Astronomers at the University of Arizona have identified five examples of a new class of star system. They are not quite galaxies and they only exist in isolation.
The new star systems contain only young blue stars, which are spread out in an irregular pattern and appear to exist in surprising isolation from any potential parent galaxy.
The star systems – which astronomers say appear through a telescope as “blue spots” and are roughly the size of tiny dwarf galaxies – are located in the relatively nearby Virgo galaxy cluster. All five systems are separated from any potential parent galaxy by more than 300,000 light years in some cases, making it difficult to identify their origins.
Astronomers discovered the new systems after another research group, led by Elizabeth Adams of the Netherlands Institute for Radio Astronomy, compiled a catalog of nearby gas clouds, providing a list of potential sites of new galaxies. Once this catalog was published, several research groups, including one led by the UArizona Associate Professor of Astronomy David Sablebegan looking for stars that might be associated with these gas clouds.
The gas clouds were thought 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 nowhere near the Milky Way, but rather in the Virgo cluster, much more distant but still very close on the scale of the universe.
SECCO1 was one of the very unusual “blue spots”, said Michael Jones, postdoctoral fellow at the UArizona Steward Observatory and lead author of a study describing new star systems. Jones presented the findings, co-authored by Sand, at the 240th meeting of the American Astronomical Society in Pasadena, Calif., on Wednesday.
“It’s a lesson in the unexpected,” Jones said. “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.”
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. Study co-author Michele Bellazzini, with the Istituto Nazionale di Astrofisica in Italy, led the analysis of the Very Large Telescope data and submitted a companion paper focused 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 gas. 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 observed that most systems lack atomic gas, but that doesn’t mean there isn’t molecular gas,” Jones said. “Actually, there must be molecular gas because they are still forming stars. The existence of mostly young stars and 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 lower mass and therefore live longer than blue stars, which burn up quickly and die young, so old red stars are usually the last ones left alive,” Jones said. “And they’re dead because they don’t have the gas left to form new stars. These blue stars are like an oasis in the desert, basically.
The fact that the new star systems are abundant in metals indicates how they could have formed.
“For astronomers, metals are any element heavier than helium,” Jones said. “It tells us that these star systems formed from gas that was extracted from a large galaxy, because the way metals are built is by many repeated episodes of star formation, and you don’t really only get that in a big 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 gravitationally pull gas and stars apart.
The other is what is known as piston pressure suppression.
“It’s like falling face down in a pool,” Jones said. “When the belly of a galaxy collapses into a cluster filled with hot gas, its gas is forced out behind it. This is the mechanism we think we see here for creating these objects.
The team prefers the ram pressure stripping explanation because for the blue drops to have become as isolated as they are, they must be moving very quickly and the velocity of tidal stripping is low compared to ram pressure stripping .
Astronomers expect that one day these systems will eventually separate into individual clusters of stars and spread out into the larger cluster of galaxies.
What the researchers learned feeds into “the larger story of gas and star recycling in the universe,” Sand said. “We believe that this belly-falling 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.”
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