A brand new approach to search out chilly gasoline flows that might make up the lacking (regular) matter within the universe

Where is all the missing matter? This question has plagued astronomers for decades because, given current theories about its composition, the universe looks emptier than it should. Most of the universe (70%) appears to be made up of dark energy, the mysterious force that increases the rate of expansion of the universe. Another 25% of the universe is dark matter, an unknown substance that cannot be seen but was theorized to explain the otherwise inexplicable gravitational forces that control the formation of galaxies. So baryonic matter – all the normal “stuff” like you, me, trees, planets and stars – only makes up 5% of the universe. But when astronomers look at the sky, there doesn’t even seem to be enough normal matter to make 5%. Part of the normal business is missing!

Where is it? It’s probably out there, but it may be hard to see, maybe in the form of clouds of gas that don’t produce enough light to be easily seen. How do you search for almost invisible matter? The discovery of Fast Radio Bursts (FRB) in 2007 provided an answer. A research team led by J.-P. Macquart and JX Prochaska theorized that if you know the distance to the origin of an FRB, you can measure how much matter the radio burst passed through on its way to Earth. This is because radio waves slow down as they pass through baryonic matter and slow down longer wavelengths more than short ones. So if there were baryonic matter between the source of the FRB and the telescope on Earth, the shorter wavelengths would arrive first and the longer ones later.

Artist’s impression of a fast radio burst detected by the ASKAP (Australian Square Kilometer Array Pathfinder) radio telescope. Photo credit: ESO / M. Kornmesser.

Testing this theory would have to wait more than a decade to develop a telescope that can determine the point of origin of an FRB. This was the Australian Square Kilometer Array Pathfinder (ASKAP), which found its first FRB source in 2017. In 2020, Macquart, Prochaska, and their team were able to use ASKAP data to confirm that the FRBs were indeed passing through some previously invisible normals of matter – the missing matter was found!

But that’s not the end of the story. There were still many questions: what is this matter? What shape does it take? How does it affect the evolution of galaxies? In early 2021, PhD student Yuanming Wang and a team of researchers, including her supervisor Tara Murphy, published a paper that addressed some of these questions. They used a slightly different method to find the missing baryonic matter: the team observed the “twinkling” of galaxies with bright quasars in their centers. Just as stars twinkle because their light is disturbed as it passes through the Earth’s atmosphere, these galaxies appear to flash as their light passes through the gas that makes up the missing baryonic matter.

ASKAP is able to observe thousands of galaxies at the same time. By observing which galaxies were sparkling and which were not, Wang and the team could not only see the missing matter, but also get a rough estimate of their shape! It appears to be a gas cloud with a long, thin shape that extends in a straight line for long stretches.

Cold streams of gas, possibly stretched out by a passing star. Photo credit: Mark Myers, OzGrav / Swinburne University.

Long, narrow gas clouds like this one can form when a black hole withdraws gas from a star that dares to get too close. But without a black hole, Wang suggests that a cold “snow cloud” of hydrogen might have been pulled into this thin, whispery shape by a passing star.

The next step is to expand the scope of the study in the hopes of seeing more of these clouds, their shapes, and how they fit into the formation of the Milky Way. It’s an exciting time to be made out of baryonic matter – we’re just beginning to see the rest of the ‘things’ like ourselves.

References:

• Renyue Cen and Jeremiah P. Ostriker. “Where are the baryons?” The Astrophysical Journal (1999) 514 1.
• JP Macquart, JX Prochaska, M. McQuinn, et al. “A count of baryons in the universe based on localized high-speed radio bursts.” nature 581, 391-395 (2020).
• Yuanming Wang, Artem Tuntsov, Tara Murphy et al. “ASKAP observations of several fast scintillators show a straight-length plasma filament.” Monthly notices from the Royal Astronomical Society, (2021) stab139.
• Yuanming Wang and Tara Murphy: “5 sparkling galaxies help us uncover the mystery of the missing matter of the Milky Way.” The conversation.
• J.-P. Macquart and JX Prochaska: “Half of the matter in the universe was missing – we found it hidden in the cosmos.” The conversation.

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