simulations

• Timescales of ~1 billion years

Much of what we know comes from computer simulations:

• Solve Newton's laws of motion for gas & stars.
• Compare predictions with observed properties of interacting galaxies.

Requires the fastest supercomputers available.

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

• Tides raised are stronger, giving greater tidal distortion
• Tear off huge "Tidal Bridges" of stars & gas
• Stars pass through without colliding, but
• Gas clouds collide, leading to a massive starburst in the galaxy disks.

Splash Encounters

Results in a tidal "splash":

• Circular density wave moving outward through the "target" galaxy.
• Wave triggers star formation in the disk gas

"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]

Starbursts

• Millions of O & B stars greatly enhance the brightness of the galaxy.
• Exhausts the available gas in a few Myrs.
• Many supernovae can drive fast "superwinds" blowing out of the galaxies.

The most intense starbursts occur in violently interacting galaxy pairs.

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

Mergers

• Wreckage merges into a single galaxy.
• Gas clouds collide and form new stars.
• Some portion of the old stars are ejected from the system (carry off orbital energy).

Galactic Cannibalism

• Smaller galaxy gets torn apart by the tides from the larger galaxy.
• Gas and stars get incorporated into the larger galaxy.
• Nuclei of the galaxies slowly spiral together.

The Milky Way & Andromeda

• Moving towards each other at ~120 km/sec
• In ~3-4 Gyr, they will have a close encounter
• Tidally distort & merge after ~1-2 Gyr

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.]