http://www.news.com.au/technology/science/dark-matter-galaxy-may-be-holding-four...Dark matter ‘galaxy’ may be holding four stars together beyond the edge of the Milky Way
5 HOURS AGO FEBRUARY 19, 2015 1:10PM
The universe it laughing at us ... This “smiley face”, recently snapped by the Hubble Space Telescope, demonstrates the effect of “gravitational lensing”, where light is bent by gravity. Source: NASA/ESO Source: Supplied
IT’s been there all along. We just didn’t know it. Astronomers believe an entire dark matter galaxy is lurking at the edge of our own.
The problem with dark matter is that it’s … dark.
Astrophysicists know it must be there: Calculations measuring the size of the universe and the influences of gravity don’t add up without it.
But where?
It’s a question of growing importance in observatories and laboratories around the world.
Boiled down, the idea is simple. We can see “ripples” in the hydrogen clouds which swirl about in the depths of space outside the Milky Way.
Like a disturbance in a pond, these “ripples” need something to cause them. Some kind of “pebble”.
But the ripples we see don’t all come paired with a visible source.
They have been caused by something unseen.
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“It’s called dark matter because it doesn’t shine, but it also doesn’t block light either making it the ultimate Easter egg hunt looking for blobs of dark matter around our galaxy that are completely invisible,” Swinburne University astrophysicist Dr Alan Duffy says.
The idea that these strange disturbances in nearby hydrogen clouds was a sign of a nearby dark matter “galaxy” was pitched as far back as 2009.
Now the idea is gaining momentum, with new evidence published in Astrophysical Journal Letters.
Something as big as a galaxy should be detectable.
Even if it is invisible.
‘Filaments’ of dark matter have been calculated to connect visible galaxies in a universe-spanning ‘web’. Source: Aquarius Project Source: Supplied
Simulations have predicted a region where such a cluster of dark matter should be, so the research team began a search …
What they found were four tightly clustered stars.
Four stars do not a galaxy make.
But there was something odd about their behaviour.
First, they were a type of star called a Cepheid variable: A star which varies in brightness as its outer atmosphere expands and contracts, like the lid on a boiling pot.
Second, it was insanely unlikely that four stars would be clustered so tightly together some 294,000 light years away — the middle of nowhere in intergalactic terms.
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“One issue at peering at distant parts of our galaxy is that dust and gas can block the view making the stars appear redder and fainter than they really are making us think that they are further away than in reality,” Dr Duffy says. “Yet these stars seem to be in a patch of sky that’s no dustier than usual making us confident that they really are far away in a separate object, and not just an extension of the Milky Way’s disk.”
Beyond the rim ... A close cluster of four stars in the void surrounding our galaxy suggests there is something there holding them in place. Source: Hubble Source: Supplied
For such stars to be together — and remain together — requires the hefty influence of a lot of nearby “stuff”.
Stuff that produces blobs of gravity which, if it isn’t caused by visible stars, must come from something else.
“You would never get them clumped together like that, it’s incredibly unlikely,” Dr Duffy says.
“That’s your clue — tantalising evidence of something else out there. The chances that these four rare Cepheid stars happen to be so close together is ridiculously small — 3.5 million to one against.“
“The stars are like flagposts letting us know that there is something bigger around them, holding the stars close together with the force of its gravity. The dark matter.”
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