Even galaxies despise being alone. While astronomers have long known that galaxies tend to cluster and form groups, the process of moving from formation to companion groupings has remained a mystery in cosmology.
An international team of astronomers reports the finding of a group of objects that appear to be a developing accumulation of galaxies in the making known as a protocluster in a publication published in the Astronomy & Astrophysics Journal.
“This discovery is an important step toward reaching our ultimate goal: understanding the assembly of galaxy clusters, the most massive structures that exist in the universe,” said Brenda Frye, an associate professor of astronomy at the University of Arizona’s Steward Observatory and a co-author of the study.
The European Space Agency’s Planck instrument was used to discover G237 in the far-infrared region of the electromagnetic spectrum. Follow-up observations with the Large Binocular Telescope in Arizona and the Subaru Telescope in Japan, as well as archival data from the Herschel Space Observatory and the Spitzer Space Telescope, have now confirmed its existence.
The Milky Way, which houses our solar system, is part of the Local Group galaxy cluster, which is part of the Virgo supercluster. But, 11 billion years ago, what did a supercluster like Virgo look like?
“We still know very little about protoclusters, in part because they are so faint, too faint to be detected by optical light,” Frye said. “At the same time, they are known to radiate brightly in other wavelengths such as the sub-millimeter.”
The protocluster detailed in the current research was first identified as part of an all-sky scan by the European Space Agency’s Planck observatory. It showed up prominently in the far-infrared area of the electromagnetic spectrum.
Researchers discovered a protocluster known as PHz G237.01+42.50, or G237, while sifting through a sample of more than 2,000 structures that could be on their way to becoming clusters. The results appeared promising, but more observations with other telescopes were necessary to establish its identify.
Each of the 63 galaxies discovered so far in G237 was like a star factory in overdrive. It’s as if the galaxies were working on overtime to the assemble stars. The rate of production was unsustainable. At such a pace, the supply chains are expected to break in the near future, and in a way that permanently shuts down the galaxy shipyard.
Brenda Frye
The team used the combined power of the Large Binocular Telescope in Arizona, which is operated by UArizona, and the Subaru Telescope in Japan, led by Mari Polletta of the National Institute for Astrophysics in Milan, Italy. The G237 protocluster has 63 galaxies, according to the researchers.
Early observations of G237 suggested that it was forming stars at an unacceptably fast and unsustainable rate. Some of the observations of star formation originated from galaxies unconnected to G237, according to the astronomers.
The first observation was made using archival data, the Herschel Space Observatory, and the Spitzer Space Telescope, and follow-up observations were made using archival data, the Herschel Space Observatory, and the Spitzer Space Telescope.
“You can think of galaxy protoclusters such as G237 as a galaxy shipyard in which massive galaxies are being assembled, only this structure existed at a time when the universe was 3 billion years old,” Frye said.
“At the same time, the genealogy may be closer than you think. Because the universe is homogeneous and the same in all directions, we think that the Milky Way may have docked at a protocluster node similar to G237 when it was very young.”
At first, measurements of G237 suggested an abnormally high total star formation rate, and the researchers struggled to make sense of the data. The G237 protocluster appeared to be producing stars 10,000 times faster than the Milky Way.
At that rate, the protocluster’s stellar fuel would be quickly depleted, and it would eventually settle down into a complicated system akin to the Virgo supercluster.
“Each of the 63 galaxies discovered so far in G237 was like a star factory in overdrive,” Frye said. “It’s as if the galaxies were working on overtime to the assemble stars. The rate of production was unsustainable. At such a pace, the supply chains are expected to break in the near future, and in a way that permanently shuts down the galaxy shipyard.”
Continuous injections of fuel, which in the case of stars is hydrogen gas, are required to maintain such high yields. According to Frye, this would necessitate a well-connected supply line that sucked in unrealistically vast amounts of new gas to power the star-forming factories.
“We don’t know where that gas was coming from,” she said.
Even after the extraneous observations were excluded, the total star formation rate remained high, at least 1,000 solar masses per year, according to Poletta. The Milky Way, on the other hand, creates around one solar mass per year.
“The picture we have pieced together now is that of a successful galaxy shipyard, which is working at high efficiency to assemble galaxies and the stars within them and has an energy supply that is more sustainable,” Frye said.
All galaxies in the universe are connected by the cosmic web, a massive structure that resembles a three-dimensional spider web. The nodes, which correspond to the galaxy shipyards in the analogy, are where the filaments of the cosmic web intersect.
“We believe that the filaments mediate the transfer of hydrogen gas from the diffuse medium of intergalactic space onto these hungry, newly forming protocluster structures in the nodes,” Frye said.
Pointing to future research, Polletta said: “We are in the process of analyzing more observations on this and other Planck protoclusters with the goal of tracing the gas that gives birth to these newly forming stars and feeds the supermassive black holes, to determine its origin and explain the observed extraordinary activity.”
Frye expressed excitement about merging data from the Large Binocular Telescope with views from NASA’s James Webb Space Telescope, which is scheduled to launch in December.
“Protoclusters offer an opportunity to investigate key questions in astronomy that only this new observatory can answer,” she said, “such as what mechanisms drive the prodigious star formation, and when will the hydrogen supply run out, forcing this galaxy shipyard to close its doors and turn into a supercluster similar to the one our Milky Way is in?”
