14-5 Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomena

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Occasionally, two galaxies within a cluster or from adjacent clusters can collide with each other. Past collisions have hurled vast numbers of stars into intergalactic space. In some cases, we can even observe a collision in progress, a cosmic catastrophe that gives birth to new stars. And astronomers can predict collisions that will not take place for billions of years, such as the collision that is fated to occur between the galaxy M31 and our own Milky Way Galaxy.

High-Speed Galaxy Collisions: Shredding Gas and Dust

When two galaxies collide at high speed, the huge clouds of interstellar gas and dust in the galaxies slam into each other and can be completely stopped in their tracks. In this way, two colliding galaxies can be stripped of their interstellar gas and dust.

The best evidence that such collisions take place is that many clusters of galaxies are strong sources of X-rays (Figure 14-21). This emission reveals the presence of substantial amounts of hot intracluster gas (that is, gas within the cluster) at temperatures between 10⁷ and 10⁸ K. The only way that such large amounts of gas could be heated to such extremely high temperatures is in violent collisions between galaxies.

Figure 14-21: X-ray Emission from a Cluster of Galaxies (a) An X-ray image of this cluster of galaxies shows emission from hot gas between the galaxies. The gas was heated by collisions between galaxies within the cluster. (b) The galaxies themselves are too dim at X-ray wavelengths to be seen in (a), but are apparent at visible wavelengths. This cluster, one of many cataloged by the UCLA astronomer George O. Abell, is about 1 billion ly from Earth in the constellation Serpens. (a: NASA, CXC, and University of California, Irvine/A. Lewis et al.; b: Palomar Observatory DSS)

In a less-violent collision or a near miss between two galaxies, the compressed interstellar gas may have more time to cool, allowing many protostars to form. Such collisions may account for starburst galaxies such as M82 (Figure 14-22), which blaze with the light of numerous newborn stars. These galaxies have bright centers surrounded by clouds of warm interstellar dust, indicating recent, vigorous star birth. Their warm dust is so abundant that starburst galaxies are among the most luminous objects in the universe at infrared wavelengths.

Figure 14-22: RIVUXG A Starburst Galaxy Prolific star formation is occurring at the center of the irregular galaxy M82, which lies about 12 million ly from Earth in the constellation Ursa Major.

The starburst galaxy M82 shown in Figure 14-22 also shows the effects of strong winds from young, luminous stars. It also contains a number of luminous globular clusters. Unlike the globular clusters in our Galaxy, whose stars are about 12.5 billion years old, those in M82 are no more than 600 million years old. These young globular clusters are another sign of recent star formation.

M82 is one member of a nearby cluster of galaxies that includes the beautiful spiral galaxy M81 and a fainter elliptical companion called NGC 3077 (Figure 14-23a). Radio surveys of that region of the sky reveal enormous streams of hydrogen gas connecting the three galaxies (Figure 14-23b). The loops and twists in these streamers suggest that the three galaxies have had several close encounters over the ages. A similar stream of hydrogen gas connects our Galaxy with its second-nearest neighbor, the Large Magellanic Cloud, suggesting a history of close encounters between our Galaxy and the LMC.

Figure 14-23: The M81 Group (a) The irregular starburst galaxy M82 is shown close up in a nearby cluster of about a dozen galaxies, dominated by the spectacular spiral M81. (b) Several of the galaxies in this cluster are connected by streamers of hydrogen gas. This wide-angle visible-light photograph shows the three brightest galaxies of the cluster, dominated by M81. The area shown is about 1° across. (c) This false-color radio image of the same region, created from data taken by the Very Large Array, shows streamers of hydrogen gas that connect the three bright galaxies as well as several dim ones.

Tidal Forces and Galaxy Mergers

Tidal forces between colliding galaxies can deform the galaxies from their original shapes. The galactic deformation is so great that thousands of stars can be hurled into intergalactic space along huge, arching streams. (This same effect has stripped material away from the Canis Major Dwarf as it orbits the Milky Way, as shown in Figure 14-16.) Supercomputer simulations of such collisions show that while some of the stars are flung far and wide, other stars slow down and the galaxies may merge.

Figure 14-24 shows one such simulation. As the two galaxies pass through each other, they are severely distorted by gravitational interactions and throw out a pair of extended tails. The interaction also prevents the galaxies in the simulation from continuing on their original paths. Instead, they fall back together for a second encounter (at 625 million years). The simulated galaxies merge soon thereafter, leaving a single object. Cosmic Connections: When Galaxies Collide explores a real-life example of two galaxies that are colliding in just this manner.

Figure 14-24: A Simulated Collision Between Two Galaxies These frames from a supercomputer simulation show the collision and merger of two galaxies accompanied by an ejection of stars into intergalactic space. It shows a small galaxy (yellow stars) being devoured by a larger, disk-shaped one (blue stars, white gas). Note how spiral arms are generated in the disk galaxy by its interaction with the satellite galaxy.

Our own Milky Way Galaxy is expected to undergo a galactic collision like that shown in Figure 14-24. The Milky Way and the Andromeda Galaxy, shown in Figure 14-2, are actually approaching each other and should collide in another 6 billion years or so. (Recall that our solar system is only 4.57 billion years old.) When this happens, the sky will light up with a plethora of newly formed stars, followed in rapid succession by a string of supernovae, as the most massive of these stars complete their life spans. Any inhabitants of either galaxy will see a night sky far more dramatic and tempestuous than our present one.

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When two galaxies merge, the result is a bigger galaxy. If this new galaxy is located in a rich cluster, it may capture and devour additional galaxies, growing to enormous dimensions by a sort of galactic cannibalism. Cannibalism, in this sense, differs from mergers in that the galaxy that does the devouring is bigger than its “meal,” whereas merging galaxies are about the same size.

Many astronomers suspect that galactic cannibalism is the reason that giant ellipticals are so huge. As we have seen, giant galaxies typically occupy the centers of rich clusters. In many cases, smaller galaxies are located around these giants (see Figure 14-8 and Figure 14-17). As they pass through the extended halo of a giant elliptical, these smaller galaxies slow down and are eventually devoured by the larger galaxy.

COSMIC CONNECTIONS: When Galaxies Collide

Although galaxies can collide at very high speeds by Earth standards, they are so vast that a collision can last hundreds of millions of years. Understanding what happens during a galactic collision requires ideas about tidal forces, star formation, and stellar evolution.