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Mysteries of the Milky Way, New Scientist, March 28, 2012


Take 10 billion tonnes of antimatter and 10 billion tonnes of matter, and stir. Our galaxy mixes up one of these explosive cocktails every second, resulting in a warm inner glow of gamma radiation. Each photon created in this process carries an energy exactly equivalent to the annihilated mass of an electron and its antimatter counterpart, a positron. But what could be pumping out so many positrons?
Most galactic radiation - including visible light, ultraviolet, infrared, X-rays, radio waves and gamma rays of other energies - comes predominantly from the Milky Way's flat disc, where brilliant, short-lived new stars are formed. But the annihilation gamma rays come mainly from the galaxy's much smaller, bulging centre. "It is a unique case," says Nikos Prantzos of the Institute of Astrophysics in Paris, France.
There are some odd suggestions for this odd origin. The positrons could be created by decaying dark matter or blobs of exotic quantum matter called Q-balls; or they could be spat out by microscopic black holes or a tangle of cosmic strings, snags in the structure of space-time. Or it might be something more familiar. Supernova explosions in the Milky Way's disc create radioactive isotopes that emit positrons as they decay, and neutron stars and black holes can make antimatter when they feast on material from a sibling star. Some of these objects also inhabit the galactic bulge, although only enough to account for a fraction of the gamma-ray emission seen there.
The picture changes if the galaxy's magnetic field can funnel positrons from the disc into the central bulge. That depends on the field's basic shape, something we could learn from observations of how radio waves from distant sources are polarised. It also depends on whether positrons can travel tens of thousands of light years before being annihilated. That will be much harder to work out, because it depends on the small-scale details of magnetic fields and interstellar gas beyond the power of our telescopes.
If positrons can travel so far, another possibility opens up. "They could come from an event that happened long ago in the central black hole," says Prantzos. Starved of fuel, our galaxy's black hole is currently quiet (see "Milky Way mysteries: Five oddities of our galaxy"). Millions of years ago, though, it might have flared up, pumping out positrons that have since pushed out through the central bulge, creating a spherical halo of annihilation. Without a more detailed picture of the gamma-ray emissions, for now the truth remains out there.
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Stephen Battersby is a consultant for New Scientist

Milky Way mysteries: Our galaxy's shape

Step out into a clear night, far from the city lights, and you'll see a creamy streak of stars splashed across the sky: the "Milky Way" that has come to stand for our island universe. We see it as we stare through the flat, star-dense disc of our galaxy where we are also quartered. But what does our home look like from outside?
The short answer is we're not sure. Our telescopes unveil other galaxies in majestic detail, but introspection is much trickier. We think we live in a spiral galaxy of the sort we see scattered throughout the cosmos, but our lowly viewpoint in the galactic disc means we struggle to trace how its arms are furled, or even count how many there are.
Interstellar dust doesn't help: it blocks our view over distances of more than a few thousand light years, so we cannot map out distant spiral arms by their stars. We instead trace clouds of hydrogen atoms, which emit radio waves with a characteristic wavelength of about 21 centimetres. This long-wave radiation penetrates the dust, and by measuring the change in its wavelength - its Doppler shift - we can work out a cloud's speed towards or away from us. Comparing that with the ways in which different parts of the galaxy should rotate allows us to pinpoint a cloud's distance.
The resulting tentative maps suggest that the galaxy is a complicated, messy, many-armed spiral (see diagram). But even that sketch is arguable. For a start, the galaxy's rotation is not precisely known, and individual clouds need not follow the average motion; different models produce different maps. And when we look towards or away from the galactic centre, where the clouds are moving almost sideways relative to us, the Doppler method is no help in determining their distance. "Arms can only be identified in segments," says galaxy mapper Robert Benjamin of the University of Wisconsin-Whitewater. "The task of piecing them together is left to the discretion of the astronomer."
A parallel mapping effort, which suffers from similar limitations, uses radio emission from carbon monoxide gas that hangs around parts of the galaxy. Since 2008, this method has revealed more details of the galaxy's structure, including what seem to be new arm segments. Better landmarks may be interstellar clouds where molecules of water or methanol act as lasers, amplifying a narrow line of microwave emission to produce bright pinpoints. These "masers" are so well localised that we can see how their position shifts as Earth orbits the sun, and thus triangulate their distance from us precisely. There are too few of them to map out the galaxy on their own, but they can be used to test the results of other methods. Maser range-finding could finally reveal the true face of the Milky Way.
Stephen Battersby is a consultant for New Scientist

Milky Way mysteries: Five oddities of our galaxy

V838 Monocerotis

In February 2002, this previously undistinguished star about 20,000 light years away briefly achieved a luminosity a million times that of our sun. The following month it happened again. And in April. It was first assumed to be a nova - a white dwarf that pulls gas off a companion until it triggers a thermonuclear explosion on its surface. But novas don't happen three times in quick succession and then go quiet.
Was it a rarely seen flare-up near the end of a giant star's life? The scream oftwo stars colliding? Or did one star swallow three giant planets? What is certain is that the triple burst of light was reflected off nearby dust to surround the object with rapidly changing light shells, making it a true cosmic beauty.

