Colliding
Galaxies

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 (Source). 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, star burst 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. Thus, the 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.

SOURCE: http://www.noao.edu/image_gallery/html/im0011.html
CREDIT: National Optical Astronomy Observatories

Example: A Fiery Galactic Collision

The following figure, which is a composite of two images in blue and near-infrared light taken with the Wide-Field and 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 another. 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 took place. Click here 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 telescope data (Ref). 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 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 NGC 6240, 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; Ref).

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 galaxy 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 (Ref). 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 Stephan's Quintet, 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 redshifts; 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 other. Credit: Kitt Peak National Observatory.

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 disk galaxy 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.

Source: http://zebu.uoregon.edu/movie.html, 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. However, 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 simulation of the collision (Source).

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