September 17, 2008—A distant blob in space long thought to be a single object is actually two galaxies in rare alignment, a newly released image from the Hubble Space Telescope reveals.

The larger background galaxy seen above is 780 million light-years away and roughly the size of the Milky Way. The foreground galaxy, seen with a core of bright white, is about ten times smaller. Together they have been named 2MASX J00482185-2507365.

Most of the stars speckled around the pair belong to the nearby spiral galaxy NGC 253, which was the initial target of the researchers who snapped the image in September 2006.

But a clear view of the two aligned galaxies behind NGC 253 offered the team a rare chance to examine "tentacles of dust" beyond the smaller galaxy's disk of starlight. Such dark outer structures are rarely visible in a galaxy, because there is usually nothing to illuminate them from behind.

"We often mark the end of a spiral galaxy by where most of the stars are," said lead author Benne Holwerda, of the Space Telescope Science Institute in Baltimore, Maryland.

"Since the advent of radio astronomy, we know there is hydrogen gas further out. But this image dramatically shows how there is cosmic dust outside the disk as well."

Astronomers have rarely seen dust this far beyond the starlit edge of a galaxy, and they do not know if it is common. A better understanding of how dust affects a galaxy's brightness can help researchers use that brightness as a measure of its distance from Earth.

—Anne Minard

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