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

How do Gas and Stars Build a Galaxy?

ALMA image of the galaxy NGC 253, with a diffuse envelope of carbon monoxide gas (in red) which surrounds star-forming regions (in yellow). The ALMA data are superimposed on a Hubble image that covers part of the same region. Credit: Credit: B. Saxton (NRAO/AUI/NSF); ALMA (NRAO/ESO/NAOJ); A. Leroy; STScI/NASA, ST-ECF/ESA, CADC/NRC/CSA

When we look up at the night sky outside of the bright city, we can see a dazzling array of stars and galaxies. It is more difficult to see the clouds of gas within galaxies, however, but gas is required to form new stars and allow galaxies to grow. Although gas makes up less than 1% of the matter in the universe, “it’s the gas that drives the evolution of the galaxy, not the other way around,” says Felix “Jay” Lockman of the National Radio Astronomy Observatory (NRAO).

With radio telescopes and surveys such as the Green Bank Telescope (GBT) in West Virginia, the Atacama Large Millimeter/submillimeter Array (ALMA), and the Arecibo Legacy Fast ALFA (ALFALFA) survey, Lockman and other astronomers are learning more about the role of gas in galaxy formation. They presented their results at the annual American Association for the Advancement of Science (AAAS) meeting in San Jose.

Although we have an excellent view of our part of the Milky Way, and we can tell that it has a disk-shaped structure — that is the origin of its name, after all — it is not so simple to study how the galaxy formed. Lockman described the situation with an analogy: if you were trying to understand how your own house was built without leaving it, you would look and listen throughout the house and you would look out the window to learn what you can from your neighbors’ homes. Andromeda is the Milky Way’s largest neighbor, and they both have “satellite” galaxies traveling around them, some of which appear to have gas.

In addition, Lockman and his colleagues found clouds of gas between Andromeda and one of its satellites, Triangulum, which could be a “source of fuel for future star formation” for the galaxies. As a dramatic example of high-velocity clouds, Lockman presented new GBT images of the Smith Cloud, which was first discovered in 1963 by a student in the Netherlands. The Smith Cloud is a newcomer to the Milky Way and could provide enough gas to form a million stars and solar systems. Based on its speed and trajectory, “we think in a few million years, splash!” as it collides with our galaxy.

Artist's impression of the Smith Cloud approaching the Milky Way, with which it will collide in approximately 30 million years. The cloud's image from the GBT can be seen at bottom. Credit: NRAO/AUI/NSF
Artist’s impression of the Smith Cloud approaching the Milky Way, with which it will collide in approximately 30 million years. The cloud’s image from the GBT can be seen at bottom. Credit: NRAO/AUI/NSF

Kartik Sheth, another scientist at NRAO, continued with a description of astronomers’ current state of knowledge of the assembly of disk and spiral galaxies, of which the Milky Way and Andromeda are only two examples. Spiral galaxies typically have many gas clouds forming new stars, often referred to as stellar nurseries, and now with ALMA, “a fantastic telescope at 16,500-ft elevation,” Sheth and his colleagues are studying them in more detail.

In particular, Sheth presented newly published results by Adam Leroy in the Astrophysical Journal, in which they examine star-forming clouds in the heart of the nearby starbursting galaxy, Sculptor, to study “the physics of how gas got converted into stars.” Sculptor and other starbursts form stars at a rate about 1,000 times faster than typical spiral galaxies like the Milky Way. “Only with ALMA can we actually accomplish observations like this” of objects outside our galaxy. By comparing the concentration and distribution of ten gas clouds in Sculptor, they find that the clouds are more massive, ten times denser, and more turbulent than similar clouds in more typical galaxies. Because of the density of these stellar nurseries, they can form stars much more efficiently.

