Posted: September 17, 2008
Hubble captured this image of a small galaxy silhouetted in front of a larger galaxy on 19 September 2006. Lanes of dust extend from the small galaxy's visible edge. Image: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
The snap shot was captured purely by chance; astronomers were using Hubble to study galaxy NGC 253, to which most of the stars scattered across the foreground of the image belong. The two galaxies looked like a single ‘blob’ from ground based telescopes, but the Advanced Camera on Hubble resolved the nucleus into two separate galaxies that although appear close together, are not close enough to interact with each other. The background galaxy is now known to be about the size of the Milky Way Galaxy and is about 10 times larger than the foreground galaxy. Together they are classified as 2MASX J00482185-2507365 and are estimated to lie 780 million light-years away.
The chance arrangement of a small galaxy silhouetted against a larger background galaxy will greatly improve astronomers’ understanding of the structure of galaxies, since they have never before seen dust extend so far out from the visible edge of a galaxy. Moreover, the tracks of dust extending from the smaller galaxy appear to be completely devoid of stars.
Given the vast cavern of space there is usually nothing positioned so fortunately to allow the right illumination conditions, so it is not yet known if these dusty structures are common features in all galaxies. Astronomers will have to play the waiting game to find out.
The fourth and final Hubble servicing mission is now due for launch on 10 October; you can read more about the mission and the planned upgrades in the October issue of Astronomy Now.