Cosmic Collisions: Galaxy Mergers and Interactions

One of the most spectacular events in the universe occurs when two galaxies collide with one another. The gravitational pull between two companion galaxies results in a sort of cosmic dance as the pair of galaxies orbit each other. Over time gravitational interactions can cause the two galaxies to get closer until they eventually merge into a single galaxy. This is illustrated in the video below, which shows a simulation by Josh Barnes, from the University of Hawaii, of two galaxies merging. At each close pass, galaxies can strip material (including stars, gas, and dust) from each other, often producing long and beautiful tidal tails and morphological disturbances (as seen in the Hubble image of 'The Mice' shown above). This process is a slow one and can take millions or billions of years, so our view of the merger timeline is based on snap shots of the many objects we observe at each of the various stages. In today’s universe, this is a rare event. Studies of objects in the nearby universe have found that only 1-3% of objects appear to be involved in such a merger event.

 Simulation of two galaxies merging

Credit: Josh Barnes (University of Hawaii)

So why do galaxy mergers, though beautiful, matter to astronomers? Even though these events are rare now, we know that they were much more common in the past. As we look at galaxies farther away from us, and thus as they were at an earlier time in the universe, we find such interactions and mergers happened more frequently. Mergers between galaxies appear to be quite important for the evolution of galaxies over the history of the universe. These events can affect many aspects of a galaxy: they can change its overall morphology, for example, two spiral galaxies can merge together and form an elliptical galaxy; they cause galaxies to grow in mass; and they change the overall number of galaxies in the universe (i.e., where once there were several small galaxies, now there might be just one large galaxy). Indeed, we now know that our very own Milky Way Galaxy will one day merge with our large spiral neighbor, the Andromeda Galaxy. But don’t worry – this won’t happen for another 4 billion years so we won’t be around to see the effects!

The Antennae Galaxies, Credit: NASA, B. Whitmore, F. Schweizer

There are other ways that galaxy mergers can be important. When two galaxies collide with one another, the chances of two stars colliding is very rare because stars themselves are tiny compared to the vast distances of space between them. However, galaxies have large clouds of gas and dust within them, and these can and do collide. When this happens, the collision can induce huge bursts of star formation and many new stars are born from the gas clouds. An example of this has been seen for the famous pair of interacting galaxies called the Antennae (see image to the right). Over 1000 star clusters have been identified in the Antennae (blue in image) as a result of this collision. In addition, much of this gas can be funneled toward the center of the galaxy and provide fuel for feeding a central black hole. One of the current open questions in galaxy evolution is how many stars are produced because of such collisions and how important are these mergers for feeding black holes and creating active galaxies.

Identifying Galaxy Mergers

Credit: ASA, ESA, the Hubble Heritage 
(STScI/AURA)-ESA/Hubble Collaboration,
and W. Keel (University of Alabama)

In order to study the effects of galaxy mergers, the first step is to try to identify as many of them as we can. We use deep images of galaxies in order to do this. From the pictures you see in this post, you might think this is easy to do! Well, even for nearby galaxies, it requires looking at a lot of individual objects to identify the few percent that might have tidal tails or surrounding debris. Such an effort has been undertaken by the Galaxy Zoo project using images from the Sloan Digital Sky Survey and the help of many citizen scientists - ordinary people who have volunteered to look at galaxy images.. We can also identify galaxies in pairs in an automatic way, but this requires knowing the distance to both galaxies very precisely, as well as how fast they are moving relative to one another, in order to ensure that they are gravitationally bound and will one day merge. Many of the galaxies that appear to be next to each other are really chance projections along the line of sight, such as in the image to the left. These two galaxies are not interacting with each other. In fact, they are not at the same distance at all! They just happen to be aligned so that they look like they are.


Challenges at High Redshift

CANDELS images of possible high redshift galaxy mergers
Credit: Jeyhan Kartaltepe

While identifying merging galaxies might seem to be straightforward when looking at the nearby universe, this becomes much more complicated as we look at more distant galaxies at high redshift. The farther away a galaxy is, the fainter it is and the smaller it is. This makes it much more difficult to see features such as tidal tails and debris, which are often much fainter than the nuclei of the galaxies. If two galaxies are very close to each other they can blend together in images and look like one galaxy. This is where the Hubble Space Telescope comes in. Since Hubble is in space, we can get around the blurring effects of the Earth’s atmosphere. Another difficulty is that at high redshift, the light we see from galaxies is actually shifted toward longer wavelengths, or redder colors, from the original bluer light that was emitted. This means we might actually be sensitive to light from different processes and parts of galaxies than we are for nearby galaxies. CANDELS is ideal for studying the properties of galaxies at high redshift, and for identifying galaxy mergers, because it is the first large survey to obtain high-resolution data in the near-infrared. This allows us to study distant galaxies in the same way that we study nearby galaxies - though it still isn’t easy! Take a look at the sample of candidate high redshift mergers and interactions in the above panel. This is made even more complicated because galaxies at high redshift can be strange and irregular for other reasons, besides galaxy mergers. In a future post, we will discuss some of these alternate possibilities.

The story of galaxy mergers is just beginning! Stay tuned to hear more about what we learn about galaxy mergers in the distant universe from the CANDELS survey.