There are many instances where galaxies appear to be interacting with
each other enough to cause obvious distortions of the galaxies
that interact. These interactions may have a significant connections with the
manner in which
galaxies evolve with time.
Example: Arp 273 Interacting Galaxies
The adjacent pair of interacting spiral galaxies in the constellation Andromeda
at a distance of about 200 million light years
is called Arp 273
The larger galaxy at the upper left is strongly tidally distorted.
The companion (seen edge-on in the lower right)
exhibits a relatively undisturbed
spiral disk, but a luminous,
nucleus (one in which there is a burst of new star formation).
The nucleus of the large spiral
has properties indicating
much less activity than the bright nucleus
of its companion.
large galaxy has a modified disk but little evidence of damage to its nucleus,
while the small galaxy has an undisturbed disk but a very active nucleus.
CREDIT: National Optical Astronomy
Example: A Fiery Galactic Collision
The following figure,
which is a composite of two images in blue and
near-infrared light taken with the
Planetary Camera (WFPC2) of the
Hubble Space Telescope,
suggests that two galaxies have actually
collided with each other, with a violent wave of star-forming material ejected
as a result (Source).
The Cartwheel Galaxy is a member of a group of galaxies
about 500 million light years away in the constellation Sculptor.
The remarkable ring-like structure seen in the right side of the image is about
150,000 light years across (larger than the Milky Way Galaxy).
The ring is a wave of star formation traveling outwards at about 200,000 miles per hour
that would not be there if the galaxy had not collided with
It is a ripple of energy deposited in the center of the galaxy by the passage of another galaxy of the
group through it. As the wave
passes outward it compresses and heats the matter
that it passes through, triggering the star formation.
The top left inset shows
a magnified portion of the ring. The bright blue regions are clusters of new star formation and the loops
and bubbles result from supernovae produced by massive star formation. The loop extending to the left
near the center of
this inset appears to be a wave of new secondary star formation on the leading edge of an expanding
bubble that is a supernova remnant. It is estimated that the wave
of star formation in the ring represents 2 billion new stars.
The lower left inset shows the nucleus and inner part
of the galaxy, which is just beginning to recover its spiral
structure. The center contains large amounts of dust and clusters of new star formation (the bright
points). We may guess that this galaxy was a spiral much like our own Milky Way before this collision
for an explanation of what you are seeing.
Possible evidence for the galaxy responsible for the collision with the Cartwheel is given in the adjacent
image which shows a neutral hydrogen map of the region around the Cartwheel Galaxy constructed from radio
The trail of neutral hydrogen from the Cartwheel Galaxy to the galaxy in the upper right
part of the image suggests that this galaxy
may have passed through the Cartwheel and initiated the vigorous
activity found there now.
Other examples of galaxies where collisions appear to be taking place include
The Sleeping Beauty
Galaxy (M64), where the center of the galaxy seems to be rotating in the
opposite direction from the outer regions, and the galaxy
where the motion of objects in the center is extremely
rapid and jumbled.
Example: The Antennae Galaxies
The following Hubble Space Telescope image shows two colliding galaxies known as the "Antennae Galaxies"
because of their peculiar shape
(Ref). The two galaxies are 63 million light
years away, in the constellation Corvus.
The long tails are thought to be due to tidal distortion following the
collision of two galaxies. The right portion of the image shows a blowup of the region outlined in green
in the left portion of the image. At least a thousand bright young star clusters can be found in the
image, the result of vigorous star formation triggered by the collision. The cores of the two colliding
galaxies are the two orange blobs.
The adjacent animation shows a supercomputer simulation of the collision of the two galaxies that
formed the Antennae Galaxies (Source: Joshua Barnes, U. Hawaii;
Example: An Elongated Spiral
The adjacent image shows one of the longest galaxies known, the SBb barred spiral
NGC 6872, as imaged by the
European Southern Observatory.
This is one of the largest barred spirals known, measuring 750,000 light years in length.
It is thought that the shape has something to do with the interaction of NGC 6872 with the S0
IC 4970, which is just above the center of the picture. The upper left spiral arm shows an unusual amount
of star formation, which accounts for its blue color
(Ref). It is thought that this star formation
is associated with the recent passage of IC 4970 through this region of NGC 6872
NGC 6872 is about 300 million light years away, in
the southern sky constellation
Pavo (the Peacock).
Example: Stephan's Quintet
An extremely interesting example of possible galactic collisions
can be seen in
shown in the adjacent figure.
This is a close grouping of the visual image of 5 galaxies
(the cores of the 5 galaxies are the brightest spots in the image).
Four of these galaxies are
probably physically close because they have similar
the 5th - in the lower left - is probably
just accidentally in the line of
sight since it has a very different redshift.
In this image, the 3 galaxies in the upper part of the image
appear to be colliding with each
Kitt Peak National
Computer Simulations of Galaxy Collisions
The largest computers may be used to simulate the interaction of galaxies and
the resulting distortions and star formation. Here are three MPEG movies
and descriptive sound files showing
computer simulations of colliding galaxies:
The first two movies show encounters between equal-mass galactic disks with two
different initial orientations of the disks. The third movie shows a simulated
encounter between a gas-rich
and a gas-poor dwarf galaxy. In all
three movies only the motion of the gas is shown---the
unseen dark matter
influences the collisions through gravitational interactions,
but does not show in the simulations. Thus, in the
third movie only the disk galaxy is visible in the movie;
the gas-poor dwarf galaxy is not seen directly. The color coding
in all three movies
represents intensity of star formation: red indicates high
rates and blue denotes low rates of star formation.
where further details on these simulations
may be obtained.
Collision of Andromeda and the Milky Way
Almost all galaxies are moving away from us because of the
Hubble expansion of the Universe.
some nearby galaxies can actually move toward us because of local motion over and
above that associated with the expansion of the Universe (this motion is called
peculiar motion). One such example of peculiar motion is that the
Andromeda Galaxy (adjacent image)
is moving toward us and it appears that Andromeda and
the Milky Way will collide in about 3 billion years.
Here is a
of the collision