Galaxy Evolution and the Language of Violence

Posted By Pamela on Jul 7, 2008 | 11 comments

One thing that has always fascinated me is how readily humans (myself certainly included) anthropomorphize everything around us. Even in the precise language of science, the tendency to use emotionally loaded words persists as we discuss the stress or strain on everything from physical structures (calculating the point of failure of building supports), and the failure of ecological niches (determining the tipping point that triggers an estuary to die). The equations we use to describe these failures differ, but the words – the same one we use when a human is breaking – are equally applied. Stressed and strained are not the only emotional states that describe the physical world. In the field of galaxy evolution, the words cannibalism, harassment, shocking, stripping and strangulation are all used on a daily basis. (image credit: NCSA)

The universe is a deadly place. (Don’t believe me? Pre-order Phil’s book!) And apparently, if you’re a galaxy, you simply have to learn how to live with abuse unless you are lucky enough to live in isolation

As we look out across the sky, we find that galaxies like people prefer not to be alone. Studies by determined scientists like Martha Haynes have found that if you look hard enough, their are large galaxies like our own with no nearby large companions, but finding these systems is hard. It is much easier to turn up pairs, groups, and clusters of all sizes and shapes – these communities of galaxies seem to be the more normal social structure for these organisms of stars and gas. But not all family units are healthy ones. In small groups, like the one our Milky Way is located within, it is possible to see active star formation and beautiful spiral structure as far back in time as we are capable of resolving structure. In these small systems, the large galaxies dance lazily, and it may be billions of years before the largest members sweep out their arms to embrace one another in a dance of death. Sadly, that deadly gravitational embrace seems to be the natural way for all galactic relationships to end.

In the largest systems – the Coma and Virgo clusters of space and time – this process is accelerated. There is more mass and thus more gravitational pull, and galaxies whip through space at higher velocities, and due to the increased density – the larger number of galaxies per volume of space – the probability of collisions and gravitational interactions is much higher. This basically means death comes earlier to galaxies that choose to live in cosmic cities.

This death can take many forms, but to understand it, we must first understand how giant galaxies are constructed.

Everything we think we understand about modern galaxy formation* is built on a simply picture: Small systems gravitationally fall together via a process known as Hierarchical Galaxy Formation. As they merge, these small systems build giant spiral galaxies. These large systems will continue to happily consume small systems while calmly producing stars for billions of years at a time.

As these giant galaxies live out their cannibalistic, baby-galaxy-eating lives, they leave little evidence of their feeding. Occasionally, there will be a warped disk or a tidal tail (or both), but generally they are good about gravitationally picking up their own messes. As small galaxies fall into larger galaxies, they are tidally disrupted. This means the difference in gravitational pull on one edge of the small galaxy is significantly different than on the other edge, such that the difference in pull generated by some external force (for instance a big galaxy) is greater than the galaxies self-gravity (the force that holds it together). This is the moral equivalent of pulling on the front edge of a cake with enough enough force to pull the cake apart. It is possible to pull on the cake just enough to just move it, but if your pull is greater than the pull of the molecular bonds inside the cake, you will break the bonds and also break the cake.

In the image at left, there is an amazing loop of excess stars surrounding the main (seen edge-on) disk galaxy. This system, called NGC 5907, is an isolated spiral that has recently consumed a smaller galaxy. As the galaxy orbited in and fell apart, it left behind a train of stars, and evidence of its death in the form of a warped disk. The stream of stars is technically referred to as a tidal tail. (image credit:David Martínez-Delgado ( IAC, MPIA), Jorge Peñarrubia (Univ.Victoria ), R. Jay GaBany (Blackbird Observatory), Ignacio Trujillo ( IAC), Steven R. Majewski (Univ. Virginia), Michael Pohlen (Cardiff)).

In addition to tidal tails, interactions between large spirals and smaller systems can lead to fascinating features, varying from grand design spiral arms (like in M51), to bars (like in NGC 1300), to star bursting rings (like in NGC 4314). These interactions do not destroy the large galaxy’s structure, but rather enhance it by changing the gravitational field.

Changing a spiral into something new requires bigger interaction.

Shaping changing events can come in many forms. Most familiar are the head on collisions of merging galaxies, such as the mice (shown at right. Image credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA). These dramatic events initially trigger massive star formation, as pockets of gas and dust are shocked into collapse and driven toward the system centers. What dust isn’t converted into stars in typically jettisoned as beautiful tails and twisted arms. As the galaxies merge into a single object, their spiral structure is destroyed, and a new elliptical shape takes hold. Once the breathtaking burst of star formation is over and the tidal arms are lost, the systems fad away over billions of years into a single dead red elliptical galaxy. This is the eventual fate of the Milky Way and its nearest companion Andromeda.

