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Two years ago, a professor at Florida Atlantic University in the US proposed that the Milky Way – our home Galaxy – had

a hidden companion. The suggestion by Sukanya Chakrabarti recalls the 19th century search for Planet X – a planet hypothesised to perturb the orbits of the outer planets. Perhaps not surprisingly, the name Galaxy X has been attached to Chakrabarti’s hidden galaxy and new research has given the idea a further boost. If found, it could shed new light on dark matter and put a final nail in the coffin for competing theories of gravity.

So what is this missing companion like? “Galaxy X is a dwarf galaxy, inferred from analysing disturbances in the outer disc of the Milky Way,” Chakrabarti explains. More than 80 dwarf galaxies are thought to be orbiting our Galaxy. You may know two of the largest – the Large and Small Magellanic Clouds, which can be seen by stargazers in the southern hemisphere.

Chakrabarti’s method relies upon the effects of gravity. In spiral galaxies, cold hydrogen gas is gravitationally confined to the galactic plane but often extends up to five times the radius of the visible stars. The key is finding small perturbations in the motion of this cold hydrogen gas in the fringes of the Milky Way. Using radio observations that map the density of the gas, the location and mass of any satellite galaxies perturbing its motion can be constrained. TH

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The inspiration for Chakrabarti’s work came from a radio map of hydrogen gas density produced by a trio of researchers at the University of California in Berkeley. Noticing the ripples in the gas, Chakrabarti was intrigued. “I wanted to see how massive a satellite would have to be, and how close it would have to get, to produce the observed disturbances,” she says. With a sophisticated model of the galaxy-satellite interaction, she explored all possible masses and locations and concluded that if such a satellite galaxy is responsible for the ripples, it has so far evaded detection.

Ripples of evidence If it exists, Galaxy X is likely to lie about 260,000 lightyears away, with a radius of about 50,000 lightyears and a mass about one-hundredth that of the Milky Way. That would make it the Milky Way’s third largest satellite, after the Large and Small Magellanic Clouds. Perhaps more interesting is that about 300 million years ago, Galaxy X almost certainly swept

> The southern hemisphere’s Large and

Small Magellanic Clouds; could there be another

such dwarf galaxy hidden in our midst?

right through our own Galaxy, just 16,000 lightyears from the Galactic Centre. This encounter accounts for most of the ripples seen in the radio observations.

Chakrabarti’s analysis has revealed enough detail to make the search for Galaxy X an interesting challenge. The main problem is its location in the sky. It lies on the other side of the Milky Way and should appear just west of the galactic centre, probably in the constellation Norma or perhaps Circinus. This area of sky is incredibly rich in stars, gas and dust since we’re looking through the plane of the Milky Way, which means our view of Galaxy X is obscured. Chakrabarti believes this is why the mysterious companion has so far remained hidden. Galaxy X is also likely to be extremely faint, with just a sprinkling of dim stars. Finding it among the clutter of the galactic plane is a daunting task.

The original prediction of Galaxy X in 2009 received a good deal of interest from astronomers. Now, Chakrabarti has published another paper demonstrating

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A dwarf galaxy lurking undetected

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that her ‘tidal analysis’ technique correctly predicts the known satellites of two nearby spiral galaxies, both about 30 million lightyears distant. Using data from recent radio surveys carried out in the US and Australia, Chakrabarti and colleagues looked at M51 (the Whirlpool Galaxy), which has a companion one-third its size, and NGC 1512, with a satellite only one-hundredth its size. In both cases the model correctly revealed the masses and locations of the satellite galaxies. The technique is believed to be good enough to predict satellite galaxies as small as one-thousandth of the mass of the primary galaxy. This success has prompted astronomers to step up the hunt for Galaxy X.

Dark matter puzzle Finding Galaxy X is important for more than just the joy of discovery. Locating it, and vindicating Chakrabarti’s technique, may shed light on a conundrum that has perplexed astronomers for decades. It’s known as the ‘missing satellite problem’.

The formation of the large-scale structure of the Universe, the ‘clumpiness’ we see in the distribution of galaxies and galaxy clusters, seems to require a huge amount of hidden material. This ‘dark matter’ apparently accounts for more than 80 per cent of the matter in the Universe, but as yet astronomers have no idea as to its nature. Even though we haven’t detected it, we know it’s there. With the best, well-tested physics we have, the structures we see in the Universe simply couldn’t exist without this missing

have a total mass of only a few per cent of the Milky Way,” says Sarkar. “Estimates of the masses of those we cannot see are not much bigger.” So, even if a vast population of dwarf galaxies is found using Chakrabarti’s technique, they are unlikely to account for the missing 80 per cent of matter. Where the rest of the dark matter resides is still a mystery.

