Galaxies Experience Winds of Change

The leading side of the disk of galaxy NGC 4921 shows the effects of strong ram pressure. NASA, ESA, and Roberto Colombari

Galaxies may seem like they're fixed and unchanging up there in the sky, but in reality, they're hotbeds of evolution!  Their sizes, shapes and even their populations of stars change over long periods of time. Astronomers are also beginning to probe many galaxies to trace the history of their collisions, events that shaped each galaxy throughout history. 

A General Look at Galaxies

Galaxies are collections of stars, planets, black holes, and clouds of gas and dust. Astronomers have long studied how they can be affected by activities inside their spiral arms and cores. Galaxies form in collisions, each one bringing more stars to the mix. However, stars themselves can change galaxies, too. For example, supernova explosions send clouds of material out to interstellar space and can shine as bright or brighter than the galaxy itself. 

Ever-changing Galaxies

However, galaxies can also be shaped by outside forces. Observers have long known that intergalactic material creates winds — called "cosmic winds" — can shape galaxies, too. The image above is one taken by Hubble Space Telescope, focused in on the Coma Cluster of galaxies. This grouping of galaxies lies some 320 million light-years away and contains more a thousand members. 

The Winds of Galactic Change

One galaxy shows evidence that strong cosmic winds swept through and eroded clouds of gas and dust at the "leading edge" (that is, the edge that the winds contacted first). This galactic wind, also called "ram pressure", is really caused as the galaxy orbits through regions of hot intergalactic gas inside the cluster. It's really more of a collision.

As the galaxy rips through the gas and dust, ridges of material build up (the dark, arc-shaped region in the upper right quadrant of the image). It seems to be surrounded by blue stars, which likely formed when pressure from the collision forced clouds of gas together, and, under pressure, they began to form stars. There are also filaments that look similar to comet heads and tails (but on light-years-long scales), shaped by the action of the winds as they collided with the clouds. 

As the winds push on these clumps of gas and dust, it strips out the gas, removing the raw material for future star formation. Even though there are stars being formed within the pillars and column-type structures, once they are born, there will be no more "building blocks of stars" to create the next generation of stellar bodies.

Eating Up Star-forming Material

If you've ever seen the famous Hubble Space Telescope image of an object called the "Pillars of Creation", you've seen a similar kind of action. There, however, columns of dust and gas in the Eagle Nebula were created by the strong ultraviolet light from a nearby star. That radiation destroyed and tore apart the clouds of gas and dust, leaving behind thick clumps of material. There were stars forming inside the left-behind clumps, and they will eventually break free of their birth cloud and shine out. The dust filaments in this distant galaxy are similar in some ways to the Pillars of Creation, except they are a thousand times larger.

In both cases, destruction is at least as important as creation. An external force is pushing away most of the gas and dust, therefore destroying most of the cloud, leaving behind only the densest material — the pillars. But even the pillars don’t last that long.

It's well known that galaxy collisions actually stimulate the formation of swarms of new stars in their participating galaxies. Astronomers have seen that across the universe. However, in this case, when a galaxy encounters a strong intergalactic wind, the process of star formation just gets choked off and completely stopped.

It's an interesting part of galaxy evolution and one that astronomers continue to study with their observations. Since all galaxies formed through collisions, it's a useful way to understand the galactic structures we see in the sky, including our own Milky Way Galaxy and its neighbors.