In large groups, not all interactions are as straight-forward. Often, death comes from a dozen off center blows.

As a spiral galaxy falls into a large cluster of galaxies, it experiences many traumas. It is shocked as it slams into the inter cluster medium. The diffuse gas, which is extremely hot in the cores of clusters, used to be part of the cluster’s member galaxies. Over time, gravitational interactions stripped this material out of its parent systems and left it orphaned in the space between galaxies. Gravitationally trapped, it is heated both through the processes you learn about in high school chemistry (combine enough moles of a gas in a given volume under sufficient pressure, and it heats up. In this case, the pressure comes from gravity compressing the gas), and also from more exotic means, such as radio galaxies blasting the inter cluster medium with high energy jets. The shock of systems colliding also heats the gas. Put together, all these pieces can bring this gas to as much as 100,000,000 degrees Celsius! This angry hot gas is just waiting to do bad things to infalling spiral galaxies. As these spirals fall in, the shock of hitting the gas can trigger star formation, and also disrupt the shape of the galaxy. This is called ram pressure stripping, and it is most destructive in cluster cores. If it blasts too much gas from the infalling galaxy, it can effectively strangle all future star formation, killing off the system.

In addition to being shocked by the inter cluster medium, our in falling spiral may also interact with other galaxies. When galaxies approach each other slowly, such as with the Andromeda and Milky Way, it is possible for them to gravitationally grab onto one another and merge. If instead, two galaxies zip past each other at high velocities, their mutual gravitational attraction will disrupt the systems, but will not cause them to merge. Over time, multiple high speed encounters can transform a spiral galaxy into dwarf elliptical or S0 system. In the most dense clusters, these interactions may occur as frequently as once per billion years. Over just the past 4 billion years, moderate sized galaxy clusters have successfully transformed blue, star bursting spiral galaxies into dead ellipticals via galaxy harassment.

Looking across the universe, we see a pattern where the smallest systems, like the one we live in, have star formation across all epochs, while the richest systems are dead and red no matter how far back we look. It is in the middle-sized systems that we see the universe evolve. This picture is defined by gravity. The large systems formed first, died quickly, and are so dense that infalling galaxies are killed off in as little as 10,000,000 years by ram pressure stripping. What the ram pressure doesn’t kill, galaxy harassment takes care of. In the smaller systems, galaxies move more slowly, generally don’t lose significant amounts of gas, and take their own sweet time getting around to merging. In the middle, something in between occurs, and we are able to see the transformation from clusters having as much as 50 percent blue galaxies, to similar systems only containing a few percent (if that many) blue galaxies over several billion years. Blue means star formation. As the blue galaxies go away, the entire system is said to die.

I have to admit that the beauty of the galaxies, coupled with my own awe at our ability to understand what is going on leaves me wishing we had chosen less violent metaphors to describe our science. Galaxy mergers are one of the most stunning things we can image. While the process does involve the collision of a lot of gas and dust, and it does bring an end to spiral structures, it also causes the birth of something new: an elliptical galaxy. Still, violence is how we are accustomed to emotionally viewing all interactions that involve sudden shape change, or removal of bits. We have been taught, when you see a car fly into another car such that they turn into a single hunk of metal (minus several dozen small bits) you are are seeing something violent, destructive, dangerous, and deadly. When I look at the universe, I see something magnificent, stunning, violent, destructive, dangerous, and deadly. We have no working metaphor of beauty for this type of process. Our language is simply insufficient to capture the glory of the cosmos, so instead, we tell a story of violence. After all, if it bleeds it leads, and the universe is nothing if not deadly.

*Some large galaxies likely formed via a different mechanism in the early universe.

I’m not sure what it says about the personalities of those of us who study galaxy clusters that these are the words we have chosen as a community to use, but I would love to work with a sociologist focused on linguistics to learn more.


  1. For not having the language you do a pretty good job of it!

  2. I think it probably comes from our drive not just to anthropomorphise events themselves, but to reduce them to our scale. In order to imagine these events we like to place ourselves in them, or beside them (everyone always asks: but if I could SEE a black hole, or if I could STAND on a neutron star, what would happen if OUR galaxy collided), and the conclusion we always have to draw after we’ve done the cool hypothetical imagining is some variation on ‘ouch’.

    So as far as a human perspective is concerned, the universe IS literally deadly, dangerous and violent… but that’s not to say that deadly, dangerous violent things can’t be majestic or awesome… on a smaller scale, Tornadoes, Volcanoes, Tsunamis, they all invoke the same sense of awe to a lesser degree.

    What I think we have problems comprehending (and this is where our negative associations with violence come in and ruin that majestic truth) is that there’s no REASON for these events, no maliciousness or anger behind the violence, no intent to cause harm… just the universe getting on with itself.
    With our constant humanicentric desire to see the intent of a mind in everything, our variously believed gods and creation stories, it’s no wonder such a hostile universe can seem violent and cruelly aimed at us, instead of violent and awesomely sweeping us along.

