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collective fluctuations in strongly correlated systems. s
Peter Schiffer is in the Department of Physics and the Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA. e-mail: schiffer@phys.psu.edu
1. Mirebeau, I. et al. Nature 420, 5457 (2002). 2. Ramirez, A. P. in Handbook of Magnetic Materials (ed. Buschow, K. J. H.) 423520 (New Holland, New York, 2001). 3. Taguchi, Y. et al. Science 291, 25732576 (2001). 4. Gardner, J. S. et al. Phys. Rev. Lett. 83, 211214 (1999). 5. Snyder, J., Slusky, J. S., Cava, R. J. & Schiffer, P. Nature 413, 4851 (2001). 6. Harris, M. J., Bramwell, S. T., McMorrow, D. F., Zeiske, T. & Godfrey, K. W. Phys. Rev. Lett. 79, 25542557 (1997). 7. Ramirez, A. P. et al. Nature 399, 333335 (1999). 8. Bramwell, S. T. & Gingras, M. J. P. Science 294, 14951501 (2001). 9. Moessner, R. & Chalker, J. T. Phys. Rev. Lett. 80, 29292932 (1998). 10. Harris, M. J., Zinkin, M. P., Tun, Z., Wanklyn, B. M. & Swainson, I. P. Phys. Rev. Lett. 73, 189192 (1994). 11. Gardner, J. S. et al. Phys. Rev. Lett. 82, 10121015 (1999). 12. Gardner, J. S. et al. Phys. Rev. B 64, 224416 (2001). 13. Tsui, Y. K., Burns, C. A., Snyder, J. & Schiffer, P. Phys. Rev. Lett. 82, 35323535 (1999). 14. Broholm, C., Aeppli, G., Espinosa, G. P. & Cooper, A. S. Phys. Rev. Lett. 65, 31733176 (1990). 15. Dunsiger, S. R. et al. Phys. Rev. Lett. 85, 35043507 (2000).

Astronomy

Close encounters of the tidal kind
Tommy Wiklind Between the Magellanic Clouds, in a region swept with tides of gas, stars are forming. The detection of carbon monoxide shows gas is condensing, and further observations may reveal the ultimate fate of the clouds.
s giant galaxies merge, small galaxies may be born in the gaseous debris thrown out from the collision. This kind of `tidal debris' is also seen in our own neighbourhood, as the Milky Way and two nearby galaxies, the Magellanic Clouds, act out a gravitational tug-of-war with each other. Writing in the Monthly Notices of the Royal Astronomical Society, Muller et al.1 announce their detection of cold, dense molecular gas in the Magellanic tidal debris, signalling the birth of new stars in this inhospitable place and possibly offering a unique window on galaxy formation. The Magellanic Clouds -- the Large

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Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) -- are dwarf galaxies and gravitationally bound satellites of our own, much larger, Milky Way galaxy. These clouds have endured a rather violent life over the past 1.5 billion years or so, which has left a number of tell-tale signatures: a long, trailing stream of neutral atomic-hydrogen gas, stretching almost 100° across the southern sky; a `leading arm' (a second stream of gas that seems to lead the motion of the clouds) also of neutral hydrogen gas; and a gaseous bridge connecting the LMC and SMC. These features appear clearly in a recent all-sky map (Fig. 1) derived from a well-known atomic-

