Astronomers detect missing 'normal' matter in the Universe using ASKAP

A research paper published in the journal Nature today describes how ASKAP’s ability to localise fast radio bursts was key to finding the “missing” baryonic matter that has puzzled astronomers for decades.

Measurements of the cosmic microwave background and our understanding of the way that baryonic matter (the stuff that makes up stars, planets, humans, everything we are familiar with) formed after the Big Bang predict how much of this material we should find in the present-day Universe. However, the total amount of matter we can see in galaxies has always fallen short of expectations.

Astronomers knew this “missing matter” should exist somewhere in the Universe and today they have directly measured its presence for the first time.

Ionised matter (existing as dissociated charged subatomic particles) is known to exist in small quantities in the space between stars inside the Milky Way but measuring how much exists in the vast space between galaxies is much more challenging. The key to the puzzle was Fast Radio Bursts – short, millisecond-duration pulses of radio energy with a broad spectrum. Discovered in 2007, these bursts are known to originate outside the Milky Way due to the large amount of dispersion they experience. Dispersion arises when different wavelengths of light travel at different speeds through an ionised medium. By measuring the arrival time of a burst at different radio frequencies, we can determine how much matter it has passed through on its journey to Earth.

Knowing the amount of matter an FRB has passed through does not help unless you know its point of origin, and this is where ASKAP’s contribution was crucial. Using a specially designed transient data buffer to capture and replay FRB signals, a team of researchers were able to pinpoint the exact location of several FRBs. This information was shared with the world’s largest optical telescopes, which were used to image the galaxies hosting these FRBs. Optical spectroscopy provides a distance to the host galaxy, which allowed the team to measure the average density of matter encountered by each of the FRBs observed.

The team found that there is enough matter between galaxies to account for the missing fraction, balancing the matter budget for the first time.

Although this is a ground-breaking result, many questions remain. Is the missing matter spread out evenly as a very thin gas, or does it group up around galaxies and clusters? Every FRB localised with ASKAP adds a new piece of information.