Galaxy formation: Dark matter, ellipticals, spirals, harassment, stripping, strangulation, and cannibalism.

Galaxies Are Everywhere. Why Don’t Astronomers Know Where They Come From?

What are astronomy's most intriguing puzzles?
Feb. 20 2014 11:19 AM

Where Do Galaxies Come From?

Cosmic harassment, stripping, strangulation, and cannibalism.

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Three of the galaxies in this famous grouping, Stephan's Quintet, are distorted from their gravitational interactions with one another. One member of the group, NGC 7320 (upper left) is actually seven times closer to Earth than the rest.
Three of the galaxies in this famous grouping, Stephan's Quintet, are distorted from their gravitational interactions with one another. NGC 7320 (upper left) is actually seven times closer to Earth than the rest.

Courtesy of NASA/ESA/Hubble SM4 ERO Team

Another way astronomers attempt to explain how galaxies transform draws on a correlation between the shape of a galaxy and the density of the galaxy’s environment, that is, the number of neighboring galaxies within a certain distance. It turns out that ellipticals are commonly located in galaxy clusters while spirals are more likely to be found in isolation or on the fringes of clusters. This implies that galaxies are shaped by their surroundings, namely that a high-density environment could be responsible for quenching star formation and aiding in a galaxy’s transition from blue to red.

How might star formation be suppressed in practice? This is where those criminal cosmic acts I mentioned earlier come into play: galaxy harassment, stripping, strangulation, and cannibalism.

Galaxies in groups, clusters, and superclusters are constantly interacting via gravity, with the strength of their interactions driven by their relative sizes, motions, and distances from each other. Galaxies harass one another gravitationally in high-speed fly-bys and head-on collisions, each distorting the other’s structure into unrecognizable shapes.


Stripping primarily affects spirals “falling” face on toward the center of a cluster. The motion essentially acts as a wind, blowing the gas and dust within the galaxy in the opposite direction of its path, stripping it of fuel for ongoing star formation.

Strangulation occurs when galaxies fall edge first, rather than disk first. Stars, gas, and dust are pulled out in elongated structures both toward and away from the cluster. With the galaxy’s gas and dust dispersed, it’s much harder for star formation to continue.

Some galaxies take things to the extreme, ripping smaller galaxies apart entirely and accreting their stars and gas, effectively cannibalizing them. The Milky Way is such a cannibal.*

The role of dark matter in shaping the evolution of a galaxy is an entirely separate can of cosmic worms. Some physicists are probing the nature of the dark matter itself, but many astronomers believe we can learn just as much about dark matter by investigating its effects on galaxies. In fact, it was by studying galaxies that we discovered dark matter in the 1930s, when astronomers observed that galaxies rotate much faster than Newton’s Law allowed for, at least based on their visible matter. Current estimates require roughly six times more dark matter than visible matter to keep a given galaxy from flying apart.

It’s not the dark matter though, that gives us the most trouble when trying to model the universe; surprisingly, it’s the visible matter. To see the effect of star formation on galaxies, we need to model them on the level of individual stars. However, there are also forces occurring on scales many times the size of individual galaxies. Powerful black holes in the centers of galaxies heat up huge amounts of gas and dust in their vicinity and eject them beyond the farthest boundaries of the galaxy. And supernovas blast waves of shock-heated gas well beyond their stellar neighborhoods. While computers grow more powerful every year, astronomers still struggle to model this range of scales at a high enough resolution to tell us anything new. On top of that, doubling the resolution of a simulation requires roughly 16 times more computing time. Theorists sit in front of their computers enough as it is. We get around this by simulating isolated pieces of the universe in detail, but by definition they only reveal pieces of the picture.

So for now, all we have is this rich vocabulary to describe galaxies for the brief time we have been able to observe them. All we have is pictures—static, narrowly framed snapshots—when what we really need is a movie.

*Correction, Feb. 20, 2014: This article originally misstated that astronomers suspect the Milky Way is a cannibal. They're quite sure that it is. (Return.)

Summer Ash is director of outreach for Columbia University’s astronomy department and the in-house astrophysicist for The Rachel Maddow Show. Follow her on Twitter.

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