A mysterious intergalactic force is drawing the Milky Way galaxy inward, according to a group of Australian astronomers.
Nobody knows what, or why, is causing the immense gravitational pull, but a swarm of hundreds of nearby galaxies may help astronomers reveal the identity of the ‘Great Attractor’.
This pack of galaxies has been spotted by astronomers using CSIRO’s Parkes Observatory in NSW. The international study involved researchers from Australia, South Africa, the US and the Netherlands, and was published today in Astronomical Journal.
Despite being ‘just next door’ in astronomical terms — a mere 250 million light years away — these galaxies have remained hidden from view because they are on the opposite side of our own.
The intensity of stars and dust crowded together along the plane of the Milky Way is directly in the line of sight — masking everything behind it from view.
That something must be there has been known for some time.
Its immense gravitational pull — the equivalent of a million billion Suns — has been observed through calculations of strange deviations in the flight path of nearby galaxies.
And our own.
In the absence of any indication as to what it may be, astronomers have simply dubbed it the ‘Great Attractor’.
Our Milky Way is just one of hundreds of thousands of local galaxies ensnared by its grasp.
And we’re hurtling towards the mysterious source of this attraction force at more than two million kilometres per hour.
“We don’t actually understand what’s causing this gravitational acceleration on the Milky Way or where it’s coming from,” says study lead author Professor Lister Staveley-Smith of the University of Western Australia.
“We know that in this region there are a few very large collections of galaxies we call clusters or superclusters, and our whole Milky Way is moving towards them.”
Essentially, all we know is that there is an immense — but probably diffuse — concentration of mass lurking some 250 million light years away.
Is it a monster-black hole? Or something else?
“Some astronomers think the Great Attractor is a super-supercluster of galaxies; some astronomers think that some regions of the universe are “darker” than others,” Professor Staveley-Smith says, referring to densities of the invisible source of gravity dubbed ‘Dark Matter’.
“Some physicists are even considering the possibility that the mass fluctuations in the universe are so significant that astronomers may be fundamentally misinterpreting the relationship between gravity and motion.”
It all remains speculation.
But observing and understanding the distribution and behaviour of the new galaxies may uncover vital clues.
“The ‘Great Attractor’ lies at the intersection of several large-scale filaments of galaxies,” says Dr Barbel Koribalski of CSIRO Astronomy and Space Science. “One could picture a giant hoover with galaxies near and far slowly streaming towards it. We can’t see much of this hoover, but we can measure the motion of the galaxies.”
What is doing the hoovering is the issue.
The Parkes radio telescope is a 64-metre dish that was activated in 1961. It was more recently modified with an innovative receiver, allowing the international team of scientists to peer past the ‘interference’ of our galactic core into unexplored space.
“The Milky Way is very beautiful of course and it’s very interesting to study our own galaxy but it completely blocks out the view of the more distant galaxies behind it,” says Professor Staveley-Smith.
Not so completely anymore.
What the survey revealed was a field of 883 galaxies, a third of which had not previously been suspected says Professor Staveley-Smith.
University of Cape Town astronomer Professor Renée Kraan-Korteweg — also part of the research team — said astronomers have been trying to map the galaxies hidden behind the Milky Way for decades.
“We’ve used a range of techniques but only radio observations have really succeeded in allowing us to see through the thickest foreground layer of dust and stars,” she said.
“An average galaxy contains 100 billion stars, so finding hundreds of new galaxies hidden behind the Milky Way points to a lot of mass we didn’t know about until now.”
So what caused this odd accumulation of galaxies?
That bit remains the problem.
Dr Koribalski says innovative technologies on the Parkes radio telescope had made it possible to survey large areas of the sky quickly. And things are about to get much, much better.
“Detecting galaxies behind the Milky Way (in the so-called Zone of Avoidance) and measuring their motions is important to pinpoint its location and total mass,” she says. “The Parkes multibeam system made this possible. With this receiver we’re able to map the sky 13 times faster than we could before and make new discoveries at a much greater rate.”
Copies of this receiver have been purchased from CSIRO by United States and Chinese astronomers to upgrade their own radio telescopes.
But this receiver is being temporarily removed from the Parkes telescope this week. A new Phased-Array Feed (PAF) is being attached for testing.
— Baerbel Koribalski (@HIgalaxies) February 5, 2016
“The PAF is a huge technological advance, a breakthrough of major proportion that will be able to do fast and sensitivities surveys of the sky, (and is) bound to make many new discoveries,” Dr Koribalski says. “How to learn more about the Great Attractor? The answer is WALLABY — the upcoming Australian SKA Pathfinder (ASKAP) HI All Sky Survey — which is expected to spot more than 500,000 galaxies.”
This will offer much faster scanning of the skies than current equipment, combined with a twenty-fold increase in resolution.
Dr Koribalski says she expects the new scope will detect an additional 10,000 galaxies tucked away behind our own.
It may also, hopefully, paint a trail to the ‘Great Attractor’ itself.