Astronomers have discovered a new type of star system: elongated and lumpy clusters of young, hot, blue stars formed from gas expelled from a galaxy as it blasts its way through a cluster of galaxies.
Everything about this phrase is great.
Although these star systems were found on purpose – in the sense that astronomers were looking for them – they turned out not to be what astronomers expected, which added a bit of fun to the whole story. [link to paper].
We live in the Milky Way, a huge galaxy that is part of a small group of a few dozen nearby galaxies that we call the Local Group. Radio wave surveys of the sky reveal many very small, fast moving clouds of atomic hydrogen that are thought to be part of the Milky Way or the Local Group. Astronomers thought these clouds could collapse to form stars, but are actually small galaxies.
However, initial observations in visible light revealed little. They just looked like blue drops. But then deep observations of one cloud showed that it did indeed form stars. It was named SECCO-1, after the StEllar analogue of compact high speed clouds. However, they concluded that it is not actually part of the Milky Way or the Local Group, but much farther away, part of the Virgo Cluster of galaxies about 60 million light-years away!
The Virgo Cluster has thousands of large galaxies and a lot of gas floating around. Encouraged by initial results, more observations of these hydrogen clouds were made using the Very Large Array for more detailed radio maps, the huge Very Large Telescope (or VLT) for spectroscopy, and the Hubble Space Telescope for imaging. They targeted the gas clouds, which also had the ultraviolet emission observed by the GALEX observatory, indicating probable star formation.
They targeted five such clouds. One turned out to be a cluster of distant background galaxies, which is a little disappointing. But the other four have been split into stars by Hubble, showing that they are actually clusters of stars in space. Although they cannot prove that they are part of the Virgo Cluster, their brightness and speed are consistent with this. It seems pretty obvious that they are indeed inside the cluster.
But what is it? They are strange. Objects are lumpy, elongated, and have warm hydrogen gas, consistent with star formation. All stars are blue, indicating that they are young and massive. Although they were originally discovered by being seen on maps of atomic hydrogen, only one of them, named BC3, seems to contain a lot of this gas; the rest lack it.
They don’t see bright red stars, which is another sign that the systems are young. When stars like the Sun die, they turn into luminous red giants. This may take billions of years, but for massive stars it may only take a few tens of millions of years. If nothing is visible, then these systems are really young.
They also have a fairly low mass: they have less than 100,000 times the mass of the Sun’s stars. This may seem like a lot, but galaxies contain millions or many billions of solar masses, and even globular clusters typically have several hundred thousand solar masses. Globular bodies are compact, a few light-years in diameter, but these objects range in length from a few thousand to about 20,000 light-years and are elongated.
So what are they? Gas clouds just floating around in intergalactic space don’t usually collapse to form things like this. But they are not just between galaxies, they are between galaxies. in a massive cluster of galaxies.
This environment is of great importance. All galaxies in a cluster revolve around each other due to their gravity. A gas-rich galaxy passing through a cluster can move at speeds of several hundred kilometers per second – millions of kilometers per hour – and when they do, they experience intense pressure created by the rarefied gas between the galaxies. It can remove the gas inside a fast moving galaxy, much like you can ventilate a car while driving by opening the windows. This type of thing is called plunger pressure cleaning.
We see this happening in many clusters of galaxies, including the Virgo cluster, and in fact, some galaxies are moving so fast that they lose a significant amount of gas. This orphan gas can compress as it is thrown back and follows its parent galaxy, forming stars in the process. It could also explain why these new systems are elongated – a natural result of them plowing through intracluster gas – and why they are also relatively isolated rather than close to other galaxies. Their parent galaxies have long since left them behind.
They formed in a dramatic way, but their fate is to dissolve. Given their individual combined masses, they are probably not bound by gravity; in other words, over time, the stars in these systems will fly apart, becoming isolated stars in the Virgo Cluster between galaxies. Astronomers estimate that star clusters will not last more than 500 million years, short compared to the lifetime of a cluster of galaxies. This also means that these systems are likely to form continuously as gas is removed from galaxies. If these objects were created by some single short-term event, then they would probably have dissipated a long time ago.
If all of this is true—and the evidence certainly points to it—then this is indeed a new type of star cluster. Formed when gas bursts from galaxies, which in turn form stars, they eventually evaporate, eventually strewn with millions of rogue stars in the cluster.
This is amazing and a great indication that we still have a lot to learn about how the universe creates stars. Keep in mind that we only saw these objects because they happened to be nearby; in more distant clusters, they would be too faint to be seen clearly with current technology.
What else is going on in corners of the universe too far away to be closely examined?
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