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Catch a galactic double feature

NASA / ESA / STScI / AURA
This Hubble image reveals a rare alignment involving a small foreground galaxy
and a larger background galaxy. The smaller galaxy's tentacles of dust are
silhouetted against the bigger galaxy's glow. Click on the image for a larger
version from the Space Telescope Science Institute.

Two galaxies, one right in front of the other, have put on a rare light show for the Hubble Space Telescope - and the backlighting reveals seldom-seen dust tentacles that may be standard equipment for starry spirals.

Until Hubble focused its Advanced Camera for Surveys on the sight in the southern constellation Sculptor, astronomers saw just one single blob in the sky. But the space telescope could make out a background galaxy about 780 million light-years away that is the size of our Milky Way - as well as a smaller, closer galaxy.

The galactic double feature is cataloged as 2MASX J00482185-2507365, and the research team's description of the pair has been submitted for publication in The Astronomical Journal. The researchers haven't yet gotten a fix on just how close the closer galaxy is, but they see no evidence that it's gravitationally interacting with the background galaxy.

The most interesting thing about the sight is the way that the closer galaxy is silhouetted against the farther-out galaxy. That rarely happens in astronomy. More typically, you merely see the galaxy's glow against the blackness of space, and the dark edges remain invisible.

In this case, the background galaxy serves to light up the foreground galaxy's outer tentacles of dark dust. Today's image advisory compares the structures to "barren branches" on a tree and says that "astronomers have never seen dust this far beyond the visible edge of the galaxy."

Astronomers don't yet know whether these dark branches are common features in galaxies. But they do know the dust doesn't account for the mysterious dark matter that makes up most of the universe's mass. "This is a known component in galaxies," Roelof de Jong of the Baltimore-based Space Telescope Science Institute, told me today.

De Jong said astronomers have long known that the dark dust was there, based on infrared emissions. They just haven't had much of an opportunity to see how it was distributed. "The silhouette effect helps you see the tiny amounts," he explained.

The double feature also helps you see why Hubble's observations are the gifts that keep on giving. This research was based on archived imagery that was captured almost exactly two years ago, before the Advanced Camera for Surveys was crippled. Next month's Hubble repair mission could return the ACS to full service - but even when the venerable space telescope gives way to the next generation, there'll still be Hubble data galore for astronomers to pore over.

The science team behind the double-galaxy observations includes de Jong as well as Benne Holwerda of the Space Telescope Science Institute; Bill Keel of the University of Alabama at Tuscaloosa; and Julianne Dalcanton and Benjamin Williams of the University of Washington. Dalcanton is a frequent blogger at Cosmic Variance.

Update for 6:15 p.m. ET: I caught up with the University of Washington's Williams and Dalcanton to find out more about the research. They're involved in a project called the ACS Nearby Galaxy Survey Treasury, or ANGST. The researchers were in the middle of reviewing Hubble imagery showing scores of galaxies neighboring our own when the galaxies popped up in the region of the sky around the better-known spiral galaxy NGC 253.

"I was the one who found them in the data," said Williams, a postdoctoral research associate. "It was one of the first observations that we took for our project. ... I just thought, 'Wow, I've never seen anything like that before.'"

Williams showed the sight to Dalcanton, and although they couldn't follow up immediately (Oh, the ANGST!) they eventually enlisted Holwerda, de Jong and Keel to do further analysis.

"There are only a handful of systems known with similar overlaps, and none are as nicely arranged as these two," Dalcanton said in an e-mail. "Bill Keel ... has been working hard on similar systems for years."