5 mins read 15 Feb 2021

Playing Connect-the-Dots with Twinkling Galaxies

Ph.D. Student Yuanming Wang from Sydney University used data from ASKAP to reveal an apparent alignment of distant twinkling galaxies, revealing a long, thin, previously unseen cloud of hydrogen gas located 10 light-years away within the Milky Way Galaxy - which is relatively close in astronomical terms.

An artist’s impression of a gas cloud being disrupted by a nearby star. Credit: OzGrav.

Did you know that we don’t know where all the matter is in the Universe? Some matter is “missing”, and is hard to find. Baryonic matter - that is, the ordinary matter that makes up people, animals, planets and stars - makes up 5 percent of the universe. This is the stuff around made up of protons and neutrons.

But, when astronomers try to account for all of this baryonic matter, their results fall short of their predictions. So where is this missing matter?

To put it plainly, this matter is too dark to see. Not to be confused with Dark Matter, the missing Baryonic matter is very dim, both visibly and on the radio spectrum. Yuanming Wang, a PhD student at the University of Sydney, has utilised a new method to be able to find this difficult-to-spot matter.

“We suspect that much of the ‘missing’ baryonic matter is in the form of cold gas clouds either in galaxies or between galaxies. This gas is undetectable using conventional methods, as it emits no visible light of its own and is just too cold for detection via radio astronomy,” said Ms Wang.

As a quick science check what is occurring here - most objects across the Universe emit electromagnetic radiation, and the rule of thumb is that the hotter the object, the more energetic its radiation will be. 

Our Sun for example, releases a wide range of EM radiation, from radio through to x-rays. The planets, which don’t produce their own energy through fusion, also radiate out into space, but in lower frequencies, like infrared, microwaves and radio waves. 

There’s even the big clouds of cold hydrogen gas that forms the stars - which radiate at 1.420 GHz frequency (21cm radio waves), produced by the result of electron’s jumping from an energetic state to a rest state. 

But the stuff that Ms Wang’s new paper is about, is even colder than this (it’s almost at absolute zero), and so she has created a breakthrough method of seeing these dark clouds by looking at the bright galaxies behind them.  

Twinkling Galaxies

The positions of the galaxies Yuanming Wang observed. The colourful stars denote the galaxies that showed variation. Credit: Yuanming Wang.

Just like the stars in the night sky that twinkle due to their light passing through our atmosphere - known as scintillation - galaxies can also twinkle. This twinkling can be seen over different lengths of time - even across years! 

This can be caused by either factors within the galaxies themselves, or by the electromagnetic waves from the galaxies travelling through different materials on their way to our telescopes and observatories. 

Ms Wang and her colleagues used data from the CSIRO Australian Square Kilometre Array Pathfinder (ASKAP), located at the Murchison Radio-astronomy Observatory in Western Australia to see the variations in radio waves that were coming from galaxies, and crossing through the any mediums between us and them. Remarkably, across the span of several hours to days, she observed a twinkling of six galaxies, with five of these galaxies forming a line from our perspective and position within the Milky Way Galaxy. 

“We found five twinkling radio sources on a giant line in the sky. Our analysis shows their light must have passed through the same cold clump of gas,” Ms Wang said. 

Snow in space

Those five twinkling galaxies that Ms Wang found in a line show that the radio waves from those galaxies were all travelling through something before they reached us - a cloud in a long narrow shape, located within the Milky Way itself. 

“We aren’t quite sure what the strange cloud is, but one possibility is that it could be a hydrogen ‘snow cloud’ disrupted by a nearby star to form a long, thin clump of gas,” Dr Artem Tuntsov, a co-author of Ms Wang’s from Manly Astrophysics, said. 

A “snow cloud” is the result of hydrogen freezing solid at a chilling minus 260 degrees celsius. These snow clouds are one of the theorised hiding spots of the missing baryonic matter, and are next to impossible to directly detect. 

“However, we have now developed a method to identify such clumps of ‘invisible’ cold gas using background galaxies as pins,” Ms Wang said.

The clever technique involves monitoring of radio light from different galaxies and catching the twinkling ones at just about the right time. To do this, ASKAP’s capabilities are perfectly suited - with its ability to observe a large portion of the sky at any one time. 

ASKAP’s field of view covers a portion of the sky roughly as big as the Southern Cross constellation (about 68 square-degrees), and in this study, a total of 30,000 galaxies were captured during seven observations of the same patch. 

Ms Wang’s discovery adds to a growing toolkit of methods for astronomers in their hunt for the Universe’s missing baryonic matter.

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site.

Read the paper, now available from the journal Monthly Notices of the Royal Astronomical Society