IN THE 1969 film Doppelg盲nger, the world鈥檚 astronomers were astonished to discover a major new planet on the far side of the Sun. Trailing precisely six months behind the Earth in its orbit, the planet had been entirely overlooked because it was lost in the dazzling solar glare.
Now, in a peculiar example of science imitating science fiction, astronomers are discovering new celestial entities on the far side of our own Galaxy, whose light has been obscured by the glare of intervening stars and dimmed by dense curtains of interstellar dust.
Among the discoveries are a nearby spiral galaxy dubbed Dwingeloo 1, and a large cluster of galaxies 65 million light years from the Earth. The shimmering stars and choking dust that blocked these objects from view are part of our Galaxy, a great fiery pinwheel 130 000 light years in diameter which is turning ponderously in the cosmic night. Since the Sun is embedded deep within the pinwheel, about 24000 light years out from the centre, the Galaxy looks like a broad white band in the night sky, hence its name, the Milky Way.
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The part of the sky concealed by the glare of the Milky Way is called the Zone of Avoidance ostensibly because galaxies appeared to avoid the area. But the name is apt in more ways than one. Astronomers had been reluctant to waste their time struggling to find objects in a region that might contain nothing worth looking at. But over the past few years new discoveries have brought about a change of heart. Though the Zone of Avoidance takes up a mere fifth of the entire sky, astronomers are realising that this fifth is vital. Lifting the Milky Way鈥檚 veil to peer inside the Zone of Avoidance will not only help us to understand what goes on in our own back yard but may help us work out how the great daisy chains of galaxies in today鈥檚 Universe first congealed out of the stuff of the big bang.
In our own neighbourhood, it is possible that the Zone of Avoidance may be hiding major concentrations of matter whose gravity could be influencing the motion of our local cluster of galaxies. This Local Group, as it鈥檚 called, is dominated by the Milky Way and Andromeda, a galaxy two million light years away. Astronomers know that two major galaxy clusters in our corner of the Universe appear to be playing a tug of war with the Local Group. But this picture could be deceptively simple 鈥� the discovery of a third major cluster, or even a fourth could complicate matters considerably.
So astronomers were excited by the discovery of a major new spiral galaxy Dwingeloo 1 鈥� literally on Earth鈥檚 doorstep. The new spiral was found last year by an international team of astronomers probing the Zone of Avoidance with a 25-metre radio telescope at Dwingeloo in the Netherlands. A radio telescope enables astronomers to 鈥渟ee鈥� right through to the other side of the Milky Way because interstellar dust is transparent to radio waves. On 4 August 1994, two members of the team, Ren茅e Kraan-Korteweg of the Kapteyn Astronomical Institute in Groningen, the Netherlands, and Andy Loan of the Institute of Astronomy in Cambridge, England, were poring over the Dwingeloo data and spotted what looked like a galaxy. Their evidence was a pronounced radio broadcast at 21 centimetres 鈥� in spiral galaxies cold neutral hydrogen broadcasts at this wavelength.
The strength of the radio emission and the large chunk of the sky that it covered 鈥� about half the diameter of the Moon as seen from the Earth 鈥� implied that the galaxy is both big and nearby. Kraan-Korteweg and Loan were stunned. How could astronomers have missed such a significant galaxy so close to home? However, other groups of astronomers were quick to confirm the existence of the galaxy at radio, infrared and even optical wavelengths. Dwingeloo 1 is a 鈥渂arred鈥� spiral galaxy. It is a third as massive as our own Galaxy or Andromeda and about 10 million light years away (This Week, 27 August 1994).
鈥淭hat placed it just beyond the Local Group,鈥� says Ofer Lahav of the Institute of Astronomy, in Cambridge, one of the members of the Dwingeloo team. 鈥淣evertheless, the mere fact that such a large galaxy had been overlooked on our doorstep, prompted the question: could the Local Group contain another Andromeda?鈥�
Dating game
According to Lahav, the existence of another Andromeda-sized galaxy in our immediate vicinity would have serious implications for both the mass and the age of the Local Group. Astronomers can determine the age by means of a 鈥渢iming argument鈥�. This means that they assume that the two biggest players in the Local Group 鈥� the Milky Way and Andromeda 鈥� were born close together in space but with a sizable relative velocity. Subsequently, they moved apart until gravity slowed them to a halt then slammed them into reverse, just like a stone that is thrown up into the air and, after reaching its highest point, begins falling back to the Earth. Knowing their masses and their relative velocity makes it possible to work backwards and decide how long it is since they were first born.
