Knowledge in a Nutshell: Astrophysics

Chapter 14 Interacting Galaxies

Galaxies are not fixed structures but alter shape over time as a result of both internal changes and interactions with the environment.

The most common form of interaction is the galaxy–galaxy collision in which two galaxies gravitationally interact with each other. At low speeds, this interaction can involve galaxy mergers and ‘cannibalism’, causing the larger of the two systems to absorb the interloper. This is also the favoured mechanism for growing larger galaxies from smaller dwarf galaxies. Astronomers think this has been the major galaxy-growing mechanism from the early history of the universe to the present time. At the typical relative speeds of nearby galaxies, about 300 km/s, galaxies can travel the average million-light year distances that separate themselves in about 1 billion years. This is enough time for many of these distant encounters to take place in the 12-billion-year history of a galaxy since the Big Bang (see pages 186–194). Dwarf galaxies, such as the Magellanic Clouds at a distance of 160,000 light years, are even now interacting with the Milky Way. They may still be around in the next 7 billion years, although gradually stripped of most of their stars.

When galaxies collide, the gravitational transformations can completely alter the individual galaxies and form an end state that is unlike that of the participating galaxies. In 4 billion years, the Milky Way and the Andromeda Galaxy, both large spirals, will collide and merge to produce a giant elliptical galaxy with a Hubble morphological class near E0, but with some gas and dust available to form a brief period of star formation. Other collisions can produce single spiral arm ‘tidal tails’ and other mixed shapes with complicated population mixtures.

Our Milky Way has been steadily cannibalizing the smaller galaxies in its neighbourhood, and is currently disrupting the Large and Small Magellanic clouds, which are Pop I irregular galaxies. A flow of hydrogen gas called the Magellanic Stream shows that these dwarf galaxies have already had one encounter with the Milky Way and, by some calculations, will probably merge with the stellar population of the Milky Way within the next few billion years. Although these galaxies themselves will become invisible, their stars and some of their dense interstellar clouds will be discernible as having a high peculiar speed relative to other stars in the halo. Astronomers have also discovered several other families of ‘high velocity stars’ that no doubt indicate other dwarf galaxies that have long since been fully cannibalized. Recently, astronomer Stefan Meingast used the billion-star Gaia satellite survey to uncover 4,000 stars left over from a collision with the Milky Way and a globular star cluster perhaps a billion years ago. There may be many of these ‘star rivers’ to be found as fossils of other encounters, hidden in plain sight among the stars of the Milky Way.

Our own Milky Way has been involved in multiple collisions with small dwarf galaxies in the past. In 2007, astronomers using the Spitzer Space Telescope identified three streams of stars connected to the nearby dwarf galaxies in Sagittarius and Canis Major, each containing 100 million stars. Currently, 12 streams have been identified as the remains of ancient dwarf galaxies. In 2012, astronomers detected a distortion in the distribution of stars above and below the plane of the Milky Way and attributed this to a collision with a very large dwarf galaxy about 100 million years ago. The encounter may not have destroyed the dwarf galaxy, which may still be present within the 54 members of the Local Group today. In 2018, the Gaia satellite measured the properties of over 1.7 billion stars near the Sun, and verified the existence of at least one cloud of stars not rotating around the Milky Way like other stars, but instead on a peculiar trajectory through the galaxy. The cloud also contains stars with higher-than-normal abundances of heavy elements. This represents a collision that may have happened about 10 billion years ago and involved a metal-rich, very massive dwarf galaxy with a mass of 10 billion suns. It may even have been this collision that triggered the formation of the spiral arms in the Milky Way.

The most active regions of space for galaxy mergers and collisions is in the cores of dense clusters of galaxies, such as the Coma Cluster, or in very compact groups of galaxies such as Stephan’s Quintet. These systems can have galaxy separations of no more than a few times the diameters of the galaxies themselves, and over millions of years this leads to frequent mergers.

High-speed encounters can be even more dramatic as the galaxies pass each other by at speeds beyond the gravitational capture speed. The result for galaxies of similar mass is the gravitational production of ‘tidal arms’ that can extend millions of light years. Across the nearby universe, we only see snapshots of such events at various stages. But modern supercomputer simulations, based upon Newton’s Law of Gravity and tracing millions of ‘mass points’, yield striking movies of what some of these encounters can look like over the course of millions of years.

