Ancient cosmic rays mapped in close galactic neighbours
Australian-led team estimates star formation rate in Large and Small Magellanic Clouds. Alan Duffy reports.
Cosmic rays from exploding stars in Milky Way’s closest galactic neighbours have been mapped for the very first time by Australia’s Murchison Widefield Array (MWA) telescope.
The measurements also reveal the rate at which the galaxies are producing new stars.
An international team of astronomers, led by Bi-Qig For of the International Centre for Radio Astronomy Research (ICRAR), based in Western Australia, have used the MWA to create low-radio frequency maps of two nearby galaxies – the Large and Small Magellanic Clouds.
Although the energy of the light detected by the MWA is low, the sources of the radio emission are anything but, with hot ionised gas (or plasma) and highly-relativistic particles known as cosmic rays spiralling in magnetic fields.
In particular, the cosmic rays detected now have an ancient source. As ICRAR’s astrophysicist Lister Staveley-Smith explains: “The number of cosmic rays that are produced depends on the rate of formation of these massive stars millions of years ago.”
For those in the Southern Hemisphere, the Large and Small Magellanic Clouds are visible to the unaided eye at optical wavelengths as faint pale white ‘clouds’.
Although just a hundredth of the mass of the Milky Way, and nearly 200,000 light years away, the objects have been resolved in exquisite detail thanks to the MWA extended ‘arms’ of receivers that form a telescope effectively three kilometres across.
The MWA mapped the galaxies across a wide range of low-radio frequencies, stretching from 19.7 megahertz (below commercial FM radio stations) to a few times higher frequency than a home microwave oven, at 8.55 gigahertz. Within this range of “very low frequencies between 76 and 227 megahertz,” says Dr For, “we could estimate the number of new stars being formed in these galaxies”.
They did this by mapping pockets of ionised hydrogen light years across, known as HII regions, that surround star-forming areas in a galaxy. The more HII regions, the more stars forming.
With this probe For “found that the rate of star formation in the Large Magellanic Cloud is roughly equivalent to one new star the mass of our sun being produced every 10 years,” while “in the Small Magellanic Cloud, the rate of star formation is roughly equivalent to one new star the mass of our sun every 40 years.”
This groundbreaking study was made possible thanks to the MWA’s high resolution with its extended arms, as well as its exquisite sensitivity to faint radio signals, by adding up 512 large ‘tiles’ of arrays.
The site in Western Australia will soon house the next-generation Square Kilometre Array (SKA).
“With the SKA the baselines are eight times longer again, so we’ll be able to do so much better,” Staveley-Smith says.
The research is published in The Monthly Notices of the Royal Astronomical Society.