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According to Wired, IT Home reported on November 9 that some galaxies seem to have all the conditions needed to form stars, yet have barely ever produced any.
Scientists have been puzzled by this phenomenon for years. Recent research suggests that these objects, known as “dark galaxies,” may be widespread throughout the universe. Dark galaxies contain gas and dark matter but have few stars or none at all. Finding them could help astronomers understand why some galaxies form stars while others remain dark, and test whether existing theories of galaxy formation are accurate.
The currently most widely accepted model of the universe is called the Lambda-CDM model. It posits 2 invisible components: dark energy, a mysterious property thought to drive the accelerating expansion of the universe; and cold dark matter, which physicists estimate makes up about 85% of all matter in the universe.
According to this model, concentrations of dark matter called dark matter halos provide the gravitational framework in which galaxies form and grow. Gas collects inside these halos, then gradually cools and collapses, eventually forming stars. Astronomers are still trying to understand why this process occurs in some dark matter halos but not others.
One possibility is that some galaxies contain large amounts of hydrogen gas but have formed almost no stars. Such objects are therefore extremely difficult to observe.
Researchers have now identified a promising dark galaxy candidate and published their findings in Monthly Notices of the Royal Astronomical Society. The object, called AGC 322753, contains hydrogen gas with a mass equivalent to about 2 billion suns, but currently appears to have no stars. The researchers note, however, that it is too massive to be likely to contain no stars at all. If confirmed, it would be an unusually massive starless galaxy.
Researchers at Australia’s Swinburne University of Technology found the object by comparing data from 2 existing radio observation projects. One set of observations was made by the Arecibo Observatory in Puerto Rico, and the other came from China’s 500-meter Aperture Spherical Radio Telescope (FAST). Both telescopes detected a massive cloud of hydrogen gas.
Other researchers have found several dark galaxy candidates over the past 10 years, but AGC 322753 has another distinctive feature: it may be rotating.
As IT Home understands it, hydrogen gas emits radio waves at a particular wavelength. When gas moves toward Earth, the signal’s wavelength becomes shorter; when it moves away, the wavelength becomes longer. The signal from AGC 322753 has 2 distinct peaks, suggesting that one side of the hydrogen cloud may be moving toward us while the other is moving away.
This could mean that the hydrogen gas has formed an orderly rotating structure, similar to the rotating disk of an ordinary galaxy, even though it appears never to have formed stars.
Marco Monaci, a researcher at Swinburne University of Technology and an author of the paper, said: “Theoretical models and simulations tell us that dark galaxies should not rotate, so this discovery is very interesting.”
If AGC 322753 really is a rotating disk of gas without stars, astronomers will have to explain how the gas could organize itself into such a regular structure without forming stars.
That is not the only mystery. Most models of galaxy formation predict that a cloud with such a vast reserve of hydrogen gas should form at least some stars, because its densest regions would collapse under their own gravity and eventually trigger star formation.
The paper’s coauthors consider it perhaps unlikely that AGC 322753 has no stars at all. They therefore favor another explanation: the object may contain some extremely faint stars that are difficult to observe, making it an “almost dark galaxy.”
If so, those stars must be extremely faint. The researchers searched deep optical images from the DESI Legacy Imaging Surveys project but still found no light from any stars. Unlike the hydrogen gas detected by radio telescopes, this survey primarily looks for visible light emitted by stars.
Based on current observations, the team estimates that the mass of hydrogen gas in AGC 322753 is at least 40 times the mass of its stars. This raises a key question: what has suppressed star formation?
The researchers say more observations are needed to confirm whether AGC 322753 really is a dark or almost dark galaxy. The Very Large Array (VLA), a collection of radio antennas in New Mexico, United States, could, for example, be used for further observations.
Robert Minchin of the National Radio Astronomy Observatory said: “Right now, learning more about this object really is the crucial next step. It is fascinating, and I want to know more.”
Minchin knows well that such observational signals can be misleading. He was involved in the discovery of another dark galaxy candidate, VIRGOHI21. It was discovered in the early 21st century, but researchers later reinterpreted the observations, concluding that it was actually an elongated stream of hydrogen gas pulled out of a nearby galaxy by gravity, known as a tidal tail.
Gas pulled out in this way may retain some of its original motion, producing a signal that resembles rotation. Minchin therefore believes researchers need more observations to rule out the possibility that AGC 322753 is such a case. So far, however, no obvious nearby galaxy has been found that could have supplied its gas.
Rhys Taylor, an astronomer at the Astronomical Institute of the Czech Academy of Sciences in Prague, thinks tidal debris is unlikely to explain the current observations. Reproducing this dark galaxy candidate’s distinctive double-peaked signal through such a process is, he said, “not technically impossible, but extremely unlikely.”
Taylor and Minchin both pointed to another issue: the data from the 2 radio surveys used by the researchers place the hydrogen cloud at markedly different positions in the sky. This means AGC 322753 may be a more extensive cloud of gas rather than a compact galaxy. Follow-up observations may help researchers resolve the issue.
Elisabeth Adams of the Planetary Science Institute said that whether AGC 322753 ultimately proves to be completely dark or merely hard to observe because its stars are extremely faint, studying such objects could help astronomers understand why some galaxies form stars while others apparently cannot.
One possible explanation is that some galaxies rotate so rapidly that their gas never gathers densely enough to collapse. Another possibility is that ultraviolet light from the universe’s earliest stars heated the gas, leaving it too hot to condense into stars.
Studying these almost dark galaxies may help scientists distinguish between those 2 possibilities and reveal more about how conditions in the early universe shaped the galaxies we see today.
Adams said: “Once an object is no longer classified as a dark galaxy, people seem to stop caring about it, and I don’t think that is right.”
If AGC 322753 is ultimately confirmed to have no stars at all, however, it will be among the most massive dark galaxies discovered to date and could help scientists explain where the “missing galaxies” predicted by the Lambda-CDM model, but never found, are hiding.
The discovery could even help astronomers understand why some gas-rich galaxies form stars while others do not. It could provide an important test of existing models of galaxy formation and offer new clues to the nature of dark matter.