Blue stragglers

The Milky Way's dense globular clusters are spherical swarms of red, lightweight and ancient stars, most of them more than 10 billion years old. A few globular-cluster stars, however, shine in blue-white light - suggesting something anomalously hot, young and bright.
We now think these "blue stragglers" are just as old as their companions, but have somehow been rejuvenated. Some may have sucked gas from a neighbouring star, compressing their central nuclear engine to make them burn faster and hotter. Others may be the offspring of stellar mergers - two cool red stars fusing to make one hot blue one.

Sagittarius A*

Sagittarius A* is a source of radio waves at the Milky Way's centre, thought to hold a huge black hole four million times the sun's mass. In some galaxies, such a black hole would be a fearsome source of radiation, blazing in light and X-rays as it feasted on nearby gas.
Not so in our galaxy. That is partly because Sagittarius A* has a much scantier supply of gas, but even so it is faint, and seems unusually inefficient at converting gas into heat and light. Some clues as to why might come next year, when a nearby gas cloud looks set to plunge into our listless giant's maw.

S2

S2 is a fast, intense, blue-white star that frankly has some explaining to do. It orbits within a whisker of the galaxy's central black hole, Sagittarius A* (see "Milky Way mysteries: Where are all the supernovae?"), swinging by at a speed of up to 5000 kilometres per secondMovie Camera, or nearly 2 per cent of the speed of light.
At this distance, the black hole's gravity should shred gas clouds before they can condense into new stars. And although a star might migrate inwards from more tranquil breeding grounds, S2 is a bright young thing no more than about 10 million years old, whose lifetime seems too brief for such a trek.

SDSS J102915+172927

Most stars today contain a moderate hoard of heavy elements inherited from earlier stellar generations. Not so this one, more than 4000 light years away.Discovered last year, it is an almost pristine blend of hydrogen and helium, with just 0.00007 per cent other stuff.
That is similar to the primordial matter emerging from the big bang. Such pure gas, lacking the carbon and oxygen that normally help clouds to cool and condense, was thought to form only colossal, short-lived stars. No one knows how this anomalous object managed to form - perhaps it was a fragment spun off during the birth of a supergiant star, back in the dark ages of the universe.
Stephen Battersby is a consultant for New Scientist

Milky Way mysteries: Andromeda, our sibling rival

The Milky Way and Andromeda are siblings: two great spirals that dominate our local group of galaxies. They have about the same total mass, and we used to think they were near-twins.
Not any more. "As we look in more detail, we see that they are quite different," says Alan McConnachie of the Herzberg Institute of Astrophysics in Victoria, Canada. Andromeda is the favourite child. It is brighter, with a wider disc of stars. The black hole at its heart is more than a hundred times as massive as ours. And while our galaxy is strewn with about 150 of the bright galactic baubles known as globular clusters, Andromeda boasts more than 400.
One might ask, a little plaintively, whether Andromeda is an exceptional specimen of galaxyhood. It seems not. In 2007, François Hammer and his colleagues at the Paris Observatory in France compared Andromeda and our galaxy with a sample of more distant galaxies. They found that whereas Andromeda is a pretty well-adjusted spiral, the Milky Way is an oddball -dimmer and quieter than all but a few per cent of its peers.
That is probably because typical spirals such as Andromeda are transformed by collisions with other galaxies over their lifetimes. These violent events shake up the galaxy's gas to form new stars and globular clusters, stir up the disc so it spreads farther out, and perhaps send some gas and stars plunging into the galactic heart to feed a more monstrous central black hole.
If this is to explain our sibling inequality, then the Milky Way must have lived relatively undisturbed. Except for encounters with a few little galaxies such as the Sagittarius dwarf, which the Milky Way is slowly devouring, we wouldn't have seen much action for 10 billion years.
Perhaps that is why we are here to note the difference. More disturbed spirals would have suffered more supernova explosions (see "Milky Way mysteries: Where are all the supernovae?") and other upheavals, possibly making the Milky Way's rare serenity especially hospitable for complex life. "We are still a long way from being able to answer that," says McConnachie, "but it's not a crazy suggestion."
Stephen Battersby is a consultant for New Scientist

