Astronomers at the University of Arizona have found five examples of a new class of galaxies. They are not actually galaxies and only exist in isolation.
The new galaxies contain only young, blue stars, which are scattered in an irregular pattern and appear to exist in surprising isolation from a possible parent galaxy.
The stellar systems — which astronomers say appear through a telescope as “blue blobs” and are about the size of small dwarf galaxies — are located in the relatively nearby Virgo galaxy cluster. The five systems are in some cases more than 300,000 light-years from any parent galaxies, making it challenging to identify their origin.
The astronomers found 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, listing potential locations of new galaxies. Once that catalog was published, several research groups, including one led by UArizona associate astronomy professor David Sand, set out in search of stars associated with those gas clouds.
The gas clouds were thought to be associated with our own galaxy, and most probably are, but when the first collection of stars, called SECCO1, was discovered, astronomers realized it was not near the Milky Way at all, but rather in the Virgo cluster, which is much further away but still very close on the scale of the universe.
SECCO1 was one of the very unusual “blue blobs,” said Michael Jones, a postdoctoral researcher at the UArizona Steward Observatory and lead author of a study describing the new stellar systems. Jones presented the findings, which Sand co-authored, at the 240th meeting of the American Astronomical Society in Pasadena, Calif., Wednesday.
“It’s a lesson in the unexpected,” Jones said. “If you’re looking for things, you won’t necessarily find what you’re looking for, but you might find something else very interesting.”
The team obtained 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 Italy’s Istituto Nazionale di Astrofisica, led the analysis of the data from Very Large Telescope and has submitted an accompanying paper focusing on that data.
Together, the team found that most stars in any system are very blue and very young, and that they 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 becoming stars.
“We’ve found that most systems don’t have atomic gas, but that doesn’t mean there isn’t molecular gas,” Jones said. “In fact, there must be molecular gas because they are still forming stars. The existence of mostly young stars and small 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 call “red and dead.”
“Stars that are born red have lower masses and therefore live longer than blue stars, which burn quickly and die young, so old red stars are usually the last ones alive,” Jones said. “And they’re dead because they’ve run out of gas to form new stars. These blue stars are actually an oasis in the desert.”
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,” Jones said. “This tells us that these stellar systems are formed from gas stripped from a large galaxy, because metals are built up through many repeated episodes of star formation, and you really get that in a large galaxy.”
There are two ways in which gas can be removed from a galaxy. The first is tidal stripping, which occurs when two large galaxies pass each other and pull gas and stars away by gravity.
The other is what is known as ram pressure stripping.
“This is like falling on your stomach in a swimming pool,” Jones said. “When the belly of a galaxy falls into a cluster that is full of hot gas, the gas is forced behind it. That’s the mechanism we think we see here to create these objects.”
The team prefers the ram pressure stripping explanation because to get the blue blobs as isolated as they are they must have gone very fast and the tide stripping rate is low compared to stripping of the ram pressure.
Astronomers expect that one day these systems will eventually split into separate star clusters and spread out over the larger cluster of galaxies.
What researchers have learned feeds into the larger “story of recycling gas and stars in the universe,” Sand said. “We think this process of rolling up the belly turns many spiral galaxies into elliptical galaxies at some level, so learning more about the general process teaches us more about galaxy formation.”
- Michael G. Jones, David J. Sand, Michele Bellazzini, Kristine Spekkens, Ananthan Karunakaran, Elizabeth AK Adams, Giuseppina Battaglia, Giacomo Beccari, Paul Bennet, John M. Cannon, Giovanni Cresci, Denija Crnojevic, Nelson Caldwell, Jackson Fuson, Puragra Guhathakurta, Martha P. Haynes, John L. Inoue, Laura Magrini, Ricardo R. Munoz, Burcin Mutlu-Pakdil, Anil Seth, Jay Strader, Elisa Toloba, Dennis Zaritsky. Young, blue and isolated galaxies in the Virgo Cluster. II. A new class of galaxies. Filed with ApJ, 2022 DOI: 10.48550/arXiv.2205.01695
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