Cosmic CluedoPosted: July 1, 2009
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So space sleuths, let me set the scene for this game of cosmic Cluedo. On the right is an image (credit: NASA, ESA, and the Hubble Heritage Team) of the famous Antennae Galaxies where the collision of two galaxies has triggered many massive bursts of star formation. Individual slightly smaller bursts like this have also been seen in isolated galaxies. In star systems such as NGC 6946 clumps or bubbles of star formation have been found. There are several competing models for the cause of these, a globular cluster passing through the disk of the galaxy, an exploding young star or hypernova or a high velocity cloud of gas hitting the disk.
In our Galaxy (the Milky Way), there are two large young clusters that formed 5-20 million years ago Stephenson 2 and BDSB 122 close to each other. So did these form as the result of some incident? The suspects in our star-forming whodunnit are,
The Reverend Omega Centauri – a globular cluster. An old massive sort of fellow most likely to be found orbiting around the centre of our Galaxy outside the plane of the Galactic disk. When his orbit sometimes takes him through the disk, any gas clouds in the region could get a gravitational kick, leading to the formation of stars.
Professor High Velocity Cloud – a mysterious wandering loaner. A cloud of gas flying through space faster than you would expect for its surroundings. Where did he come from in or outside our Galaxy? Nobody knows. If he were to collide with another cloud of gas then it could lead to a massive burst of star formation.
Colonel Hypernova – hot-blooded, explosive character likely to blow at any time. A high mass star that goes bang sending a shockwave into the surround interstellar gas. This could cause a burst of star formation.
Let’s focus on the first of these suspects. Globular clusters are some of the oldest objects known in our Galaxy, typically older than 12 billion years and containing hundreds of thousands of stars. While most stars in the Galaxy orbit in a disk around the galactic centre, globular clusters are part of an older population of stars known as the galactic halo. These go round the Galaxy in orbits that can take them far above and below the disk. Of course to get from one side of disk to another the globular cluster must pass through the mass of gas and dust in the plane of our Galaxy. The gravitational interaction of a massive globular cluster slamming into the disk can compress gas clouds near the point of impact. They get so compressed that in parts they can collapse to form dense clumps of gas. These clumps then go on to form stars.
So do we have any evidence that this crooked cluster is our criminal? Well a paper that came out recently used a model of the Galaxy to trace back the orbits of Omega Centauri and the two young clusters. Within the errors of their models Omega Centauri could have been in the place where Stephenson 2 and BDSB 122 were when it crossed the galactic disk 24 million years ago.
So the suspect was present at the scene of the crime, but was it there at the right time? Estimates put the age of Stephenson 2 at 12-17 million years and that of BDSB 122 at 7-12 million years. This means the stars in the clusters would have to have formed about 15 million years after their progenitor gas clouds, this isn’t an outrageous suggestion.
So it is possible that this dastardly disk impactor was at the scene of the crime when it was committed. However with an improved model perhaps it will be revealed that the good Reverend Omega Centauri missed the site of the young clusters’ birth. So was it Omega Centauri in the galactic disk with the gravitationally induced star formation? Possibly, but we’ll need more accurate models before we can finally reveal the cards.
Salerno, G., Bica, E., Bonatto, C., & Rodrigues, I. (2009). On the possible generation of the young massive open clusters Stephenson 2 and BDSB 122 by ω Centauri Astronomy and Astrophysics, 498 (2), 419-423 DOI: 10.1051/0004-6361/200911737