  3. Language is strange. After all, a similar thing happens in planetary formation. Lots of small objects ‘harrass’ and ‘swallow’ each other, and eventually ‘die’ as large spherical objects, but because it suits our prejudices we think of that as a creative process. Nobody mourns those poor baby chondrules that gave themselves to become planets, though no doubt for a while they formed beautiful swirling spiral patterns.

    Whether something is destructive or creative depends on where you want to end up, and galaxies don’t want anything. They just are.

  4. This is where the flexibility of language should come in. Perhaps we should invent a word for the awe and majesty of such a seemingly violent merger of two galaxies. It’s not that mergers are inherently distructive, because they produce the energy required to instigate new star growth and extend the growth phase of the galaxy.

    I propose calling it a shivastic event. Since the Hindu belief in Shiva as both the creator and destroyer of the universe makes for such a metaphorical analogy.

  5. Followed the link from Bad Astronomy and, I’ll echo Mr. Cooper, continue to not have the language! 🙂 Very nice article and thanks for all the embedded links. Mmmmm….NGC 4314….mmmmmm.

  6. All kinds of connections. It’s almost as if we identify with the galaxies, rather than with what they will become. (It’s not as if galaxies we see are some kind of stable, permanent state whose transformation is unnatural). I’d bet some of the language (hostile takeovers, harassment) got its start in talks with someone trying to be clever and riffing on current news items, and caught on. The same thing would happen when these terms are used in talking to reporters or even in classes, trying to make the basic picture a little easer to grasp. Especially in classes, we really have to watch language which leads to anthropomorphic images, as much as we might think it – dynamical friction makes the more massive objects want to sink to the core, galaxies like to live in dense neighborhoods – because that can shortchange the precision of physical behavior in students’ minds.

  7. “…single dead red elliptical galaxy. This is the eventual fate of the Milky Way and its nearest companion Andromeda.”

    I’ve been wondering how that is known.

    I know that spectroscopy tells us that light from Andromeda is blueshifted, so we know the radial component of Andromeda’s motion is toward rather than away from us. However, how can we know that the sideways motion is too small to have the Andromeda & Milky Way galaxies miss each other?

  8. Follwed the link from BA…

    Fascinating subject. The anthropomorphizing aspect is easy to understand and others have said it. We relate to galaxies as living things because they move and change. We relate human characteristics, feeling and emotions to things that move and change because that’s our experience and because it helps us to (think we) understand the reasons things happen.

    However, I think the language and the words we choose absolutely give us insight into our prejudices and preferences. That is, we say spirals are destroyed because we think spirals are better. I would suggest that if ellipticals evolved into spirals we would use words like that: evolved. We wouldn’t say the ellipticals were destroyed to make the spirals. It gets back to what we would prefer to have. Using Vagueofgodalming’s example of planet formation, we don’t say that planetesimals are “eaten” to make a planet. Rather we say they were “swept up.” Hey, we’re just tidying up here… making the solar system neat and orderly.

    Still I wonder if anyone would notice if we changed the language and started using different words.

    What if galaxy collisions were marriages? Depending on your point of view (and perhaps personal experience) that might be considered a much nicer union.

    What if the newcomer joined the neighborhood rather than slammed into the cluster?

    What if galactic gas was “cleaned out” of the incoming galaxy instead of stripped?

    Maybe a red elliptical galaxies are “calm and quiet”, instead of dead.

    Gee Pamela, maybe you could start a trend or even a language. Hey, if quarks can be called charm, surely galaxies can get married…

  9. Watching a war movie during WW II there was a night firing of battleships that was extremely beautiful. I was young and did not realize the amount of death involved and exclaimed how beautiful. An older women behind me felt otherwise. I didn’t find the firings as beautiful up close.

  10. Unix programmers talk of “children” (child processes)being “killed”. Apparently we have not become a tribe of ax-murderers.

    And even stars are “born”.

    In other news, there was a city council meeting recently, in which one member, talking about how parking tickets seem to disappear without ever being paid, spoke of them “disappearing into a black hole”.

    Another councilman, black, instantly arose to voice his indignation, and demanded an immediate apology.

    I put that down to the sad state of science education today. Probably more accurately, over the last 20 years, which is when the offended councilman went through school.

    Comparing a car collision with a galaxy collision: the thing is, we see the one in real time; the other takes rather longer. If we could see the other played out in a matter of seconds, we might appreciate the effect. I think there are in fact computer simulations of such things.

  11. Unfortunately our beautiful universe does bruise and bash as well. Another of life’s paradoxes. Your warmhartedness shines through too.


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