鈥淗owever, this timing argument is based on the assumption that the Milky Way and Andromeda are the major galaxies in the Local Group,鈥� says Lahav. 鈥淚f a new massive galaxy is found, everything could fall apart.鈥�
The discovery of another Andromeda would not just upset our neat picture of the age of the Local Group. It could alter our estimate of its velocity relative to the overall expansion of the Universe. Since this so-called peculiar velocity is determined by the net gravitational pull of large concentrations of mass in our corner of the cosmos, the existence of another huge galaxy could alter the perspective completely.
Such a galaxy would throw a spanner in the works because the peculiar velocity of the Local Group cannot be measured directly. Astronomers start by measuring the peculiar velocity of the Sun and then separate out the component of the Sun鈥檚 motion that depends on the velocity of the Local Group. They measure the velocity of the Sun relative to the expansion of the Universe by observing the cosmic microwaye background radiation, the cooled 鈥渁fterglow鈥� of the big bang fireball which still permeates all of space 15 billion years after the moment of creation.
Since the background radiation expands with the Universe, it gives a handy universal 鈥渞eference frame鈥� for measuring the velocity of the Sun. The Sun鈥檚 motion through the background radiation 鈥渂lue shifts鈥� or boosts its energy in the direction of the Sun鈥檚 motion, making it appear hotter, and 鈥渞ed shifts鈥� the radiation, or makes it appear colder, in the opposite direction. The effect is small, amounting to only a few thousandths of a degree, but reveals that the Sun is moving at 370 kilometres per second in the direction of the constellation of Leo.
The Sun鈥檚 peculiar velocity is partly due to the gravitational pull of the other galaxies within the Local Group and partly to the motion of the Local Group relative to the expansion of the Universe. So estimating the component given by the pull of the other galaxies means that you can calculate the component given by the Local Group鈥檚 velocity. Until now astronomers have assumed that Andromeda is the only galaxy in the Local Group to exert a significant pull on the Milky Way, and hence on the Sun, and have deduced that the Local Group is actually moving at 600 kilometres per second roughly towards the constellations of Hydra and Centaurus. This velocity is within 10 or 20 degrees of the direction predicted if the Local Group is moving solely under the net gravitational influence of all the visible matter in the form of clusters in our cosmic neighbourhood, with the motion dominated by the gravity of two large superclusters at comparable distances but on opposite sides of the sky. In this picture, the Local Group is the prize in the tug of war between the Perseus-Pisces supercluster on one side and the Great Attractor on the other 鈥� a mysterious and unidentified mass that exerts an influence on large numbers of galaxies.
So far so good but an Andromeda-sized galaxy nearby could toss a spanner in the works. The estimate of the peculiar velocity of the Local Group could be quite different, perhaps affecting the ultimate fate of the Local Group in the tug of war.
And the Zone of Avoidance doesn鈥檛 just hold secrets about our own back yard. It could help us to understand the structure of the rest of the Universe. Curiously, the Perseus-Pisces supercluster is partly hidden behind the zone. So, too, is the great conglomeration of galaxies that astronomers think might be at least a large part of the Great Attractor. 鈥淚t鈥檚 odd that nature has contrived to hide two of the biggest galaxy groupings in our cosmic neighbourhood behind the Zone of Avoidance,鈥� says Lahav.
A powerful hint of what else we might have overlooked came in 1993 when a group of astronomers from Britain, Japan and the Netherlands found a major new galaxy cluster in the Zone of Avoidance. The cluster, which is 65 million light years away in the direction of the constellation of Puppis, was discovered by Lahav, Kraan-Korteweg, Caleb Scharf of the Institute of Astronomy in Cambridge and Toru Yamada of Kyoto University in Japan (New 杏吧原创, Science, 10 April 1993).
