University of Arizona astronomers have identified five examples of a new class of star systems. They are not really galaxies and only exist in isolation.
The new star systems contain only young, blue stars, which are distributed in an irregular pattern and appear to exist in surprising isolation from all potential parent galaxies.
The star systems – which astronomers say appear through a telescope as “blue bubbles” and are about the size of small dwarf galaxies – are found in the relatively nearby galaxy cluster Virgo. The five systems are separated from all potential parent galaxies by more than 300,000 light-years in some cases, making it difficult to identify their origin.
Astronomers discovered the new systems after another group of researchers, led by Elizabeth Adams of the Dutch Institute for Radio Astronomy, compiled a catalog of nearby gas clouds and provided a list of potential sites for new galaxies. When that catalog was published, several research groups, including one led by UArizona’s associate professor of astronomy, David Sand, began looking for stars that could be associated with these gas clouds.
The gas clouds were considered to be associated with 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 at all, but rather in the Virgo Cluster, which is much further away. but still very close in the scale of the universe.
SECCO1 was one of the very unusual “blue bubbles,” said Michael Jones, a postdoctoral fellow at the Arizona Steward Observatory and lead author of a study describing the new star systems. Jones presented the results, which Sand co-authored, 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 look for things, you will not necessarily find what you are looking for, but you can find something else very interesting.”
The team received its 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 data from the Very Large Telescope and has submitted a supplementary document focusing on this data.
Together, the team was told that most of the stars in each system are very blue and very young and that they contain very little atomic hydrogen. This is important because star formation begins with atomic hydrogen, which eventually evolves into dense clouds of molecular hydrogen before forming into stars.
“We observed that most of the systems lack nuclear gas, but that does not mean there is no molecular gas,” Jones said. “In fact, there must be some molecular gas because they still form stars. The presence of mostly young stars and small gas signals that these systems must have recently lost their gas.”
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 refer to as “red and dead”.
“Stars that are born red have a lower mass and therefore live longer than blue stars, which burn quickly and die young, so old red stars are usually the last ones left,” said Jones. “And they are dead because they have no more gas to form new stars with. These blue stars are basically like an oasis in the desert.”
The fact that the new star systems are rich in metals suggests how they may have formed.
“For astronomers, metals are all the elements heavier than helium,” Jones said. “This tells us that these star systems were formed by gas removed from a large galaxy, because how metals are built depends on many repeated episodes of star formation, and you really only get that in a large galaxy.”
There are two main ways to remove gas from a galaxy. The first is tidal drift, which occurs when two large galaxies pass each other and gravitationally tear away gas and stars.
The second is what is called frame pressure stripping.
“This is like flipping into a pool,” Jones said. “When a galaxy’s stomach flops into a cluster full of hot gas, its gas is forced out behind it. That’s the mechanism we think we see here to create these objects.”
The team prefers the explanation for pressure relief because in order for the blue lumps to have become as isolated as they are, they must have moved very quickly, and the speed of tidal stripping is low compared to pressure relief.
Astronomers expect that one day these systems will eventually be divided into individual star clusters and spread over the larger galaxy cluster.
What scientists have learned is fed into the larger “story of recycling gas and stars in the universe,” Sand said. “We believe that this belly-floating process transforms many spiral galaxies into elliptical galaxies on some level, so learning more about the general process teaches us more about galaxy formation.”
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