Some scientists have suggested the laws of gravity are somehow modified for very small gravitational forces, such as those encountered where dark matter is thought to exist. This Modified Newtonian Dynamics, or MOND, would do away with the need to postulate dark matter. “To test this radical idea we need an independent probe of dark matter,” says Subir Sarkar, an Oxford physicist working on dark matter. “Studying tidal effects on the gaseous discs of galaxies seems quite promising.” So not only is the technique a new probe of the mass distribution in galaxies, but it may well find the missing population of invisible dwarfs, thus vindicating our current models of the Universe and discounting MOND.

However, the number of invisible dwarf galaxies in the Universe may provide clues to the nature of dark matter itself. It will help us to determine whether the particles that make up dark matter interact with each other or not. “If dark matter

is totally collisionless, a huge number of invisible dwarfs is expected to be lurking out there. But if the dark matter is self-interacting then this can suppress the number of dwarfs,” explains Sarkar. If Chakrabarti’s prediction comes true, we may have a new tool for studying the nature of dark matter.

Chakrabarti now has plans to further test the tidal analysis technique. “Our paper is a proof of principle,” she admits. “We are now in the process of analysing another 30 galaxies to determine the viability of the method.” Although confident in her prediction of Galaxy X, she acknowledges there may be shortcomings in the technique. “My personal guess is that if Galaxy X isn’t found, it’s because the model we have assumed is too simple,” she says.

The fabled Planet X was never found. We now know that bodies way out in the Solar System don’t have enough mass to appreciably change the orbits of the other planets. On the other hand, Galaxy X, with a mass only one-hundredth that of the Milky Way, would have an easily measurable effect.

Whether or not Galaxy X is found, we still stand to learn something fundamental. It represents another step on our quest to understand the nature of dark matter and the evolution of the Universe.

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material. It is commonly supposed that dark matter is made primarily of particles predicted by subatomic theories but which have not yet been observed. There is hope that we may soon discover these particles at the Large Hadron Collider at CERN.

One flavour of the dark matter hypothesis, known as Cold Dark Matter (or CDM) proposes that dark matter, whatever it is, moves slowly (ie much less than the speed of light). If this is the case, then the structure of the Universe has evolved from the bottom up. Small objects condensed first and merged to form larger structures. But CDM makes one crucial prediction that may make or break the theory. It predicts there should be many more dim dwarf galaxies in the Universe than we actually see. This is the ‘missing satellite problem’, and it’s a major stumbling block in modern cosmology.

Dwarf galaxies are very faint, probably consisting of old, dim stars and a lot of gas and dust. It appears that they are dominated by dark matter, accounting for some of the Universe’s missing material. However, dwarf galaxies do not contain all the dark matter. “The ones we can see

Armed with a prediction of where to look for Galaxy X, the search is now on. A proposal to use the Spitzer Space Telescope has been made by Barbara Whitney, an astrophysicist working at the University of Wisconsin in the US. “We will look for an enhancement of red giant stars at the galactic longitude predicted by Sukanya. If it’s there, I think we have a good chance of detecting it,” she explains.

Spitzer is an infrared space telescope, one of NASA’s Great Observatories. Infrared radiation will most easily reveal Galaxy X since it is less affected by the intervening gas and dust of the galactic plane. An estimate of the infrared brightness of red giant stars within Galaxy X has shown that Spitzer is ideally suited to the search.

Another survey known as UKIDSS also has a good chance of finding Galaxy X. This would be performed with the UK Infrared Telescope (UKIRT) on Hawaii. But for the moment the mysterious dwarf galaxy that’s stirring up the fringes of the Milky Way remains hypothetical.

How to find a missing galaxyþ A dwarf galaxy in the constellation of Fornax; will these enigmatic objects help unravel the mystery of dark matter?

Halley’s Comet (upper left) streaks through the Constellation of Norma. This rich, star-filled region is the most likely hiding place for Galaxy X

The Spitzer Space Telescope will be able to gaze through intervening gas and dust to spot Galaxy X

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