Lahav and his colleagues discovered the Puppis cluster after analysing images obtained by the Infrared Astronomical Satellite of a region lying within 5 degrees of the galactic plane 鈥� like radio waves, infrared radiation can penetrate the Milky Way鈥檚 dust haze. They were able to identify 32 galaxies but suspect there may be a great many more since big elliptical galaxies, lacking dust, tend not to show up well in the near-infrared.
The Puppis cluster may not be the only cluster hidden behind the Zone of Avoidance. Kraan-Korteweg says she has now found evidence of another big cluster, which she thinks is the core of the Great Attractor. The discovery is so new that she has not yet published her findings.
Just as the discovery of Dwingeloo 1 prompted the question 鈥� is there another Andromeda? 鈥� the discovery of the Puppis cluster and Kraan-Korteweg鈥檚 raises the possibility of other major clusters that may have been overlooked. This is of more than passing interest; it is crucially important for understanding how galaxies were born out of the cooling stuff of the big bang.
Most astronomers are convinced that the process of galaxy formation was governed almost entirely by the matter in Universe that we can鈥檛 actually see. Not only did the dark matter鈥檚 mass, and therefore gravitational pull, completely overwhelm that of visible matter, so the theories say, but it started to clump together under gravity far sooner after the big bang than visible matter. What determined the characteristic size of those clumps was the precise nature of that dark matter. Cold dark matter, made of particles moving slower than the speed of light, formed tight clumps of galaxies, whereas hot dark matter, composed of particles moving close to the speed of light, formed galaxies that were spread out into 鈥渇ilaments鈥�.
The way galaxies are distributed in today鈥檚 Universe therefore contains vital information about the precise mix of hot and cold dark matter, hence the importance of finding any clusters hidden in the Zone of Avoidance. It turns out that both the Great Attractor and Perseus-Pisces supercluster form filaments extending perpendicularly from the 鈥淪upergalactic Plane鈥�, a long chain of galaxies that forms the largest galactic grouping in our neighbourhood. Maddeningly, the Zone of Avoidance cuts right across the Supergalactic Plane. Astronomers are tempted to assume that is continues behind the Zone but they cannot be sure. The discovery of yet more clusters, in addition to the Puppis cluster and Kraan-Korteweg鈥檚 as yet unnamed cluster, could settle the question. If the Supergalactic Plane is continuous, that would imply lots of hot dark matter in the Universe. If it is not, then the theorists can make do with less.
Dusty secrets
Lots of work still needs to be done in exploring the Zone of Avoidance. One of the critical things is to understand the properties of the interstellar dust that obscures the view of the far side of our Galaxy. Only if astronomers know how effectively it blocks out light at each and every wavelength and position in the sky will they be able to estimate the true brightness, and therefore distance, of any new galaxies that do turn up.
Astronomers also need to carry out far more systematic searches of the Zone of Avoidance. Until now, they have pointed their telescopes wherever they fancied in the hope of bagging a new galaxy or cluster of galaxies. Two major systematic surveys are now being planned. In the US, a team led by John Huchra of the Center for Astrophysics in Cambridge, Massachusetts, and Susan Kleinman at the California Institute of Technology in Pasadena are planning to search the zone at a near-infrared wavelength of 2.2 micrometres. A French team led by Nicolas Epchstein of the Meudon Observatory plans a survey at the same wavelength.
So far, all the observations of the Zone of Avoidance have been limited to radio and near-infrared wavelengths. But astronomers must point their telescopes at a much greater range of wavelengths to have any hope of finding out what is really out there. A promising region of the spectrum covers X-ray wavelengths 鈥� X-rays are emitted by very hot active stars and galaxies and are not absorbed very easily by the dust in the Milky Way. The problem here will lie in telling the difference between distant X-ray sources and the ones in the Milky Way itself. Unlike radio sources, extra-galactic X-ray sources that are bright enough to be seen from the Earth tend to be point-like, and can thus look very like those in our own Galaxy.
According to Lahav, in ten years the Zone of Avoidance should be mapped out to moderate distances. 鈥淲e鈥檒l know whether there is anything big 鈥� or if there isn鈥檛,鈥� he says. 鈥淚 have a sneaking hope that we鈥檒l find something dramatic.鈥�(see Diagram)
