Astronomers at the University of Arizona have identified five examples of a new class of star systems. They are not completely galaxies and they exist only in isolation.
The new star systems contain only young blue stars, which are scattered in an irregular pattern and appear to exist in surprising isolation from any potential parent galaxy.
The star systems – as astronomers say, appear through a telescope as “blue spots” and are roughly the size of small dwarf galaxies – are located in the relatively nearby Virgo galaxy cluster. All five systems are separate from any potential parent galaxy by more than 300,000 light-years in some cases, making it difficult to identify their origin.
Astronomers discovered the new systems after another team of researchers, led by Elizabeth Adams of the Dutch Institute of Radio Astronomy, compiled a catalog of nearby gas clouds that provided a list of potential sites for new galaxies. When this catalog was published, several research groups, including one led by UArizona associate astronomy professor David Sand, began searching for stars that could be associated with these gas clouds.
It was thought that the gas clouds were connected to our own galaxy, and most of them probably are, but when the first collection of stars, called SECCO1, was discovered, astronomers realized that it was not near the Milky Way, but rather in the Virgo cluster, much more distant, but still very close to the scale of the universe.
SECCO1 was one of the very unusual “blue spots,” said Michael Jones, a postdoctoral fellow at the Arizona Steward Observatory and lead author of a study describing new star systems. Jones presented the results, co-authored by Sand, at the 240th meeting of the American Astronomical Society in Pasadena, California, on Wednesday.
“It’s a lesson in the unexpected,” Jones said. “When you are looking for things, you may not find the thing you are looking for, but you may find something else very interesting. »
The team got their observations from the Hubble Space Telescope, the Very Large Array Telescope in New Mexico and the Very Large Telescope in Chile. The study’s co-author Michele Bellazzini, with the Istituto Nazionale di Astrofisica in Italy, led the analysis of the Very Large Telescope data and submitted an accompanying paper focusing on the data.
Together, the team learned that most stars in each system are very blue and very young and contain very little atomic hydrogen. This is important because star formation begins with atomic hydrogen gas, which eventually evolves into dense clouds of molecular hydrogen gas before turning into stars.
“We’ve observed that most systems lack nuclear gas, but that does not mean there is no molecular gas,” Jones said. ‘In fact, there has to be molecular gas because they still form stars. The existence of mostly young stars with little gas indicates that these systems must have lost their gas recently. »
The combination of blue stars and lack of gas was unexpected, as was the lack of older stars in the systems. Most galaxies have older stars, which astronomers call “red and dead”.
“Stars that are born red have a lower mass and therefore live longer than blue stars, which quickly burn up and die young, so old red stars are usually the last ones left alive,” Jones said. “And they died because they had no more gas to form new stars with. These blue stars are like an oasis in the desert, basically.”
The fact that the new star systems were abundant in metals indicates how they could have been formed.
“For astronomers, metals are any element that is heavier than helium,” Jones said. “It tells us that these star systems are formed by gas extracted from a large galaxy, because the way metals are built is due to many repetitive episodes of star formation, and you don’t really get that in a large galaxy. »
There are two main ways to extract gas from a galaxy. The first is tidal stripping, which occurs when two large galaxies intersect and gravity pulls gas and stars apart.
The second is what is called piston pressure suppression.
“It’s like falling face down into a pool,” Jones said. “When a galaxy’s belly collapses into a cluster filled with hot gas, its gas is expelled behind it. This is the mechanism we think we see here to create these objects.”
The team prefers the explanation of ramp stripping because in order for the blue droplets to have become as isolated as they are, they must move very fast and the speed of tidal stripping is low compared to ramp stripping.
Astronomers expect that these systems will one day eventually separate into individual clusters of stars and spread into the larger galaxy cluster.
What the scientists learned is part of “the larger story of gas and star recycling in the universe,” Sand said. “We believe that this gut-wrenching process transforms many spiral galaxies into elliptical galaxies at some level, so learning more about the general process tells us more about how galaxies form.”