Evrim Yazgin
Cosmos science journalist
High school science tells us that planets orbit stars and stars are found in galaxies. For the most part, that’s right. But astronomers have identified a new type of star system which doesn’t emerge in any galaxies.
In a press release, University of Arizona astronomers have noted their identification of five examples of the new stellar system. The new system is not quite a galaxy and seems to exist only in isolation – outside any parent galaxy.
And the systems have a fun name. After their appearance as seen through telescopes, the astronomers have been referring to the structures as “blue blobs”.
The examples discovered are located within the relatively nearby Virgo galaxy cluster (a mere 54 million light years away) – a collection of more than 2000 galaxies. The blue blobs are “tiny” – the size of “dwarf galaxies” which contain anywhere from 1000 to several billion stars (compared with our Milky Way galaxy and its 200–400 billion stars).
But, beguilingly, the blue blobs of bloated gas balls belong to no “parent” galaxy. Some of the blue blobs are as much as 300,000 light years from the nearest potential host.
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Blue blobs are made predominantly of very blue and very “going” stars – an indication that the systems contain very little hydrogen gas. But atomic hydrogen gas is the stuff which condenses to form stars in the first place.
So, where did the blue blobs come from and how did they form? This remains a mystery.
The blue blobs were discovered by accident when another research group based in the Netherlands compiled a list of gas clouds which may give rise to new galaxies. The University of Arizona team and others analysed the list, looking for stars in the gas clouds using data from the Hubble Space Telescope, the Very Large Array telescope in New Mexico and the Very Large Telescope in Chile.
The first collection of stars, named SECCO1, was found to be further away than originally thought – the first of the “blue blobs”.
Lead author of a study on the blue blobs, University of Arizona postdoctoral fellow Michael Jones presented the findings at the 240th American Astronomical Society meeting, which wrapped up at the Pasadena Convention Center on June 16.
“It’s a lesson in the unexpected,” Jones says. “When you’re looking for things, you’re not necessarily going to find the thing you’re looking for, but you might find something else very interesting.
“We observed that most of the systems lack atomic gas, but that doesn’t mean there isn’t molecular gas.
“In fact, there must be some molecular gas because they are still forming stars. The existence of mostly young stars and little gas, signals that these systems must have lost their gas recently.”
Gas can be stripped from stellar systems, like a galaxy, in two main ways: tidal stripping, when big galaxies pass and tear gas and stars away from each other gravitationally; and ram pressure stripping. “This is like if you belly flop into a swimming pool,” Jones explains. “When a galaxy belly flops into a cluster that is full of hot gas, then its gas gets forced out behind it.
“That’s the mechanism that we think we’re seeing here to create these objects.”
The team believes blue blobs formed in this latter method because the speed of the process explains the systems’ isolation.
Also surprising is the lack of older, redder stars in the systems.
“Stars that are born red are lower mass and therefore live longer than blue stars, which burn fast and die young, so old red stars are usually the last ones left living,” Jones says.
“And they’re dead because they don’t have any more gas with which to form new stars. These blue stars are like an oasis in the desert, basically.”
Jones says that the abundance of metal in the systems may hint at how they might have formed.
“To astronomers, metals are any element heavier than helium,” Jones says. “This tells us that these stellar systems formed from gas that was stripped from a big galaxy, because how metals are built up is by many repeated episodes of star formation, and you only really get that in a big galaxy.”
As the blue blobs drift through space, astronomers predict they will break apart into individual star clusters.
Lead researcher and co-author David Sand, astronomy associate professor at the University of Arizona, says the team’s findings add to the broader “story of recycling of gas and stars in the universe”. “We think that this belly flopping process changes a lot of spiral galaxies into elliptical galaxies on some level, so learning more about the general process teaches us more about galaxy formation,” Sand says.