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A giant among galaxies?

Michael West / ESO / ESA / NASA
The elliptical galaxy ESO 306-17 looks like a fuzzball with a bright center and a
large halo in this image, captured by the Hubble Space Telescope's Advanced
Camera for Surveys. Click on the image to see a larger version.

Swirls of stars seem to surround a huge galaxy half a billion light-years from Earth. But that's just an illusion: In reality, the hazy galaxy spotted by the Hubble Space Telescope is a big fat loner. The big fat mystery is ... why?

The Hubble image takes in a patch of southern sky that is chock-full of galaxies, but dominated by the elliptical galaxy ESO 306-17. See how many spirals you can spot when you scan this 5.6-megabyte high-resolution Hubble image or watch this zoom-in video from the European Space Agency's Hubble team.

Most of those galaxies are much closer or much farther away than ESO 306-17, however. As described in today's image advisory, a team of astronomers led by the European Southern Observatory's Michael West found that the giant elliptical galaxy is actually sitting by itself in "an enormous sea of dark matter and hot gas."

ESO 306-17 is a type of galaxy known as a fossil group, because it appears to represent the fossilized remains of a larger cluster of galaxies. So what happened to ESO 306-17's neighbors? Did they just fade away, like old soldiers, or were they cannibalized in the bigger galaxy's gravitational maw?

Gravitational gobbling is par for the course for large galaxies. Thanks to Hubble and other telescopes, astronomers have witnessed a long list of galactic mergers (see below). Our own Milky Way has taken on a fair number of dwarf galaxies. Astronomers also suspect that our big next-door neighbor, the Andromeda Galaxy, has swallowed up some dwarfs as well and will eventually be coming for us (after it gulps down the Triangulum dwarf galaxy).

ESO 306-17 appears to be an extreme example of a "bully" galaxy that grows by pushing its weight around, the Hubble team said. But to solve the mystery behind the galaxy's evolution, astronomers are taking an ultra-close look at the picture.

One kind of clue is the presence of globular clusters that can be seen within the galaxy's halo. These are tightly bound knots of stars that the big bully hasn't yet managed to pull apart. Another clue would come if astronomers can find ultra-compact dwarf galaxies in the vicinity of ESO 306-17. Astronomers believe that these galaxies have been whittled down to their extremely dense cores through interactions with larger galaxies.

ESA's Hubble team notes that most of the ultra-compact dwarf that have been found so far are located near giant elliptical galaxies in large clusters. "It will be interesting to see if researchers find similar objects in fossil groups," the team says in today's advisory.

Update for 4 p.m. ET: ESO astronomer Michael West got back to me via e-mail from Chile with some answers to a few questions about the Hubble image. Here's the Q&A:

Q: Where is ESO 306-17 located?

A: ESO 306-17 is located in the southern constellation Columba ("the dove").  It's a rather faint constellation. It's ironic that the "bully galaxy" is located in a constellation that's a symbol of peace.

Q: How wide is the galaxy in light-years?

A: The galaxy's stars actually extend beyond the boundaries of our HST image in the press release.  Previous measurements have shown that the galaxy is more than 1 million light-years wide.  To put it in perspective, our Milky Way galaxy is only about one-tenth as wide, which means that you could fit roughly 1,000 Milky Way galaxies inside ESO 306-17!

Q: Is there a publication that relates to these observations?

A: We don't have a publication yet, graduate student Karla Alamo is working on the data for her Ph.D. thesis and we expect to submit a paper for publication in one of the astronomical journals sometime in the next few months.

The European Southern Observatory's facilities are in Chile, and West works in Santiago as the head of the ESO's Office for Science in Chile. One of West's jobs is to manage the ESO's scientific visitor program - which has been affected by the earthquake in Chile. The ESO has been advising scientists to hold off on travel to Chile while the country recovers from the seismic shock. However, the ESO's observatories are located far north of the quake epicenter and were undamaged.

Here's what West had to say about the situation:

"All ESO staff are safe here in Chile, although some have damage to their homes and apartments.  ESO offices in Santiago had only minor damage, we are able to return to work. We continue to feel the occasional aftershock here in Santiago, which makes people understandably nervous.

"The earthquake was definitely scary, it was amazing how much (and how long) the ground shook in Santiago; I can't even imagine what it must have been like nearer to the epicenter.  And, sadly, the resulting tsunamis devastated towns and villages along the Chilean coast. The magnitude of the disaster is becoming clearer, it's going to take a long time for Chile to recover. But Chileans are resilient.

"We received news that our astronomer colleagues at the Universidad de Concepcion, one of the hardest-hit areas (and Chile's second-biggest city) are reportedly safe, although the situation is obviously difficult and fluid for everyone there."

For more about gobbling galaxies, check out these links:

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