Bare-Naked Galaxies

Pity the ravaged galaxy known as C153. As far as astronomers can tell, this eviscerated body once resembled a grand spiral galaxy like our Milky Way, its several slender, starlit arms wrapping around a brilliant disk. That was before the distant galaxy began crashing through a crowded cluster of galaxies, an encounter that has ripped away most of C153’s gas and left the galaxy with a single, skeletal arm. The galaxy appears destined to lose even that vestige over the next 100 million years, turning into a featureless disk of old stars.

For the past decade, William Keel of the University of Alabama in Tuscaloosa and his colleagues have been meticulously documenting the travails of C153. The team uses data from a battery of telescopes that view the galaxy at wavelengths ranging from radio to X-ray.

“The galaxy is a laboratory for studying how gas can be stripped away when it flies through a cluster,” Keel said last month at a meeting of the American Astronomical Society in Atlanta.

Observations of the galaxy, which lies 1 billion light-years from Earth, may solve a long-standing riddle: Why were spiral galaxies abundant in clusters long ago but are rare today?

Under pressure

A galaxy can get gutted in two ways. The most familiar is the gravitational tug of a nearby, massive object. Gravity exerts a stronger force on a galaxy’s near side than its far side. That uneven tug can stretch and shear the galaxy, over time, tearing out long streamers of stars and gas.

But there’s another way that galaxies such as C153, which are hurtling through a galactic cluster, can be torn apart. The rushing galaxy is subject to enormous pressure from the vast cloud of gas that envelops the cluster. The galaxy’s stars can withstand this impact, known as ram pressure, but its fluffy gas clouds are easily torn from their gravitational moorings and blown outward.

In the case of C153, “we basically have all the possible signatures [of ram pressure] at once,” says Keel. That may be because the galaxy’s unusually high velocity of 5.4 million kilometers per hour favors a strong interaction with the cluster gas. “It’s really nice to see an object where nature has beefed up the process,” Keel says.

In most cases, gravity and ram pressure act simultaneously, so it’s next to impossible to determine which is more important in altering a particular galaxy’s shape. But the gas in C153 is moving independently of the galaxy’s stars, a disconnect that would appear only when ram pressure dominates. “This is a benchmark case of [ram pressure] acting almost solo,” says Keel.

Stripped-down history

The current interest in C153 has its roots in observations that date from the early 1970s. While peering at the some of the most distant galactic clusters that telescopes could then discern, researchers found a puzzling phenomenon. Galaxies rich in blue-colored emissions were more common in the past than they are now. It took until 1994, when the vision of the newly repaired Hubble Space Telescope became razor-sharp, to determine the shapes of those blue galaxies. A landmark set of Hubble images, analyzed by Alan Dressler of the Carnegie Observatories in Pasadena, Calif., revealed that all these galaxies were spirals, a shape that’s less common in clusters today. Scientists haven’t known what became of the once-abundant spiral galaxies.

That’s about the same time that Keel and his colleagues began studying a cluster of galaxies called Abell 2125, located some 1 billion light-years from Earth. Observations with the Very Large Array radio telescopes near Socorro, N.M., showed that the cluster contains an unusually large number of galaxies that are high-intensity radio emitters. C153 is among them. The radio emissions indicated that the galaxies either contain supermassive black holes or have recently undergone a burst of star formation.

Using the ROSAT X-ray satellite, astronomers found that the environment of the cluster hosting C153 was even more violent than expected. Amidst hundreds of galaxies whizzing past and sometimes through each other, ROSAT detected two enormous clumps of 20-million-kelvin gas. That suggested that the cluster actually consists of two smaller clusters in collision. Galaxy C153 seemed to be located at the heart of it all.

Yet more telescope data soon weighed in. Visible-light spectra taken with telescopes at Kitt Peak near Tucson, Ariz., indicated that C153 had undergone a strong burst of star formation in its recent past, a sign that it had been subject to some cataclysmic event. Kitt Peak observations also identified a bright tail extending from the galaxy.

But it was Hubble images taken in 1999 that showed just how unusual C153 is. With its torn-out tail, distorted shape, and gas shoved to one side, the galaxy “looked like it had been hit with a hammer,” says Keel.

