After we discovered that the obscured young open cluster Westerlund 1 is much more massive than any
other young cluster in the Milky Way (with a mass around 100,000 solar
masses), many astronomers quickly concluded that our Galaxy contained
many clusters much more massive than those normally seen in the Solar
neighbourhood. These young, compact clusters, with masses higher than
10,000 solar masses are sometimes called massive clusters, or also
Soon afterwards, Don
Figer and his collaborators found a
cluster that contained a large number of red supergiants, and - with a
strong appeal to imagination - called it the Red Supergiant Cluster.
This cluster is hidden behind so much insterstellar obscuration that it
can only be seen in the infrared. Actually only the red supergiants can
be seen at all in infrared photometric catalogues and deep observations
will be needed to reveal the rest of the population. In any case, based
on the rarity of red supergiants, Ben Davies and co-workers have
concluded that the cluster must have around 30,000 solar masses in
unseen stars. Shortly afterwards, Ben found that the cluster Stephenson
2, which is also hidden behind huge amounts of obscuration, contains
even more red supergiants and is even more massive than the original
Red Supergiant Cluster, now re-christened as RSGC1, with Stephenson 2
becoming RSGC2 (as a matter of fact, Ben beat us to this discovery by a
narrow margin; Simon had already found the cluster in GLIMPSE/Spitzer
images and we had a proposal approved to observ it).
Now we proudly announce the discovery of a third cluster full of red
supergiants, which boringly will have to be called RSGC3. This cluster
is very close in the sky to the other two and we all think that the
three of them are part of a huge region of star fomation that has been
excited at the point where the Galactic Long Bar hits the inner spiral
arm (the Scutum-Crux Arm).
Our discovery is reported in
J. S. Clark, I. Negueruela, B. Davies, V.M. Larionov, B.W. Ritichie, D. F. Figer, M. Messineo, P. A. Crowther, A.Arkharov. "A third red supergiant rich cluster in the Scutum-Crux Arm ", 2009. Astronomy & Astrophysics, in press (also at arXiv:0903.1754)Here is an image of the cluster core. It is a 3-colour composite I made combining UKIDDS JHK images and was later enhanced by Simon's sister-in-law, who is a graphical designer. The ugly blue water-drops are due to the saturation of the infrared detector by the red supergiants, which are so bright that they leave behind long-lived blemishes.
All these discoveries are prompting a real race between professional astronomers to find the most massive clusters in the Galaxy. Indeed we beat a rival group to this discovery by a mere two weeks. They have now also published a paper on the cluster. Fortunately, their conclusions are very similar to ours. We all think that this cluster may have around 30,000 solar masses. Another heavyweight!
We will be working hard to calculate the properties of these
clusters and find new massive clusters in the near future.