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The dark side of galactic radio jets

Last Updated on Thursday, 09 July 2015 12:06
Published on Tuesday, 07 July 2015 16:17


Cosmic microwave radiation points to invisible ‘dark matter’, marking the spot where jets of material travel at near light speed, according to an international team of astronomers. Lead author Rupert Allison of Oxford University presented their results yesterday (6 July) at the National Astronomy Meeting in Venue Cymru, Llandudno, Wales.


Active galaxy, Hercules A, showing extensive radio jets (Image credit: NRAO)Active galaxy, Hercules A, showing extensive radio jets (Image credit: NRAO). Click for a full-size image

Currently, no one knows for sure what dark matter is made of, but it accounts for about 26% of the energy content of the Universe, with massive galaxies forming in dense regions of dark matter. Although invisible, dark matter shows up through its gravitational effect – a big blob of dark matter pulls in normal matter (like electrons, protons and neutrons) through its own gravity, eventually packing together to create stars and entire galaxies.


Many of the largest of these are ‘active’ galaxies with supermassive black holes in their cores. Some of the gas falling towards the black holes is ejected out as jets of particles and radiation. Observations made with radio telescopes show that these jets often stretch for millions of light years from their host galaxy – far larger in extent than the galaxy itself.


Scientists therefore expected that the jets would live in regions where there was an excess, higher-than-average concentration of dark matter. But since dark matter is invisible, testing this idea is not straightforward.


Einstein’s general theory of relativity describes how light feels the effect of gravitational fields, giving away the presence of dark matter through an effect known as ‘gravitational lensing’. Observing how dark matter distorts light allows astronomers to deduce its location and measure its mass.


The Universe also has an ideal reference map – the Cosmic Microwave Background (CMB)  covering the entire sky. This is a relic of the formation of the cosmos, and is a ‘snapshot’ of the universe as it was just 400,000 years after the Big BangThe light from this epoch has taken more than 13 billion years to reach us.

Sample CMB lensing map (top) and radio overdensity map (bottom)Sample CMB lensing map (top) and radio overdensity map (bottom). Click for a full-size image 

Light coming from this very early time travels through most of the universe unimpeded. The lumpy dark matter, however, exerts a small gravitational tug on the light, deflecting it slightly from a straight-line path, rather like a lens does in a pair of glasses.


By analysing subtle distortions in the CMB, the team of Mr Allison, Dr Sam Lindsay (Oxford) and Dr Blake Sherwin (UC Berkeley) were able to locate dense regions of dark matter. As suspected, this is where the powerful radio jets are more common – a deep-lying correlation between the most massive galaxies today and the afterglow of the Big Bang.


Mr Allison commented: “Without dark matter, big galaxies wouldn’t have formed and supermassive black holes wouldn’t exist. And without black holes, we wouldn’t see intergalactic jets. So we have found another signature of how dark matter shapes today’s universe.”


The scientists now hope to use new instruments to improve their measurements and more clearly understand how radio jets and their host galaxies change over the history of the Universe. Future telescopes such as Advanced ACTPol and the Square Kilometre Array will provide the complementary data to make this hope a reality. 


Media contacts


Dr Robert Massey
Royal Astronomical Society
Mob: +44 (0)794 124 8035
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Ms Anita Heward
Royal Astronomical Society
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Dr Sam Lindsay
Royal Astronomical Society
Mob: +44 (0) 7957 566 861
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Science contacts


Mr Rupert Allison
University of Oxford
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Further information


Original scientific publication:


The researchers are part of a collaboration of scientists working on the Atacama Cosmology Telescope high in the Atacama Desert in Northern Chile (). They measured the lensing effect of dark matter on the Cosmic Microwave Background, and compared this to the positions of radio jets found using the Very Large Array radio telescope in New Mexico, USA.



Notes for editors


The Royal Astronomical Society National Astronomy Meeting (NAM 2015) will take place in Llandudno, Wales, from 5-9 July. NAM 2015 will be held in conjunction with the annual meetings of the UK Solar Physics (UKSP) and Magnetosphere Ionosphere Solar-Terrestrial physics (MIST) groups. The conference is principally sponsored by the Royal Astronomical Society (RAS) and the Science and Technology Facilities Council (STFC). Follow the conference on Twitter


The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognises outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter


The Science and Technology Facilities Council (STFC) is keeping the UK at the forefront of international science and tackling some of the most significant challenges facing society such as meeting our future energy needs, monitoring and understanding climate change, and global security. The Council has a broad science portfolio and works with the academic and industrial communities to share its expertise in materials science, space and ground-based astronomy technologies, laser science, microelectronics, wafer scale manufacturing, particle and nuclear physics, alternative energy production, radio communications and radar. It enables UK researchers to access leading international science facilities for example in the area of astronomy, the European Southern Observatory. Follow STFC on Twitter