Hubble also found that most of the light in the galaxy’s tail emanates from a recent episode of star formation. That finding, says Keel, is directly linked to the stripping of the galaxy’s gas as it passed through the cluster’s busy core. By compressing cold gas along the galaxy’s leading edge, the ram pressure ignited a flame of star birth, boosting the brightness of the galaxy at least sevenfold, Keel’s team calculates. Ram pressure also blew out the rest of the cold gas, depleting the galaxy of raw material to make any newer stars.

Last summer, ultrasharp spectra of the galaxy taken with the 8-meter Gemini North Telescope on Hawaii’s Mauna Kea revealed the star-forming history of C153 as never before. The spectra showed that most of the bright stars in the galaxy had formed in one distinct episode, 100 million years earlier. According to calculations by Keel and his team, that’s exactly when C153 had plunged through the densest part of the cluster’s core and ram pressure would have been strongest.

The Gemini observations also showed that while groups of stars in C153 circle the galaxy’s center as expected, several clouds of gas are moving along different courses. Because stars and gas move in the same direction in response to gravitational attraction, some force other than gravity—namely, ram-pressure stripping—is probably at work.

Chronicling the saga of C153 has taken longer than Keel and his colleagues ever imagined. “Every time we thought we were ready to [publish], another piece of data came along that added so much more to our whole picture,” Keel says.

Most recently, images taken with NASA’s Chandra X-ray Observatory have shown that cooler material lies embedded within some of the very hot gas in the galaxy cluster, Q. Daniel Wang of the University of Massachusetts in Amherst reported at the January astronomy meeting.

“In essence, this is telling you that a cooler component of gas has intruded into the cluster’s [vast supply of] hot gas,” explains Keel. What’s more, this cooler material “lies just at the same place that we have documented gas [being pulled] out of the galaxy.” That’s yet another piece of evidence that ram pressure exerted by the cluster’s gas deformed the galaxy.

The diversity of telescope observations provides convincing evidence that C153 has been experiencing strong ram pressure that has ripped away much of the galaxy’s star-forming gases, comments Bernd Vollmer of the University of Strasbourg in France.

However, Vollmer cautions that other processes may also be contributing significantly to C153’s ragged appearance. Gravitational interactions between C153 and other nearby members of the crowded cluster should also be considered, he notes. “Ram-pressure stripping may represent one channel of transforming spiral galaxies . . . . but certainly not the only one,” he says.

Dressler agrees. He notes that, unlike C153, the vast majority of stripped-down spiral galaxies, known as S0 galaxies, don’t reside in clusters. These loner galaxies “must have been born without the benefit of ram-pressure stripping, [and] it makes no sense to think that the processes that make S0s in these less dense environments are not at work in clusters,” he asserts. “Ram pressure is not the dominant mechanism.”

But Vollmer adds that although strong ram-pressure events are rare and last less than 100 million years—a brief episode in the 10-billion-year lifetime of a galaxy—they may represent “a decisive moment in a galaxy’s evolution.”

Past and present

Observations of C153 reveal the influence of ram pressure during a period when the universe was about a billion years younger than it is today. Ram pressure may have played a stronger role in the universe’s distant past than it does currently, says Vollmer. When galaxy clusters were first assembling, two properties that increase pressure—the density of cluster gas and the speed of the galaxies coming together—were both high, he explains.

Five billon years ago, adds Dressler, large clusters were at the peak of their formation, pulling in the biggest amount of matter. At the same time, the spiral galaxies falling into these newborn clusters “were in their heyday, full of gas and ripe for interactions” to ignite star formation, he says.

“In contrast, the galaxies falling into clusters today are pretty old, and, in a way, worn out,” Dressler concludes.

“We now have to link these observations of ram-pressure stripping from the past . . . to the local universe, where we have more information but where the effects [of ram-pressure stripping] are less violent,” Dressler says. “Once we have a large-enough sample, we might be able to put the mosaic together to unveil the physics and the importance of these interactions.”

To that end, Vollmer and his colleagues in France and Germany recently began studying two nearby clusters, Coma and Virgo, and have already found evidence of past and present ram pressure in both.

His team has now detected atomic hydrogen gas some 65,000 light-years from the disk of a Virgo galaxy called NGC 4388. The gas appears to have been pulled from the galaxy, most likely under the influence of ram pressure.

That suggests that ram pressure, although more prominent in the cosmos at earlier times, still plays a role in the universe.