a possible dark galaxy in the Virgo Cluster

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a possible dark galaxy in the Virgo Cluster

No Stars Shine in This Dark Galaxy

June 14th, 2007

An international team of astronomers have conclusive new evidence that a recently discovered "dark galaxy" is, in fact, an object the size of a galaxy, made entirely of dark matter. Although the object, named VIRGOHI21, has been observed since 2000, astronomers have been slowly ruling out every alternative explanation.
In a new research paper, entitled 21-cm […]

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Dark galaxy VIRGOHI21. Image credit: NAIC
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An international team of astronomers have conclusive new evidence that a recently discovered "dark galaxy" is, in fact, an object the size of a galaxy, made entirely of dark matter. Although the object, named VIRGOHI21, has been observed since 2000, astronomers have been slowly ruling out every alternative explanation.

In a new research paper, entitled 21-cm synthesis observations of VIRGOHI 21 – a possible dark galaxy in the Virgo Cluster, researchers provide updated evidence about this mysterious galaxy.

They have now performed a high resolution observations of VIRGOHI21 using the Westerbork Synthesis Radio Telescope (WSRT), to better pin down the quantities of neutral hydrogen gas. They also did followup observations with the Hubble Space Telescope, looking for any evidence of stars.

Astronomers first suspected there was an invisible galaxy out there when they spied galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision. All the normal evidence is there: gas is being siphoned away into a tenuous stream, and one of its spiral arms is being stretched out.

But the other partner in this collision is nowhere to be seen.

The researchers' calculated that an object with 100 billion solar masses must have careened past NGC 4254 within the last 100 million years, creating the gas stream, and tearing at one of its arms. This was the clue that an invisible dark matter galaxy might be lurking nearby.

A detailed search turned up a mysterious object called VIRGOHI21, located about 50 million light-years from Earth. Were it a normal galaxy, you would be able to see it in a powerful amateur telescope. But there's nothing there. Even in the Hubble Space Telescope, not a single star is shining from this massive region of space.

It was only visible in radio telescopes, which could detect the radio emissions from neutral hydrogen gas located in the cloud.

When they first published their research a few years ago, the astronomy community was understandably skeptical, and proposed several alternative theories to explain the mysterious object.

For example, there could be additional mass associated with VIRGOHI21, and not just dark matter. The discovery of red giant stars in the region would give some indication that this was a more normal interaction. But Hubble turned up nothing.

Dr. Robert Minchin, lead researcher from the Arecibo Observatory, said, "not even the power of Hubble has been able to see any stars in it."

It's possible that VIRGOHI21 has always been this way, formed from primordial dark matter and neutral hydrogen after the Big Bang. It's been cruising the Universe ever since, disrupting galaxies as it goes.

However, there do seem to be ways that galaxies and their dark matter can be separated. Only a few months ago, a ring of dark matter was found surrounding a group of colliding galaxy clusters by the Hubble Space Telescope. Perhaps VIRGOHI21 is the wreckage from one of these cluster collisions; a shred of dark matter hurled out into space.

It could be that there are many of these dark galaxies out there. A new sky survey, carried out with the 305-metre (1000-foot) Aricebo radio telescope in Puerto Rico should tease out more of these objects in the future. The survey is called the Arecibo Galaxy Environment Survey (AGES).

This most recent paper has been accepted for publication in the Astrophysical Journal.


Cpt_pineapple
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Dark matter has always

Dark matter has always eluded physicists, now that there is an entire galaxy of it out there could help us tell more about it.


deludedgod
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But Dark Matter doesnt

But Dark Matter doesnt cluster. How is this possible?

"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.

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Vastet
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Very interesting.

Very interesting.

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Wow! Just wow! Very

Wow! Just wow!

Very cool. 


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Update: 'Dark galaxy'

Update:

'Dark galaxy' continues to puzzle astronomers
15:19 20 June 2007
NewScientist.com news service
Stuart Clark

The Hubble Space Telescope has failed to reveal the expected number of stars in the mysterious, galaxy-sized cloud of hydrogen known as VIRGOHI21. The research bolsters the idea that the gas cloud is the only known example of a 'dark galaxy' that never kick-started star birth.

Galaxies are thought to coalesce from normal, or baryonic, matter that has collected in clouds of hypothetical dark matter. But surveys have turned up fewer galaxies than expected, suggesting that – for unknown reasons – some galaxies are stillborn, and simply fail to form stars.

The discovery of VIRGOHI21 in 2005 seemed to provide the first evidence that dark galaxies existed. However, a number of researchers suggested that VIRGOHI21 was pulled out of the nearby galaxy NGC 4254 when another galaxy called NGC 4262 shot past it at 900 kilometres per second. Indeed, NGC 4254 has a single prominent arm of stars that curls round towards VIRGOHI21, suggesting some sort of link between the two.

But Robert Minchin of the Arecibo Observatory discounts such "hit-and-run" models. "If the hydrogen in VIRGOHI21 had been pulled out of a nearby galaxy, the same interaction should have pulled out stars as well," says Minchin.

He and colleagues used Hubble to observe a patch of sky 50,000 by 50,000 light years across, centred on the hydrogen cloud's position. They found just 119 red giant stars. That is the number found in a typical region of the same size in intergalactic space and three times fewer than expected if the cloud were a large piece of celestial wreckage.

Heavyweight galaxy
Instead, Minchin believes NGC 4254's arm of stars was created by the gravity of VIRGOHI21 itself. He came to this conclusion after studying the object with the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands, which detects emission from atoms of hydrogen. Watch an animation showing VIRGOHI21 and its two galactic neighbours from a variety of perspectives.

The new, relatively high-resolution WSRT measurements suggest that VIRGOHI21 is indeed a single object, ruling out previous suggestions that its rotation was an illusion caused by two passing gas clouds.

But they do confirm a mystery raised by previous studies. The object's normal matter weighs a few hundred million times the mass of the Sun. But its dark matter – inferred by studying the rotation speed of the cloud – appears to weigh at least 100 times as much.

That ratio is much higher than expected – in all other galaxies, dark matter outweighs normal matter by a factor of only 10. "Even if this is a dark galaxy, it is not what you expect to find. The number of baryons is too low," says Michael Merrifield of the University of Nottingham in the UK, who was not on Minchin's team.

Minchin acknowledges that this is a puzzle. A number of surveys at Arecibo and other radio observatories aim to find more examples of dark galaxies, which could shed light on how much dark matter they contain. So far, however, the surveys are finding little to match the characteristics of VIRGOHI21.

http://space.newscientist.com/article/dn12100-dark-galaxy-continues-to-puzzle-astronomers.html

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Maybe that thing is God

Maybe that thing is God himself?Smiling