In healthy galaxies, new stars are formed from the interstellar medium (ISM), the supply of gas and dust between stars. But sometimes cosmic winds—the movement of material between galaxies—can sweep a galaxy's interstellar medium clean, putting a damper on new star production there.

A lot of the specifics about this sweep-out are unknown—specifics like how easily it happens and under what physical conditions, and whether dense, star-forming clouds can survive these winds for any length of time. However, a new study on cosmic winds has observed the process with unprecedented detail, revealing complex and fascinating subtleties never before witnessed.

The study made use the Hubble Space Telescope, which imaged a spiral galaxy 300 million light-years away. The galaxy’s orbital path within the Coma Cluster takes it close to the cluster's center, pushing it through the hot, ionized gas (or plasma) of the intracluster medium (ICM). The plasma winds from the ICM then blow against the galaxy’s ISM, creating what’s called "ram pressure"—the pressure an object experiences as it moves through a fluid.

That pressure can push the galaxy's dust back, just as a swimmer's hair is swept back by the water. The image above clearly shows the “front” edge of the galaxy (the side facing the direction of the galaxy’s motion) being blown by these winds, causing the material to pile up.

“On the leading side of the galaxy, all the gas and dust appears to be piled up in one long ridge, or dust front. But you see remarkable, fine scale structure in the dust front,” explained Yale astronomer Jeffrey Kenney, the paper’s lead author. “There are head-tail filaments protruding from the dust front. We think these are caused by dense gas clouds becoming separated from lower density gas.”

As some of the lower-density gas is cleared out by the wind (piling up to form the dust front), the higher-density gas clumps together, forming pillar-like structures. It seems that something’s holding these patches together as most of the lower-density material is blown away; some of the lower-density stuff clings to the clumps.

Magnetic fields

The researchers suggest that this could be due to magnetic fields, which influence the shape of the clouds. Without these fields, the researchers predict the features along the edge would have shapes that are based only on density—they would look something like a continuous mountain range with peaks suddenly rising and falling in quick succession. Instead, we observe these coherent pillars rising out of a mostly flat surface beneath. These are the result of some of the lower density material "sticking" to the higher-density stuff—a good match for the gas' behavior under the influence of magnetic fields.

The discovery will help researchers better understand the effects of ram pressure stripping on galaxies throughout the Universe. One of the main points of the study was to determine how efficient that process is at stripping galaxies of their gas. And if, as the researchers argue, magnetic fields are an important factor in that process, they could decrease its efficiency. That's because they tend to hold the dust line together, preventing holes from being punched through its edge by the wind. Any holes that did form would increase the surface area of dust exposed to the oncoming wind, speeding up the stripping process.

High-resolution images like the one used in this study will help guide the creation of future simulations which in turn can give an even more detailed look into how the process works. "In order to understand the efficiency of stripping one needs to study the behavior of the ISM in the disk as it is being stripped," the authors write in their paper. "In face-on galaxies like NGC 4921 it is possible to see how the ISM in the disk is affected by ram pressure."

There are some suggestions that other processes are more significant than magnetic fields during the stripping process, and further work is needed to confirm the researchers' hypothesis. Whatever the case, the effect produces complex and beautiful patterns, as you can see. Many of these features are similar to the star-forming pillars seen in the famous photograph called the “Pillars of Creation,” only much larger—by about a thousand times.

Even the denser structures won’t last forever, though. They’re not perfectly immune to the ravages of the galaxy’s passage through the ICM, and will ultimately disperse, preventing the formation of further stars there. Sadly, we’re witnessing the last generation of stars to form in these literal pillars of creation.

The Astronomical Journal, 2015. DOI: 10.1088/0004-6256/150/2/59 (About DOIs)