New Type of Stellar System Discovered

by johnsmith

Astronomers have spotted five blue stellar systems — which they say appear through a telescope as blue blobs and are about the size of dwarf galaxies — in the direction of the relatively nearby Virgo galaxy cluster, analogous to an enigmatic object called SECCO 1.

This Hubble image shows a blue stellar system. Image credit: Michael Jones.

This Hubble image shows a blue stellar system. Image credit: Michael Jones.

“SECCO1, also known as AGC 226067, was one of the very unusual blue blobs,” said Dr. Michael Jones, a postdoctoral researcher with the Steward Observatory at the University of Arizona.

Using data from the NASA/ESA Hubble Space Telescope, NSF’s Karl G. Jansky Very Large Array and ESO’s Very Large Telescope, Dr. Jones and colleagues detected five blue stellar systems within the Virgo galaxy cluster.

They found that these structures contain only young, blue stars, which are distributed in an irregular pattern and seem to exist in surprising isolation from any potential parent galaxy.

The five systems are separated from any potential parent galaxies by over 300,000 light-years in some cases, making it challenging to identify their origins.

The researchers also learned that the systems contain very little atomic hydrogen gas.

This is significant because star formation begins with atomic hydrogen gas, which eventually evolves into dense clouds of molecular hydrogen gas before forming into stars.

“We observed that most of the systems lack atomic gas, but that doesn’t mean there isn’t molecular gas,” Dr. Jones said.

“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.”

The combination of blue stars and lack of gas was unexpected, as was a lack of older stars in the systems. Most galaxies have older stars, which astronomers refer to as being ‘red and dead.’

“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,” Dr. Jones said.

“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.”

The fact that the new stellar systems are abundant in metals hints at how they might have formed.

“To astronomers, metals are any element heavier than helium. 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,” Dr. Jones said.

There are two main ways gas can be stripped from a galaxy. The first is tidal stripping, which occurs when two big galaxies pass by each other and gravitationally tear away gas and stars.

The other is what’s known as ram-pressure stripping.

“This is like if you belly flop into a swimming pool. 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,” Dr. Jones said.

The scientists prefer the ram-pressure stripping explanation because in order for the blue blobs to have become as isolated as they are, they must have been moving very quickly, and the speed of tidal stripping is low compared to ram-pressure stripping.

They expect that one day these systems will eventually split off into individual clusters of stars and spread out across the larger galaxy cluster.

“What we have learned feeds into the larger story of recycling of gas and stars in the Universe,” said Professor David Sand, an astronomer with the Steward Observatory at the University of Arizona.

“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.”

The study will appear in the Astrophysical Journal.


Michael G. Jones et al. 2022. Young, blue, and isolated stellar systems in the Virgo Cluster. II. A new class of stellar system. ApJ, in press; arXiv: 2205.01695

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