Bridge LMC

Stream

SMC Leading arm

Figure 1 Evidence of a violent past. The distribution of atomic-hydrogen gas in the southern sky is captured in this image from the Parkes radio telescope in Australia2. Two concentrations of gas form the Large and Small Magellanic Clouds (LMC and SMC), and behind them the long tail of the Magellanic stream stretches more than 100 across the sky. Two hundred million years ago, a close encounter between the LMC and the SMC led to the formation of the Magellanic bridge between them. Muller et al.1 have detected cold, dense molecular gas in the bridge, which shows that stars are forming in this region. (Image adapted from ref. 2.)
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hydrogen emission line (the 21-cm line) using the Parkes telescope in Australia2,3. An explanation for these features has emerged through computer modelling of a three-body gravitational interaction between the LMC, the SMC and the Milky Way4. The long Magellanic stream and the leading arm are, in this model, the result of a close encounter between the SMC and the Milky Way about 1.5 billion years ago. The gas making up the stream comes from the inner regions of the SMC, but the stream does not seem to contain any stars. The bridge connecting the two Magellanic Clouds is thought to result from a particularly close encounter between the LMC and the SMC a `mere' 200 million years ago. Again, the gas is believed to have been pulled from the SMC, the less massive of the two galaxies. But in contrast to the stream, the bridge does contain stars, even stars much younger than 200 million years. So these stars cannot have formed before the encounter between the two Magellanic Clouds, but instead must have formed in the bridge itself. Star formation in galaxies is directly associated with the presence of cold, dense molecular gas. As stars are being born in the bridge between the Magellanic Clouds, it is expected that molecular gas exists there, in cohabitation with the more diffuse and dispersed atomic gas. The main constituent of a molecular cloud is molecular hydrogen, which is a poor emitter of photons because of its symmetrical structure. Molecular gas is therefore usually observed through its rarer constituents, such as carbon monoxide (CO) and other species. For these tracer molecules to be present the gas must be enriched in `metals' (in astrophysical language, anything heavier than helium is referred to as a metal). The gas in the bridge is believed to have been drawn from the SMC, which is rather metalpoor, with metal content only around 10% that of the Sun. In fact, measurements indicate that the bridge contains a lower amount of metals than the SMC, so there are considerably fewer C and O atoms to make up observable CO molecules. Nevertheless, Muller and colleagues set out to search for the telltale CO emission in selected regions of the Magellanic bridge, using both the 22-m MOPRA telescope in Australia and the 15-m SEST telescope in Chile. Out of six candidate regions, they had time to survey only one, but were rewarded with a successful detection. The emission is associated with a region where young stars, dust and atomic hydrogen are all mixed together. But, compared with the SMC, the molecular emission is weaker and the inferred gas velocity has a smaller spread of values. Moreover, atomic gas in the SMC has two distinct velocity components, only one of which has associated CO emission5. But the CO emission in the bridge seems to be

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associated with the other velocity component, implying that it condensed from atomic gas in situ, rather than being extruded as molecular gas from the inner regions of the SMC itself. The fate of the gas and stars in the Magellanic bridge is presently unknown. Will it fall back into either of the Magellanic Clouds? Or will it be absorbed into the Milky Way? A third, though more remote, possibility is that the bridge will form a more or less independent system -- the Very Small Magellanic Cloud (VSMC). To assess the likelihood of these possibilities, the total mass of the bridge, as well as its kinematics, must be measured and this is best done by observing its atomic and molecular gas components. Should the third scenario indeed be the final outcome (before the Magellanic systems merge with the Milky Way, which is the ultimate outcome), it would mean that a small galaxy has been born right before our eyes. This would indeed be a remarkable opportunity: one of the outstanding problems in modern astrophysics is to understand how galaxies form. The most successful model of galaxy formation so far has galaxies surrounded by haloes of mysterious, cold dark matter (CDM) -- the unseen, `missing' matter that pervades our Universe; matter as we know it, in the form of stars and gas, is just a small part of the entity that we call a galaxy. In this model, small galaxies form first, dominated by CDM, and are then assembled into larger and larger complexes through gravitational attraction and subsequent merging, following a scheme of so-called hierarchical clustering. According to the CDM model, in early times our Universe contained relatively large numbers of small galaxies, and this certainly seems to have been the case. The present-day Universe, on the other hand, should contain large galaxies that occasionally grow by `swallowing' one of the surviving small galaxies. This also seems to be in accordance with observations and the Magellanic system is an example of this process. But should the Magellanic bridge become free of the LMC and SMC, it would constitute yet another method of galaxy formation. In fact, tidal-debris formation of small galaxies is known to occur in the Universe. Dwarf galaxies formed during violent gravitational interaction between large galaxies --Tidal Dwarf Galaxies, or TDGs -- are relatively common, and some of these TDGs are known to contain both stars and molecular gas in a manner similar to the Magellanic bridge6. There are even cases where tidally created galactic entities seem to contain only gas, both atomic and molecular, but no stars7,8. All of these systems are generally more metal rich and more massive than the Magellanic bridge, but the observations by Muller et al.1 show that the process of tidal galaxy formation seems to be efficient for very small systems as well (even if the Magellanic bridge itself may be a bit too small to become a free galactic entity). Tidally formed galaxies do not constitute a large population either in numbers or in mass and so will not have any great impact on the future evolution of our Universe. Their ultimate fate is probably to be swallowed by their parent galaxy, or any other large nearby galaxy. Despite their fleeting existence (from a cosmological perspective), they may still be very useful in studies of the structure of the large and extended haloes of their parent galaxies. How TDGs form, their dark-matter composition and their kinematics can tell us how the large parent galaxies themselves formed and how their dark-matter haloes are distributed. Although it is too early to draw conclusions about the CDM haloes from studies of these newly born tidal
Medicine

