Andromeda extended disk_of_dwarfs
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Lenz et al. discovered that soft particles at the nano- and microscale can form "clumpy crystals" where particles partially overlap and form regular lattices of clumps. This challenges ideas that soft materials will behave similarly to atomic and molecular systems. The discovery provides another example of how soft materials display unconventional behavior. An analysis of the satellites orbiting Andromeda found that about half are rotating coherently in a thin planar structure, providing a new constraint on galaxy formation theories. Further evidence suggests organized planar distributions of satellites may be common for nearby galaxy groups. The findings compound issues with the number of predicted versus observed satellites and suggest the structures themselves are not ancient.
Andromeda extended disk_of_dwarfs
- 1. e-mails: francesco.sciortino@uniroma1.it; emanuela.zaccarelli@cnr.it 1. Lenz, D. A., Blaak, R., Likos, C. N. & Mladek, B. M. Phys. Rev. Lett. 109, 228301 (2012). 2. Klein, W., Gould, H., Ramos, R. A., Clejan, I. & Mel’cuk, A. I. Physica A 205, 738–746 (1994). 3. Mladek, B. M., Gottwald, D., Kahl, G., Neumann, M. & Likos, C. N. Phys. Rev. Lett. 96, 045701 (2006). 4. Narros, A., Moreno, A. J. & Likos, C. N. Soft Matter materials. For example, glasses can be melted by applying a ‘shear’ deformation force paral- lel to a sample’s surface (an effect known as shear melting). Analogous to what has been reported for clumpy crystals9 , the viscosity of shear-melted clumpy glasses should increase with the intensity of the applied deformation, a phenomenon known as shear thickening.This behaviour is at odds with that of most materi- als, in which the application of shear decreases a sample’s viscosity. Finally, the link between soft particles and quantum mechanics should benoted:bosonparticleshavebeenpredicted10 to form supersolids, the quantum analogue of clumpy crystals, although such supersolids have not yet been observed. Perhaps an exper- imentally realized clumpy crystal could give insight into some aspects of such mysterious quantum solids. The unconventional behaviour of soft matter has often surprised scientists. Lenz and colleagues’ study provides yet another example of how soft particles at the nano- and microscale do not simply reproduce phe- nomena known to occur in the atomic and molecular world. ■ Francesco Sciortino is in the Department of Physics, Sapienza Università di Roma, Rome I-00185, Italy. Emanuela Zaccarelli is at CNR-ISC, Institute of Complex Systems, Rome I-00185, Italy. a b Figure 2 | The formation of clumpy crystals in two dimensions. a, At high density, soft particles (shown as transparent spheres; diameter corresponds to each particle’s typical size) might adopt an arrangement in which they partially overlap with several neighbours. The total repulsion exerted on each particle by its neighbours is high. b, Alternatively, the same particles might form a regular lattice of ‘clumps’; in this case, each clump contains an average of three overlapping particles. Particles belonging to distinct clumps do not interact, so that the overall repulsion exerted on each particle is less than that in a. Particles might also be able to hop between lattice sites (arrow). Lenz and colleagues’ numerical simulations1 reveal that dendrimeric soft particles form clumpy crystals. (Graphic courtesy of Lorenzo Rovigatti.) 6, 2435–2441 (2010). 5. Li, Y. et al. Nature Mater. 3, 38–42 (2004). 6. Seeman, N. C. Nature 421, 427–431 (2003). 7. Zhang, K., Charbonneau, P. & Mladek, B. M. Phys. Rev. Lett. 105, 245701 (2010). 8. Coslovich, D., Bernabei, M. & Moreno, A. J. J. Chem. Phys. 137, 184904 (2012). 9. Nikoubashman, A., Kahl, G. & Likos, C. N. Phys. Rev. Lett. 107, 068302 (2011). 10. Cinti, F. et al. Phys. Rev. Lett. 105, 135301 (2010). ASTRONOMY Andromeda’s extended disk of dwarfs Deep-imaging observations of the Andromeda galaxy and its surroundings have revealed a wide but thin planar structure of satellite galaxies that all orbit their host in the same rotational direction. See Letter p .62 R. BRENT TULLY I n this issue, Ibata and colleagues1 report that roughly half the dwarf companion galaxies of the Andromeda galaxy are rotating coherently about it in a thin plane. Their finding provides a fascinating new constraint on theories of galaxy formation. First, the observational facts. Androm- eda, also known as Messier 31, is the nearest giant galaxy to our Galaxy. It is so near that a census of its companions by deep imaging with the Canada–France–Hawaii Telescope has been completed over a large area and to a faint level of detection. Because the system is close, distances to the companions can be measuredandthevelocitiesofconstituentstars determined. Such a complete census for the Milky Way is impossible because candidates could be anywhere in the sky, even behind a zone obscured by the Galaxy. And other giant galaxies are too far away to be studied to such a level of detail. Ibata et al. found that 13 of 27 dwarf companions (satellites), at distances from Messier 31 of between 35 and 400 kiloparsecs (114–1,305 light years), lie in a thin plane 13 kpc thick and share a coherent velocity pattern: those to the north of Messier 31 are moving away from Earth relative to the galaxy, and those to the south are moving relatively towards Earth. No theorist of galaxy formation would have dared to predict such a situation. What’s more, the Milky Way is in the same plane as the 13 satellites. The discovery of this plane is a spectacular result, and the authors avoid the risk of diluting their message by not mentioning more speculative matters that add to the intrigue. Although the disk of Messier 31 is tilted by about 50° from the plane of the satellites, the rotation in the galaxy is in the same direction of motion as the satellite-velocity pattern. Looking beyond Ibata and colleagues’ survey region, the three galaxies that lie 250–500 kpc from Messier 31 — IC 1613, IC 10 and LGS 3 — and which were known before the advent of deep-imaging surveys, all reside in the same satellite plane. This is particularly interest- ing because these three more-distant galax- ies contain substantial interstellar gas and are still forming stars, and so might be recent arrivals on the scene. All the satellites in the authors’ survey region are gas deficient (except Messier 33, which is not on the plane being discussed) and, according to standard galaxy- formation models, would be presumed to have beeninthevicinityofMessier 31forsometime and to have complex orbits. But things are even stranger than Ibata and colleagues suggest. The remaining known Messier 31 satellites can be split roughly in equal numbers into those at lower and higher Galactic longitude. All of those at higher lon- gitude than Messier 31, including the Local Group’s third-largest galaxy Messier 33, lie in a separate common plane. This secondary plane is offset and tilted by about 13° from the primary plane through Messier 31. Ibataet al.remarkonearliersuggestionsthat satellites of the Milky Way also seem to lie in a plane2,3 . It occurred to me that perhaps there was enough information in the data archives to evaluate the distribution of companions in the next-nearest groups of galaxies — those around the dominant galaxies Centaurus A and Messier 81. The regions around these gal- axies have been closely studied in surveys for satellite candidates and in follow-up observa- tionswiththeHubbleSpaceTelescope4,5 .Inthe case of Centaurus A, 22 of 24 satellites within 600 kpc of the galaxy’s centre separate into two equally populated planes that are roughly par- allel but offset by 280 kpc. Centaurus A lies in 3 J A N U A R Y 2 0 1 3 | V O L 4 9 3 | N A T U R E | 3 1 NEWS & VIEWS RESEARCH © 2013 Macmillan Publishers Limited. All rights reserved
- 2. one of these planes. Most of the companions to Centaurus A are gas poor, but there are several systems that contain gas and in which star formation is occurring in each plane. The situation in the Messier 81 Group is less compelling but still suggestive. Here, there is a distinction between the distribution of the gas-poorsatellitesandthatofgas-richsatellites that are undergoing star formation. The gas- poor systems lie in a flattened distribution that have characteristic dimensions of 60 × 120 kpc, with the flattening coincident with the ‘Local Sheet’ structure6 that harbours all the galax- ies mentioned above and which extends over a long dimension of 10 Mpc and with a thick- ness of 1 Mpc. The gas-rich satellites typically lie farther from Messier 81, and loosely align to a plane of their own. This discussion of the organized distribu- tion of satellites is anchored in the solid evi- dence reported by Ibata and colleagues for a thin plane with coherent kinematics. There are hints that structure in the distribution of satellites is the norm. The subject deserves further attention, but it should be noted that the planes that have been discussed on scales of 300–500 kpc have a general alignment with the Local Sheet. This sheet forms a wall of an anti-structure, the ‘Local Void’, that strongly affects the development of nearby structure6 . Ibata et al. only touch on possible scenarios underlying the formation of the planar struc- tures. The new information compounds a familiar galaxy-formation problem — a defi- ciency in the numbers of satellites found com- pared with theoretical expectations7,8 . Now, it seems, not only is there a paucity of satel- lites, but also most of those that do exist are in these organized structures. The very organiza- tion suggests that the structures (possibly as distinguished from their constituents) are not ancient. Current ideas about galaxy formation pro- pose that material (both gas and already con- structedgalaxies)fallsintotheextendedhaloes around galaxies as flows along filaments. The orbital angular momentum of the infalling material over time tends to cause motion that has the same direction of rotation as that of the dominant galaxy in the halo, resulting in the build-up of a spiral disk in the galaxy. It is reasonable to assume that newly accreted satellites would share the sense of rotation, but that after a few orbits they would tend to become scrambled. Because infalling galaxies around Messier 31 adhere to such a thin plane, it would seem that they do not take many excursions before they are absorbed in the central galaxy. ■ R. Brent Tully is at the Institute for Astronomy, University of Hawaii, Honolulu, Hawaii 96822, USA. e-mail: tully@ifa.hawaii.edu 1. Ibata, R. A. et al. Nature 493, 62–65 (2013). 2. Lynden-Bell, D. Mon. Not. R. Astron. Soc. 174, 695–710 (1976). 3. Pawlowski, M. S., Pflamm-Altenburg, J. & Kroupa, P. Mon. Not. R. Astron. Soc. 423, 1109–1126 (2012). 4. Karachentsev, I. D. et al. Astron. Astrophys. 385, 21–31 (2002). 5. Chiboucas, K., Karachentsev, I. D. & Tully, R. B. Astron. J. 137, 3009–3037 (2009). 6. Tully, R. B. et al. Astrophys. J. 676, 184–205 (2008). 7. Klypin, A., Kravtsov, A. V., Valenzuela, O. & Prada, F. Astrophys. J. 522, 82–92 (1999). 8. Moore, B. et al. Astrophys. J. 524, L19–L22 (1999). DEVELOPMENTAL BIOLOGY Segmentation within scale Irrespective of an organism’s size, the proportional sizes of its parts remain constant. An experimental model reveals size-dependent adjustment of segment formation and gene-expression oscillations in vertebrates. See Letter p .101 NAAMA BARKAI & BEN-ZION SHILO D eveloping organisms face a major challenge: their body pattern must be adjusted — scaled — to their body size. But how is tissue size ‘measured’? And what conveys general size information to the local settings of each cell? Despite intense interest, the mechanistic basis of scaling is poorly understood. On page 101 of this issue, Lauschke et al.1 report that scaling persists in a tissue-culture model that simulates early segmentation in the vertebrate embryo. The simple, two-dimensional geometry of this system, and the fact that it can be visualized in real time and manipulated, opens exciting avenues for studying the formation and scaling of vertebrate segmentation*. The segmented organization of vertebrates is set up in the early embryo. As the embryo elongates along an anterior–posterior axis, segmented structures called somites bud regularly from the anterior end of its imma- ture presomitic mesoderm (PSM) tissue2–4 . The number of segments differs between spe- cies, but varies little between individuals of the same species. Seminal work by the develop- mental biologist Jonathan Cooke showed that surgical manipulations that reduce embryo size generate smaller yet well-proportioned embryos that are patterned normally along both anterior–posterior5 and dorso-ventral6 axes. In particular, somites become propor- tionally smaller, but their number and relative position are maintained5 . The observation that somite number and size are regulated independently prompted the ‘clock and wavefront’ model, which postulates that spatial and temporal inputs are combined todefinesomitesizeandposition7,8 .According to this model, the position at which a somite can be formed at a given time is defined by molecular concentration gradients that are positioned at a fixed distance from the pos- terior pole (the wavefront), and that move posteriorly through the PSM towards the pole as the embryo elongates. In parallel with this, cell-autonomous oscillations in gene expression that are coordinated across the tissue define the timing of segment formation. The overall outcome is sequential segment formation in an anterior-to-posterior direction. The predicted oscillations in gene activity have been visualized in chemically fixed and in live embryos, and correlate with the pro- gressive pattern of somite formation. Further- more, genetic manipulations of wavefront velocity or oscillation frequency modulate segment size3 , as predicted by the clock and wavefront model. In the intact embryo, oscillations are syn- chronized between adjacent cells, probably through the activity of the Notch signalling pathway. The frequency of oscillation in gene expression decreases towards the anterior PSM, so that anterior cells reach maximal signalling activity later than posterior cells. Therefore, the pattern of Notch activity seems to propagate from the posterior to the anterior PSM. This wave of molecular activity is not partoftheoriginalclockandwavefrontmodel. So, what could be the function of such waves? Do they contribute to somite differentiation or scaling? And what is the molecular basis for these dynamics? Answering these and related questions is greatly facilitated by the ability to visualize9 and perhaps perturb the differentiation process as it progresses10 . Although methods for live imaging of intact embryos have been developed, they are lim- ited, in part because of the complex geom- etry of the embryo. Lauschke et al. present an ex vivo (tissueculture) modelthatrecapitulates *This article and the paper under discussion1 were published online on 19 December 2012. 3 2 | N A T U R E | V O L 4 9 3 | 3 J A N U A R Y 2 0 1 3 NEWS & VIEWS RESEARCH © 2013 Macmillan Publishers Limited. All rights reserved