Other astronomers at the AAAS meeting, such as Claudia Scarlata (University of Minnesota) and Eric Wilcots (University of Wisconsin), presented a larger-scale picture of how spiral galaxies collide with each other to form more massive elliptical-shaped galaxies. These galaxies typically appear older and have stopped forming stars, but they can grow by “merging” with a neighboring galaxy in its group. “I will contend that most galaxy transformations take place in groups,” says Wilcots. In a paper based on ALFALFA data published in the Astronomical Journal, Kelley Hess and Wilcots find gas-rich galaxies distributed primarily in the outskirts of groups, and therefore these systems tend to grow from the inside out.

In a related issue, both Priyamvada Natarajan (Yale University) and Scarlata discussed how the evolution of massive black holes at the centers of galaxies appear to be related to that of the galaxy as a whole, when astronomers follow them from “cradle to adulthood.” In particular, Natarajan explained how mature galaxies’ black holes can heat the gas in a galaxy and drive gas outflows, thus preventing continued star formation in the galaxy.

Finally, astronomers look forward to much more upcoming cutting-edge research on gas in galaxies. Ximena Fernández (Columbia University) described the COSMOS HI Large Extragalactic Survey (CHILES) of hydrogen gas in galaxies with the Very Large Array. They have completed a pilot survey so far, in which they have obtained the most distant detection so far of a galaxy containing gas. They plan to peer even further into the distant past than previous surveys, expecting to detect gas in 300 galaxies up to 5 billion light-years away—250 times further than the galaxy observed by Leroy.

Fernández also described MeerKAT, a radio telescope under construction in South Africa, and the Deep Investigation of Neutral Gas Origins (DINGO) in Australia, both of which will serve as precursors for the Square Kilometer Array in the 2020s. These new telescopes will add to astronomers’ increasingly complex view of the formation and evolution of galaxies.

NGC 6240: Gigantic Hot Gas Cloud Sheaths Colliding Galaxies

Credit: X-ray (NASA/CXC/SAO/E.Nardini et al); Optical (NASA/STScI)

Looking almost like a cosmic hyacinth, this image is anything but a cool, Spring flower… it’s a portrait of an enormous gas cloud radiating at more than seven million degrees Kelvin and enveloping two merging spiral galaxies. This combined image glows in purple from the Chandra X-ray information and is embellished with optical sets from the Hubble Space Telescope. It flows across 300,000 light years of space and contains the mass of ten billion Suns. Where did it come from? Researchers theorize it was caused by a rush of star formation which may have lasted as long as 200 million years.

What we’re looking at is known in astronomical terms as a “halo” – a glorious crown which is located in a galactic system cataloged as NGC 6240. This is the site of an interacting set of of spiral galaxies which have a close resemblance to our own Milky Way – each with a supermassive black hole for a heart. It is surmised the black holes are headed towards each other and may one day combine to create an even more incredible black hole.

However, that’s not all this image reveals. Not only is this pair of galaxies combining, but the very act of their mating has caused the collective gases to be “violently stirred up”. The action has caused an eruption of starbirth which may have stretched across a period of at least 200 million years. This wasn’t a quiet event… During that time, the most massive of the stars fled the stellar nursery, evolving at a rapid pace and blowing out as supernovae events. According to the news release, the astronomers who studied this system argue that the rapid pace of the supernovae may have expelled copious quantities of significant elements such as oxygen, neon, magnesium and silicon into the gaseous envelope created by the galactic interaction. Their findings show this enriched gas may have expanded into and combined with the already present cooler gas.

Now, enter a long time frame. While there was an extensive era of star formation, there may have been more dramatic, shorter bursts of stellar creation. “For example, the most recent burst of star formation lasted for about five million years and occurred about 20 million years ago in Earth’s time frame.” say the paper’s authors. However, they are also quick to point out that the quick thrusts of star formation may not have been the sole producer of the hot gases.

Perhaps one day these two interactive spiral galaxies will finish their performance… ending up as rich, young elliptical galaxy. It’s an act which will take millions of years to complete. Will the gas hang around – or will it be lost in space? No matter what the final answer is, the image gives us a first-hand opportunity to observe an event which dominated the early Universe. It was a time “when galaxies were much closer together and merged more often.”

Original Story Source: Chandra X-Ray Observatory News Release.