By Stuart Ryder
[email protected]
(This blog post is dedicated to the memory of our dear friend and colleague, A/Prof Jean-Pierre (“J-P”) Macquart, who tragically passed away shortly after the publication of his Nature paper.)
The origin of Fast Radio Bursts remains a mystery, and yet that hasn’t stopped us using them as powerful cosmological probes. In a recent paper in the prestigious journal Nature, a team led by A/Prof. Jean-Pierre Macquart from Curtin University and the International Centre for Radio Astronomy Research (ICRAR) has used FRBs to reveal the previously-missing baryons between galaxies.
Every day, at least 1,000 brief pulses of radio emission called Fast Radio Bursts (FRBs) occur somewhere in the Universe, and yet before 2007 no-one had ever noticed them. This was due to several reasons, including:
Complicating matters further, the pulse from an FRB arrives at a later times at lower frequencies due to a phenomenon known as dispersion; the more ionised particles there are between us and the FRB, the larger this effect becomes. We don’t know in advance how much dispersion a non-repeating FRB will experience, so detecting one in “real-time” through a continual sweep of likely dispersion ranges requires serious computing power. Thanks to telescopes like Parkes, UTMOST, ASKAP and CHIME we now know of hundreds of FRBs.
One of the other great mysteries in astronomy has been the “missing baryon problem”. Observations of the cosmic microwave background left over from the Big Bang allow cosmologists to get a fairly good estimate of the fraction of the Universe made up of baryons, i.e. everything other than dark energy and dark matter. But when they conducted a “census” of all the baryons that they could detect, they found barely half of the expected 5%. They hypothesised that the “missing” baryons most likely exist in a very hot, diffuse and ionised state between galaxies, making them next to impossible to detect in traditional ways.
It wasn’t long after the discovery of the first FRBs that scientists including Jean-Pierre Macquart realised that the very property that makes FRBs so hard to detect, namely the amount of dispersion induced by their passage through any ionised material, would make FRBs an ideal tracer of these missing baryons. But an FRB detection by itself tells us only how many baryons in total the burst passed through – to convert this to an actual density, one needs to know how far the burst has traveled. And for that you’d need to firstly identify the galaxy it originated from, and then measure its distance via its redshift and the Hubble constant. But until 2 years ago hardly any FRBs had been traced back to their host galaxies. This all changed on 24 Sep 2018, when the Australian-led Commensal Real-time ASKAP Fast Transients (CRAFT) survey team successfully used the ASKAP array in Western Australia to pin down the location of an FRB from just a single pulse to an accuracy of less than 1 arcsecond.
Macquart and the CRAFT team used mainly the ESO VLT, as well as the Keck, Gemini, and Magellan telescopes to measure the redshifts for the host galaxies of the first half-dozen FRBs localised with ASKAP. When they plotted the FRB total dispersion against redshift they got the image below:
