Friday, 25 February 2005

Dark Matters

From the BBC via the ABC :
An international team of astronomers says it has discovered an object that appears to be a galaxy without stars.

The galaxy seems to be an astronomical case of arrested development.

Galaxies are usually vast cosmic islands of stars, with the milky way galaxy home to around 100 billion stars.

But the newly discovered galaxy, in the constellation Virgo, seems to have none.

However it does have hydrogen gas, from which stars are made, and there is enough there to make a 100 million stars.

But for some reason stars never formed and astronomers using radio telescopes have instead found a giant gas cloud.

They have also worked out that for every tonne of hydrogen in the galaxy, there is another 1,000 tonnes of so-called dark matter whose identity is one of the great puzzles of modern astronomy.
What is Dark Matter? Well although the blog The Stupid Shall Be Punished says this :
I expect A. E. Brain will have a more substantive post on this later
He's really covered it in sufficient detail, yet remained concise and pithy. I can't subtantially improve on his article, with the URLs on the theoretical background, and I recommend you go there and read the whole thing. I'll wait.

One caution : from the New Scientist :
But far from answering all the questions, VIRGOHI21 is throwing up a number of new ones. One concerns its mass. While the newly discovered galaxy is certainly dark, it may not be the dwarf that astrophysicists were hoping for.

If galaxies were made up only of ordinary matter, their speed of rotation would tear them apart. The extra mass needed to provide the gravitational pull that holds them together is generally thought to come from what is called dark matter.

When Minchin's team measured the speed of rotation of the hydrogen gas in VIRGOHI21, they found that it would have to contain about one-tenth of the dark matter of the Milky Way. But if that is so, it should also have a hundred times as much hydrogen gas as they actually detected. Far from being a dwarf, VIRGOHI21 seems to be a giant in its own right.

Merrifield says that the shortfall in the observed amount of hydrogen may mean that what Minchin and his team have seen is not a dark galaxy after all. "Their story doesn't quite hang together, and I would speculate that they have been fooled by two passing hydrogen clouds." The difference in speed as one passes the other would give the illusion of rotation, he says.

But Minchin is sticking to his guns. "There are so few known hydrogen clouds that to find two together would be extremely unlikely." He thinks they may have underestimated the mass of hydrogen in the dark galaxy. If ultraviolet light from distant quasars were ionising a large proportion of the hydrogen atoms, the gas would be rendered invisible to radio telescopes.
The numbers don't add up: either we, for some unlikely and arcane reason, aren't detecting the amount of hydrogen that's actually there, or our understanding of the whole dark matter (pardon the pun) is fundamentally wrong, or it's an improbable cosmic coincidence. It's certain that our knowledge is incomplete, but by no means certain that it's that incomplete.

Until we find evidence of quaser-UV-induced ionisation (but how would we know?), or setup a really long baseline interferometer (which would give a much higher definition picture), I think this one will remain a puzzle. Considering the distances involved, it's not exactly likely we'll be going to the immediate vicinity any time soon to have a squizz at it "up close and personal".

And by "Really" Long, I don't mean merely a VLB (Very Long Baseline) one, I mean something rather larger. More like what some people are contemplating for optical work. Something with a baseline not in kilometres, or megametres, but Astronomical Units. It's doable with today's technology - and a few billion dollars.

Make it Radio-frequency rather than optical (so data rate and hence power requirement is low), and you can use a brace of quite small birds, each with an inflatable reflective dish antenna. Put them at 1 AU distance instead of hundreds (so you can get power with even a small solar cell array), and the cost might even be in the low hundreds of millions, including launch costs. Put them only as far as Geostationary Orbit, so you can piggy-back on a ComSat heading out that way, and the cost could be less than a hundred million for a 5-year lifespan, based on my experience with FedSat. That close, and we could use GPS for really accurate 3-D positioning too, making them even cheaper. Now they'd not be as spectacular as Hubble, but possibly just as useful for Radio Astronomers, and as a project, vastly cheaper and less risky. Nowhere near as good as a 200-AU baseline array, but dramatically better than anything we could ever build on Earth.

If only Australia hadn't abandoned its space programme again, this is the type of thing that we could and should be doing. Something that might even pay its way or (Gadzooks!) make a profit from on-selling the data.

Of course an array of such birds, with antennae pointed the other way, might make very good SigInt (Signals Intelligence - Eavesdroppers) military satellites too. So we may even be able to use off-the-shelf parts, assuming some of these Dark Stars and Black Projects already exist. And if not, well, maybe the Military might be interested in subsidising a few extra birds, for their own use.

Dark matters indeed.

1 comment:

Dr. Charles said...

i find astronomy fascinating. the dark matter conundrum to me hints that our current laws may need tweaking. i can't remember superstring theory long enough to know if this question is ever answered by that paradigm. a lot of words - all i meant was "cool"!

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