Milky Way mysteries: Fast-moving clouds defy gravity

Among the many faint companions of the Milky Way (see "Milky Way mysteries: Disappearing dwarf galaxies") are two shining exceptions. The large and small Magellanic clouds are by far the largest of our dwarf-galaxy entourage. They are complex and active places where bright young stars are being born and others die, as the most recent nearby supernova, which occurred in the Large Magellanic Cloud in 1987, revealed.
That is all well and good - except that these galaxies might not be our satellites at all.
Observations in 2006 by the Hubble Space Telescope showed that these two dwarfs are crawling across the sky at a little shy of a millionth of a degree per year. That might not sound speedy, but the galaxies are more than 150,000 light years away, so it translates to a velocity of more than 100 kilometres per second. That might be too fast for the clouds to be held by the Milky Way's gravity.
Whether it is too fast depends on our galaxy's total mass. Most of the Milky Way's mass is thought to reside in its surrounding halo of dark matter, which stretches far beyond the bright disc of stars. The best way to estimate the mass of the whole galaxy is by following the motions of other, smaller satellite galaxies that are more conclusively in orbit, to see how they respond to our gravity. But these objects are fainter and slower than the Magellanic clouds, making them even harder to catch in motion. Our best estimates of the Milky Way's mass are somewhere between one trillion and three trillion times that of the sun.
That allows three possibilities. If the mass is near the top end, then we should easily hold onto the Magellanic clouds, and they have probably orbited the Milky Way a couple of times since it formed. If it is somewhere in the middle, the clouds are likely to be making their first close approach to us. Over a few hundred million years they will begin to head further away, but they will return to us eventually, like a comet on an elongated orbit. If the Milky Way is right at the lightweight end of the scale, however, the Magellanic clouds are just passing, and we will have to wave goodbye to this charismatic pair of space tourists.
Stephen Battersby is a consultant for New Scientist

Milky Way mysteries: Disappearing dwarf galaxies

Pacing out their slow circuits over a billion years or more, 26 small galaxies are known to orbit the Milky Way. That might seem an impressive band of loyal followers. Astrophysicists, though, think we should have an armyMovie Camera.
That expectation is based on the prevailing model of how dark matter helps to form galaxies. Dark matter's composition is unknown, but it is thought to outweigh ordinary matter by five to one. In simulations of the early universe, the gravitational pull of cold clumps of dark matter draws in ordinary gas to form the first galactic building blocks.
The theory works well on large scales, reproducing the spongy pattern of galaxies and voids seen across the cosmos. On smaller scales, however, the simulations show that around every large spiral galaxy, dark matter clumps should sculpt thousands of dwarf galaxies.
One possible explanation for the discrepancy is that dark matter is not cold and clumpy, but a hotter gas of lightweight particles that forms small blobs less easily. Or perhaps dark matter doesn't exist at all: if the strength of gravity were to change at long range, that would do much that dark matter does without requiring so many dwarfs.
Tinkering with gravity is controversial. A less radical idea is that all those small dark-matter blobs do exist, but we just can't see them. Because their gravitational grip is weak, the gas could have been pushed out before many stars could form; a few giant stars may have blasted it out with their fierce heat and explosive deaths, for example. This theory is hard to check in simulations, says astrophysicist James Binney of the University of Oxford, because it depends on details such as the local gas density. "That has very fine structure which is way beyond the resolution of any simulation."
If the idea is right, though, it has a startling implication. Thousands of dark galaxies are arrayed around the Milky Way: a legion not lost, but invisible.
Stephen Battersby is a cons

Milky Way mysteries: Where are all the supernovae?

Read more: "Mysteries of the Milky Way"
We can see the fiery brilliance of supernovae from more than halfway across the universe, billions of light years away. So why are we missing them in our own backyard? Comparison with similar spiral galaxies suggests that the Milky Way should host about three stellar explosions per century, but in the past millennium and a bit we have seen only five or six.
All of these have been within about 15,000 light years of us, while the Milky Way's disc is 100,000 light years across. We are missing the more distant explosions for a simple reason: our lowly viewpoint means most of the galactic disc is hidden behind interstellar dust.
As the blast wave of a supernova ploughs out into space, it energises particles to emit radio waves, which are not absorbed by intervening dust. Radio astronomers are now finding evidence of many of these supernova remnants. In 2008, one near the centre of our galaxy, called G1.9+0.3, was calculated to be the youngest known. Its light probably reached Earth about 110 years ago - perhaps marking Queen Victoria's funeral, had it not been for the black veils of dust in betweenMovie Camera.
There is still a dearth of remnants from the past 2000 years, but these explosions occur in complex regions of star formation where ionised gas clouds are also emitting radio waves, confusing the picture. "I think they're out there," says David Green of the University of Cambridge, who discovered G1.9+0.3 in the 1970s. "We have just not found them yet."
Stephen Battersby is a consultant for New Scientistultant for New Scientist
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