January 2016 – Violent Galaxies

Heavens Above! is the astronomy section of the Sci@StAnd website, updated each month to highlight a particular phenomenon in the night sky. Last month, we examined some of the largest stars in our galaxy. In this issue, we will discuss the most violent acts of galaxies.


Figure 1. CL0024+17 is a massive cluster of galaxies, containing many smaller galaxies. The strangely curved galaxies are merely illusions caused by the immensity of the cluster which gravitationally warps background galaxies. These kind of systems are some of the most violent galactic offenders. (Credit: NASA/HST)

When we peer into the vastness of space, we find galaxies clumped together in massive clusters, forming some of the largest things in the universe. But life in these dense environments is not easy. Galaxies experience forces from neighbouring galaxies, or worse from rogue galaxies passing by. We are about to explore the realm of violent galaxies.

Figure 2. The Antennae G

Figure 2. The Antennae Galaxies are some of the most well-studied. Their rare circumstances allow astronomers to better understand the nature of merging systems, and the destruction they produce. The tidal tails stream outwards, with knots of intense starbursts. (Credit: NASA)


We start off with the simplest example of the violent lives of galaxies: head-on collisions. In certain environments such as smaller collections of galaxies known as groups, the conditions are just right such that gravity can coerce two galaxies to collide. When this happens, the two galaxies are contorted into vastly different shapes. Often, streams of gas are shot out along tidal tails, which undergo high pressures and henceforth star formation. The Antennae galaxies are a great example of this. It should also be noted that stars do not collide (on average) in galaxy mergers. The space between the stars is simply too huge compared to even the most massive stars.

If they don’t collide right away, two unsuspecting galaxies can interact and fly-by one another at very close distances. Over time, these relatively mundane interactions can warp the galactic disks and eventually lead to a galaxy merger. It is thought that our own galaxy, the Milky Way, and our closest neighbour, the Andromeda Galaxy, will one day collide in a spectacular merger. This, of course, is on astronomical timescales, so don’t expect anything within a few billion years. Some astronomers believe that this won’t be the first time that the two have interacted, but rather one of many episodes between two galaxies bound by gravity.

Dynamical Friction

Some of the largest galaxies known to exist are created by so-called dynamical friction. This process involves a galaxy moving through a field of smaller galaxies, eventually slowing it down. Once slowed down, it is significantly easier for the smaller galaxies to collide and merge with the larger galaxy. Over successive mergers, the larger galaxy will grow in size. This behaviour is observed mainly in clusters, as mentioned earlier, whereby large galaxies collect together. Many of these galaxies are red and dead, meaning that they are composed of old stars which produce a red hue, and lack gas, limiting any further star formation. Because of these characteristics and the effects of dynamical friction, this larger galaxy is usually the largest galaxy in a cluster, and can sit there and collect other galaxies, growing without making more stars. This is in sharp contrast to the mention earlier of tidal tails, which requires two gas-rich galaxies to collide. Both fates, however, are likely very red and very dead.


Figure 3. NGC1427A is a relatively nearby irregular galaxy undergoing ram-pressure stripping as it enters the Fornax Cluster. The “frisbee” like shape is due to the super-heated cluster gas pushing on it’s own gas. Will it survive? (Credit: ESO)

Galaxy Strangulation

Looking towards the larger picture, we see an even darker side to galaxies. Ganging up in clusters provides galaxies with a lot of influence. In falling galaxies suffer from a series of interactions, and if they aren’t big enough, will likely perish. This particular process happens when the gravity of a large cluster of galaxies pull at the gas of an incoming galaxy, possibly ripping it out and claiming it for their own. The now gas-free galaxy can no longer produce stars, and is left for dead, floating through the intra-cluster gas.

Ram-Pressure Stripping

Even if a galaxy survives the gravitational pull of the cluster gravity, it still faces new threats. The loose collection of gas between the galaxies of a cluster is super-heated. To an incoming galaxy, it’s like trying to walk against a jet engine. The effect is similar to galaxy strangulation, but is a bit more up front. Instead of pulling on the gas, the gas is pushed away and usually trails behind the galaxy as it traverses the inner regions of the cluster. Small enough, and the galaxy will go out in an extraordinarily bright blip of star bursting. A nearby example of this is NGC1427A in the Fornax Cluster.

Galaxy Harassment

In what could be dubbed the “Galaxies Strike Back”, interactions between galaxies can severely alter their shape and behaviour. As briefly mentioned in the collisions section above, this process involves two or more galaxies interacting with each other. Sometimes the playing field is vastly uneven, whereby multiple smaller galaxies play at the bigger galaxy, slowly warping it and likely slowing it down as per dynamical friction. It can strip gas from galaxies, and vice versa. Whole galaxies can shoot through one another at immense speeds. Such is the case with Cartwheel Galaxy. In a sense, this is the “junk drawer” of galaxy interactions, and it links together much of what we observe in galaxy clusters.

So kids, galaxies are not role models. They gang up on one another. Larger galaxies annoy and harass the smaller galaxies. Sure, sometimes the little guys get a break, but size and strength in numbers are the two key ingredients to surviving in the universe, as on the playground. To pass the tribulations of galactic “initiation” is not commonplace, and those that do can eventually claim their prize as members of the great galaxy clusters.

Next Month: The History of Our Moon

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