Mysterious Blobs In Our Milky Way Could Be Part Of The Missing Matter

A plethora of missing matter problems besieges astronomy. Most famously, dark matter, which was postulated to address the problem of why galaxies spin so fast( among other things ), is thought to comprise some 23% of the mass-energy of the universe.

It is yet to be detected by direct means and remains one of the most significant puzzles in modern astronomy and physics.

Even putting aside dark matter, astronomers are still unable to account for approximately 5% of the universe thought to be made of normal matter, known as baryons. This question, known as the missing baryon problem is of particular interest here.

Where is all the stuff?

Well, we may have found at least some of those baryons, with the details on how published in Science today. But first, a little bit of history.

A Wine Glass In The Sky

In 1987, astronomers received the radio wave from a quasar that was normally reasonably constant, varying wildly for a period of a few months. They immediately concluded that the cause of the difference was not the quasar itself, but refraction through a blob of plasma in our own Milky Way.

You can imagine the blob as a wine glass, sitting on a table during a sunny day. The lighting from the sun shines through the glass, and makes a pattern on the table.

Note the pattern of sunlight through the wine glass. Flickr/ John Melancon, CC BY-NC-SA

That pattern is the illumination from the sunlight being focused and defocused by refraction through the wine glass. As you move though the specific characteristics, you ensure the sunshine as brighter or fainter, depending on whether youre closer, or further from the focus of the glass.

Thats exactly what we have with these blobs, but scaled up to cosmic scales. The radio wave from the quasar glisten through the blob, which act as an interstellar lens and which then makes a pattern in space. As the Earth moves through the pattern, the quasar appears brighter or fainter.

What Shape Are The Blobs ?

Now heres where it gets interesting. If one assumes the lenses are spherical balls of plasma( think meatball ), one can calculate that the cloud is so highly over-pressured, it is appropriate to quickly disperse.

How, then, are the clouds held together long enough for us to find them? A sheet of plasma( like lasagne) viewed edge-on solves this problem, by spreading the plasma out over a large distance.

Another, intriguing proposal, is that the lenses are cold clouds surrounded by a shell of plasma( like a hazelnut ). If these cold clouds are held together by their own gravitation, then they must be pretty massive. If this is the case, then these cold cloud could represent a substantial fraction of the missing baryons in our galaxy.

A New Technique

Before our run, merely a handful of events had been discovered with old telescopes, and largely on an ad-hoc basis using archival data obtained for other purposes. The traditional approach, called the sun curve method, looked at the brightness of a few hundred quasars roughly every day at one or two different radio frequencies.

Its helpful to think of these old telescopes like your automobile radio: you can listen to only one radio station at a time. If you want to listen to another radio station, you have to turn the dial. If you want to find a plasma lens, you have to come back to that quasar every day, to see if its changed.

But, theres a better style. Lets get back to the wine glass. If you seem closely at the pattern on the table, youll see that the white illuminate from the sunshine is also split into different colours by the glass: which means that some parts of the specific characteristics will have a redder tinge, and some a bluer tinge.

Note the differing colouring from the lighting through the wineglass. Flickr/ anataman, CC BY-NC-ND

If were in a certain part of the pattern, instead of watching white sun coming from the quasar, we watch some colours more strongly focused than others. All we needed was a telescope that could tune not to a single radio station at a time, but all the radio stations at the same time.

Fortunately, we have this wonderful machine: the Australia Telescope Compact Array, at Narrabri in rural New South Wales. With the the Compact Array, we dont get only two channels for every measurement, we get 9,000 channels.

Excited Observations

When we received our event, “were in” super aroused. Not merely had our notion worked, but it felt like wed assured colour Tv for the first time, after living with black and white for more than 20 years!

We also knew the game was on and we had to act fast. So we pointed a few other telescopes to assure what we could see. First, we got the Gemini 8m telescope in Chile, and a 1. 3m telescope, also in Chile, to monitor the brightness of the quasar in optical light.

We didnt expect the plasma to do anything to optical illuminate, but we thought there might be dust in the lens that might tell us what its made of. We didnt ensure any dust, which is also interesting from the dark matter standpoint: previous surveys for Massive Compact Halo Objects( MACHOs) in the optical never considered any dusty lenses either.

The second pair of telescopes were radio telescopes, but spaced all over Australia, and the US. These telescopes use a technique called Very Long Baseline Interferometry( VLBI) to stimulate the most detailed possible images of the sky( even better than the Hubble Space Telescope ).

We expected to see the quasar vacillating around( look at a light globe through a wine glass and youll ensure what I mean ), but we didnt see as much as wed hoped.

What does it all mean? Well, some clever modelling of my fellow members Artem Tuntsov and Mark Walker, at Manly Astrophysics, showed that the lens was either flat, like a sheet of lasagne, or hollow, like a hazelnut or noodle. It definitely wasnt a bent sheet, and definitely not filled in the middle, like a meatball.

But is it the missing baryons? Well, its too early to tell. Pinning down the geometry using the VLBI is the Holy Grail right now.

The future of this work lies in CSIROs Australian Square Kilometre Array Pathfinder( ASKAP ), which will see tens of lenses per year. With those lenses, well be busy measuring shapes and chemical compositions. A large number of detectings will yield the first in-depth survey of the distribution of lenses over the sky.

Such measurements represent a huge leap in our understanding these lenses, a better understanding of the gas conditions in the interstellar medium, and maybe even a solution to the missing baryon problem.

Keith Bannister, Astronomer, CSIRO

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