Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 29: When Galaxies Collide:
Readings: Ch 26, section 26-7
- 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?
Galaxies are large compared to the distances between them:
- 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
Galaxies interact via Gravitation.
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.
Tidal stretching along the encounter line.
Color Photo of the Whirlpool Galaxy (17k), an
example of a tidally interacting pair.
- Near side feels stronger gravitational pull from the companion
- Far side feels weaker gravitational pull and lags behind the
Galaxy interactions are very slow
- 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.
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
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'
Transformations Page at the University of Hawaii's
Institute for Astronomy.
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.]
- 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.
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":
- Circular density wave moving outward through the "target"
- 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]
Case of intense star formation in a galaxy.
- 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
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
- 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).
Slow encounter between a large & a small galaxy:
May be the way giant Ellipticals grow.
- Smaller galaxy gets torn apart by the tides from the larger
- Gas and stars get incorporated into the larger galaxy.
- Nuclei of the galaxies slowly spiral together.
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
- 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.]
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Updated: 2006 February 11
Copyright © Richard W. Pogge, All Rights Reserved.