dwarfs, continued observations may very well reveal something of this elusive dark matter that dominates our Universe. s
Tommy Wiklind is in the European Space Agency Telescope Division, and currently at the Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA. e-mail: wiklind@stsci.edu
1. Muller, E., Staveley-Smith, L. & Zealey, W. J. Mon. Not. R. Astron. Soc. (in the press); Preprint astro-ph/0209523 (2002), http://arXiv.org 2. Putman, M. E., Staveley-Smith, L., Freeman, K. C., Gibson, B. K. & Barnes, D. G. Astrophys. J. (in the press); Preprint astro-ph/0209127 (2002), http://arXiv.org 3. Putman, M. E. et al. Nature 394, 752754 (1998). 4. Gardiner, L. T. & Noguchi, M. Mon. Not. R. Astron. Soc. 278, 191208 (1996). 5. Rubio, M. et al. Astron. Astrophys. Suppl. Ser. 118, 263275 (1996). 6. Braine, J. et al. Astron. Astrophys. 378, 5169 (2001). 7. Brouillet, N., Henkel, C. & Baudry, A. Astron. Astrophys. 262, L5L8 (1992). 8. Lisenfeld, U. et al. Astron. Astrophys. (in the press); Preprint astro-ph/0208494 (2002), http://arXiv.org

Tackling multiple sclerosis
Hartmut Wekerle New work in mice finds that certain anti-cholesterol drugs can reduce symptoms of disease in brain autoimmune disorders that are akin to human multiple sclerosis. There are also hints as to how the drugs might work.
ultiple sclerosis owes its enormous socioeconomic importance to several factors. Worldwide, as many as one million people are affected by the disease. It tends to afflict sufferers for most of their lives, often taking a severe, disabling course. And there are no effective treatments that stop multiple sclerosis in its tracks (although there are some that slow its progression). New therapies are desperately needed, and on page 78 of this issue Youssef and colleagues1 investigate one attractive candidate -- atorvastatin, a drug that is already used to reduce blood cholesterol levels in people with atherosclerosis or heart disease2. Multiple sclerosis is generally believed to develop when the body's immune cells -- led by so-called helper T cells -- attack myelin, the insulating, fatty sheath around nerve cells. This damages the myelin and the underlying neurons in both the brain and the spinal cord (Fig. 1, overleaf ), leading to impaired transmission of nerve impulses and progressive physical disability. Treatments available today include one that involves engineered interferon- proteins, which reduce the inflammation associated with nerve damage. Another is based on copaxone, a random composite of basic peptides, which probably activates brainprotein-detecting T cells that inhibit rather than support the autoimmune attack. Both drugs reduce the number of clinical relapses and the damage to the central nervous system (CNS). But both also come at a price --

M

quite literally in one sense, as they are very expensive. Moreover, they must be administered frequently by injection, which is a severe bother and carries a risk of sideeffects. New treatments are needed that can be given orally and that, hopefully, also have a greater effect on the disease. Enter Youssef et al.1, who have looked at the effects of atorvastatin. This is a member of the statin group of molecules, which are commonly used to treat atherosclerosis and coronary disease2. The authors find that the drug is effective against experimental autoimmune encephalomyelitis (EAE), an experimentally induced rodent autoimmune disease that is widely used as a model of human multiple sclerosis. By itself, this finding is perhaps not all that striking. After all, statins have already been used to reduce the rejection of human heart transplants by the immune system, and there have even been reports of a protective effect of injected statins in models of brain autoimmunity similar to EAE3. But what is particularly compelling about the new paper is that it provides a clue to the mechanisms by which statins might have anti-inflammatory effects. Youssef et al. used three different models of mouse EAE. These differ in their genetics, the myelin-associated proteins targeted by the immune response, and the resulting `clinical' diseases -- they share basic features, but represent different phases of brain inflammation. After inducing EAE, the authors fed atorvastatin to the animals once a day for
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