The details of the collision also affect the final outcome. For example, if the collision is exactly head-on so that the nucleus of one galaxy passes through the nucleus of another, beautiful ring galaxies can form like ripples on a pond. An expanding ring of star formation forms a near-perfect circle about the nuclear region.

The most dramatic effects of galaxy collisions are in the behaviour of the interstellar gas and dust clouds. These components of a galaxy have a far-greater extent than individual stars, and so collisions between them are frequent and intense. The likelihood that individual stars actually collide is extremely low due to their very small sizes compared to the vast interstellar spaces that separate them. For example, the pair of galaxies in Arp 299 located 134 million light years away shows a mash-up of cloud collisions triggering the formation of massive stars, and a dramatically heated interstellar medium. This is detectable by the Chandra Observatory by its X-ray emissions. This galaxy is one of the most active star-forming galaxies in the local universe. Since 1990, eight supernovae have been detected in Arp 299.

The abundance of dust clouds forming massive luminous stars makes these collision and merger environments among the brightest infrared sources in the universe.

Another feature of a galaxy’s environment that influences its evolution is the presence of gas between the galaxies within a cluster. In Arp299, the production of some of this high-temperature medium is just beginning. However, the largest quantities are produced when spiral galaxies rich in interstellar material collide at high speed. The gas is far too hot to remain trapped by the galaxies, and so diffuses very quickly into the void between galaxies within a cluster. For some clusters, this medium is dense enough that as galaxies pass through it, it acts like a resisting medium and sweeps out the ISM housed within them. An example of this is ESO 137-001, which is falling into the Abell 3627 galaxy cluster.

These interactions were previously discovered in the 1960s and 1970s as elongated radio sources such as Perseus A, which seemed to be streamlined by their host galaxy’s motion. Modern telescopes such as the Chandra X-ray Observatory can now directly image these stripped gases.

Dark Matter

The most elusive material in the cosmos today is called Dark Matter. Originally discovered by astronomer Fritz Zwicky in 1933 within clusters of galaxies, and then in the 1970s by Vera Rubin in individual galaxies, it is the gravitating substance that dominates the universe. It is five times more abundant than ordinary luminous matter: the stars, nebulae, ISM and other space material we can actually see. This has huge consequences for the way galaxies behave as they collide, as the majority of what is holding them together is an invisible, massive halo of dark matter in which the galaxy is embedded. Based on the motions of star clusters in its Halo region, our Milky Way has a mass of about 1.5 trillion suns out to 130,000 light years from its core, but only 10 per cent of this is in stars and the interstellar medium.

The Bullet Cluster (1E0657-558) is located 3.7 billion light years from the Milky Way, and is one of the classic examples of dark matter in clusters of galaxies. We see the two groups of galaxies embedded in their own dark matter halos (labeled A and B in the figure opposite) after a collision that took place about 150 million years ago. It left behind the hot intergalactic medium (the cloud located half way between A and B) seen emitting in X-rays. The dark matter follows the galaxies (it only interacts via its gravity) and is completely unaffected by the collision, unlike the normal matter seen at the collision site.

Galaxy collisions can eject gas into the intra-cluster space of a galaxy cluster and can also eject stars. Although we cannot see the individual stars, we can detect their combined optical emission. Recently, astronomers Mireia Montes and Ignacio Trujillo at the University of New South Wales used Hubble Space Telescope images of six galaxy clusters to detect this faint light. Because these stars followed the gravitational wells created by dark matter and cluster galaxies, they were able to use this starlight to study the distribution of dark matter in these clusters to far higher resolution than previous methods allowed.

Key Points

• When galaxies collide, individual stars do not crash into each other, but the vastly larger interstellar medium and dense clouds do encounter each other.

• Galaxy collisions are powerful triggers of star-forming activity as interstellar clouds collide and portions of them are forced into collapse, leading to copious numbers of massive stars being formed.

• Galaxies found in clusters often encounter an intra-cluster medium created by previous galactic collisions. This very hot medium can dramatically affect the evolution of galaxies, and is often detected by its X-ray emission.

• Dark matter is a ubiquitous but invisible component to galaxies, but it can be detected by studying the collisions between clusters of galaxies and the motions of the galaxies and intra-cluster medium.