Galactic Stripping Mystery Uncovered

Artist’s impression showing the increasing effect of ram-pressure stripping in removing gas from galaxies, sending them to an early death. Credit: ICRAR/NASA/ESA/Hubble Heritage Team (STScI/AURA)

It’s what you might call a case of galactic homicide (or “galacticide”). All over the known Universe, satellite galaxies are slowly being stripped of their lifeblood – i.e. their gases. This process is responsible for halting the formation of new stars, and therefore condemning these galaxies to a relatively quick death (by cosmological standards). And for some time, astronomers have been searching for the potential culprit.

But according to a new study by a team of international researchers from the International Center for Radio Astronomy Research (ICRAR) in Australia, the answer may have to do with the hot gas galactic clusters routinely pass through. According to their study, which appeared recently in The Monthly Notices of the Royal Astronomical Society, this mechanism may be responsible for the slow death we are seeing out there.

This process is known as “ram-pressure stripping“, which occurs when the force created by the passage of galaxies through the hot plasma that lies between them is strong enough that it is able to overcome the gravitational pull of those galaxies. At this point, they lose gas, much in the same way that a planet’s atmosphere can be slowly stripped away by the effects of Solar wind.

‘Radio color’ view of the sky above the Murchison Widefield Array radio telescope, part of the International Center for Radio Astronomy Research (ICRAC). Credit: Natasha Hurley-Walker (ICRAR/Curtin)/Dr John Goldsmith/Celestial Visions.

For the sake of their study, titled “Cold gas stripping in satellite galaxies: from pairs to clusters“, the team relied on data obtained by the Sloan Digital Sky Survey and the Arecibo Legacy Fast (ALFA) survey. While the SDSS provided multi-wavelength data on 10,600 satellite galaxies in the known Universe, ALFA provided data on the amount of neutral atomic hydrogen they contained.

By measuring the amount of stripping that took place within each, they deduced that the extent to which a galaxy was stripped of its essential gases had much to do with the mass of its dark matter halo. For some time, astronomers have believed that galaxies are embedded in clouds of this invisible mass, which is believed to make up 27% of the known Universe.

As Toby Brown – a researcher from the Center for Astrophysics and Supercomputing at the Swinburne University of Technology and the lead author on the paper – explained:

“During their lifetimes, galaxies can inhabit halos of different sizes, ranging from masses typical of our own Milky Way to halos thousands of times more massive. As galaxies fall through these larger halos, the superheated intergalactic plasma between them removes their gas in a fast-acting process called ram-pressure stripping. You can think of it like a giant cosmic broom that comes through and physically sweeps the gas from the galaxies.”

The Arecibo Observatory in Puerto Rico, where the Arecibo Legacy Fast ALFA Survey is conducted. Credit:

This stripping is what deprives satellites galaxies of their ability to form new stars, which ensures that the stars they have enter their red giant phase. This process, which results in a galaxy populated by cooler stars, makes them that much harder to see in visible light (though still detectable in the infrared band). Quietly, but quickly, these galaxies become cold, dark, and fade away.

Already, astronomers were aware of the effects of ram-pressure stripping of galaxies in clusters, which boast the largest dark matter halos found in the Universe. But thanks to their study, they are now aware that it can affect satellite galaxies as well. Ultimately, this shows that the process of ram-pressure stripping is more prevalent than previously thought.

As Dr. Barbara Catinella, an ICRAR researcher and co-author on the study, put it:

“Most galaxies in the Universe live in these groups of between two and a hundred galaxies. We’ve found this removal of gas by stripping is potentially the dominant way galaxies are quenched by their surroundings, meaning their gas is removed and star formation shuts down.”

Another major way in which galaxies die is known as “strangulation”, which occurs when a galaxy’s gas is consumed faster than it can be replenished. However, compared to ram-pressure stripping, this process is very gradual, taking billions of years rather than just tens of millions – very fast on a cosmological time scale. Also, this process is more akin to a galaxy suffering from famine after outstripping its food source, rather than homicide.

Another cosmological mystery solved, and one that has crime-drama implications no less!

Further Reading: Royal Astronomical Society, MNRAS

A Snapshot of a Galactic Crash

This image combines NASA/ESA Hubble Space Telescope observations with data from the Chandra X-ray Observatory. As well as the electric blue ram pressure stripping streaks seen emanating from ESO 137-001, a giant gas stream can be seen extending towards the bottom of the frame, only visible in the X-ray part of the spectrum. Credit: NASA, ESA, CXC

Some galaxies shine with a red ghostly glow. Once these galaxies stop forming new stars, they can only host long-lived stars with low masses and red optical colors. Astronomers often call these ghostly galaxies “red and dead.” But the basics behind why some form so quickly is still a mystery.

“It is one of the major tasks of modern astronomy to find out how and why galaxies in clusters evolve from blue to red over a very short period of time,” said lead author Michele Fumagalli from Durham University in a news release. “Catching a galaxy right when it switches from one to the other allows us to investigate how this happens.”

And that’s exactly what Fumagalli and colleagues did.

The team used ESO’s Multi Unit Spectroscopic Explorer (MUSE) instrument mounted on the 8-meter Very Large Telescope. With this instrument, astronomers collect 90,000 spectra every time they look at an object, allowing them to gain a detailed map of the object’s motion through space.

This chart shows the location of the distant galaxy ESO 137-001 in the constellation of Triangulum Australe (The Southern Triangle). This is a rich area of the sky close to the Milky Way, but this galaxy is faint and needs a large telescope to be visible. Credit: ESO, IAU and Sky & Telescope
The location of the distant galaxy ESO 137-001. Credit: ESO / IAU / Sky & Telescope

The target, ESO 137-001, is a spiral galaxy 200 million light-years away in the constellation better known as the Southern Triangle. But more importantly, it’s currently hurtling toward the Norma Cluster and embarking on a grand galactic collision.

ESO 137-001 is being stripped of most of its gas due to a process called ram-pressure stripping. As the galaxy falls into the galaxy cluster, it feels a headwind, much as a runner feels a wind on even the stillest day. At times this can compress the gas enough to spark star formation, but if it’s too intense then the gas is stripped away, leaving a galaxy that’s empty of the material needed to form new stars.

So the galaxy is in the midst of a brilliant transformation, changing from a blue gas-rich galaxy to a red gas-poor galaxy.

The observations show that the outskirts of the galaxy are already completely devoid of gas. Here the stars and matter are more thinly spread, and gravity has a relatively week hold over the gas. So it’s easier to push the gas away.

In fact, dragging behind the galaxy are 200,000 light-year-long streams of gas that have already been lost, making the galaxy look like a jellyfish trailing its tentacles through space. In these streamers, the gas is turbulent enough to compress small pockets of gas and therefore actually ignite star formation.

The center of the galaxy, however, is not yet devoid of gas because the gravitational pull is strong enough to hold out much longer. But it will only take time until all of the galactic gas is swept away, leaving ESO 137-001 red and dead.

Surprisingly the new MUSE observations show the gas trailing behind continues to rotate in the same way that the galaxy does. Furthermore, the rotation of stars at the center of the galaxy remains unhindered by the great fall.

Astronomers remain unsure why as this is only a snapshot of one galactic crash, but soon MUSE and other instruments will pry more out of the cosmic shadows.

The results will be published in the journal Monthly Notices of the Royal Astronomical Society and are available online.