A monster so huge it is capable of slowly devouring whole galaxies at a time. Sounds incredible doesn’t it? But that is what astronomers working on the Hubble Space Telescope think that the giant elliptical galaxy NGC 1132 is – a cosmic cannibal if you will. In this stunning new image from the NASA/ESA Hubble mission astronomers are seeing the vast hulk of a galaxy, 320 million light years distant, whose past is much darker than it might at first seem. That’s because whilst the stunning elliptical in Hubble’s new image looks serene and peaceful, it is in fact the aftermath of gravitational dance which saw the death of many smaller galaxies; and it all clinches on how astronomers think galaxies form.
One of the most popular current theories is that giant galaxies like NGC 1132 are made from the merger and assimilation of lots of smaller galaxies. Over time these vast elliptical giants like NGC 1132 emerge as enormous conglomerations of stars. Sounds all very vicious but in fact this galactic cannibalism is probably quite commonplace in the Universe if our theories of galaxy evolution are correct. Indeed Hubble scientists believe that our own Milky Way may have been partial to devouring the odd dwarf galaxy which strayed too close to it.
Yet the one question that we are bound to ask is how do we know? Well the answer comes from two main lines of evidence. The first is globular clusters. Galaxies like the Milky Way are home to globular clusters which reside above and below the disc of the galaxy. These are extremely ancient (and fairly compact) balls of stars and are useful tools for studying the evolution of stars. If you know where to look you can spot them through a small telescope on a clear night.
When Hubble scientists looked at NGC 1132 they noticed something interesting. A vast collection of globular clusters around the massive galaxy. They believe that what they are seeing are the globular clusters of NGC 1132’s victims – whole globular clusters that have been cast away as NGC 1132 merges with their parent galaxies. Since the stars in globulars are packed much more densely than the normal stars in the unfortunate galaxies their collective gravity holds the globular together. This means they can survive the huge gravitational disruptions involved in the merger and breakup of their parent galaxy.
The second piece of evidence comes from material we can’t see in this image – dark matter. Observations have shown that NGC 1132 is surrounded by a truly enormous cloud of dark matter. The dark matter cloud is thought to hold quantities of dark matter that are normally found residing in whole galaxy clusters of between ten and a few hundred galaxies – not one galaxy as seen with NGC 1132! NASA’s Chandra X-ray observatory was able to show that the galaxy is also surrounded by a glow from X-rays emitted from hot gas – about 120,000 light years in diameter – roughly the size of a galaxy cluster, giving yet more support to the idea that NGC 1132 is the result of the merging of one entire galaxy cluster.
Above: NGC 1132 from the HST
Image credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgment: M. West (ESO, Chile)