April 2009 Archives

Only a handful of new stars form every year in the Milky Way, a typical quiescent galaxy. But the cosmic history of star formation is dominated by galaxies in their brief starburst phases, when prolific star formation makes them glow in the far-IR with thermal radiation around 100 μm from cocoons of dust warmed by very bright young stars. Earth's atmosphere blocks far-IR radiation at wavelengths below 800 μm, and no space telescope has yet had adequate resolution above 24 μm to resolve into discrete sources the diffuse cosmic far-IR background (FIRB) discovered 15 years ago by the COBE orbiter. Now the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST) has produced maps of patches of sky that reveal more than 500 starburst galaxies luminous at submillimeter wavelengths from 200 to 600 μm, where the FIRB is brightest. The BLAST collaboration, led by Mark Devlin (University of Pennsylvania), concludes that the entire FIRB can be accounted for by individual galaxies. The group finds that at those submillimeter wavelengths, the FIRB is dominated by distant, high-redshift starburst galaxies from long ago. Augmented by redshift measurements of the starburst galaxies at other wavelengths, the BLAST observations make it possible to document the falling cosmic rate of star formation since its heyday at redshifts around 3,  just 2 billion years after the Big Bang (see the figure). (M. J. Devlin et al., Nature 458, 737, 2009; E. Pascale et al., http://arxiv.org/abs/0904.1206v1.) — Bertram Schwarzschild

The Indus Valley civilization, in what is now eastern Pakistan and northwestern India, flourished circa 2500-1900 BCE. To this day its writing, as in the figure, has not been deciphered. Indeed, scholars are unsure if the Indus script represents a language. Other, superficially similar ancient texts are thought to be either rigidly prescribed strings, such as a hierarchical list of deities, or nonlinguistic strings in which order is unimportant. Now computer scientist Rajesh Rao (University of Washington) and colleagues from several Indian institutions have studied the correlations of neighboring tokens (symbols or words) with a statistical tool—the conditional entropy—that reliably distinguishes natural languages from token strings in which the ordering is rigid or unimportant. The Indus script, they conclude, has the structure of a language. Like the conventional entropy, the conditional entropy involves the logarithm of a probability—in this case the conditional probability that a specified token appears, given its immediate antecedent. Rao and colleagues identified the N most common tokens in the Indus script, various languages, and nonlinguistic systems and plotted the conditional entropy against N. The curves for the Indus system and the natural languages bunched in the middle and were clearly distinct from those corresponding to rigid or unimportant orderings. And the conditional entropy of the Indus system seemed especially closely related to Old Tamil, consistent with the conclusions of scholars who have analyzed the Indus script with more conventional means. (R. P. N. Rao et al., Science, 2009, doi:10.1126/science.1170391. Photo courtesy of J. M. Kenoyer / Harappa.com.) — Steven K. Blau

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Ancient Scripts: Indus Script

The unusual stiffness or sponginess of dead and decaying biological tissue is readily apparent to the human touch. However, early detection of such mechanical property changes in a tissue's extracellular matrix could signal the onset of disease. To measure the elasticity of tissue in living patients, needle-based indentation methods are more direct and less expensive alternatives to MRI, ultrasound, and electrical impedance. Such a probe has recently been developed by University of California, Santa Barbara, physicist Paul Hansma, an atomic force microscopy expert, and his collaborators. The handheld tissue diagnostic instrument (TDI) consists of a stainless steel probe, 175 µm to 1 mm in diameter depending on the tissue sample, which longitudinally oscillates at 4 Hz in a needle-thin stationary sheath. The force from the magnetically controlled oscillation of the probe produces a corresponding displacement in the tissue. The tissue's elastic modulus, or stiffness, is proportional to the slope of the force-displacement curve, and energy dissipation in the tissue is proportional to the area under that curve. The researchers measured, with millimeter spatial resolution, healthy and diseased tissue samples ranging in elastic moduli from around 1 kPa to 12 GPa. Among them were mouse breast tissue, which hardens when it becomes tumorous, and human tooth dentin (see schematic), which softens and decays when infection sets in. The researchers say the instrument could be used in the future to simultaneously test and biopsy a tumor or, if the probe is coated with antibodies, to measure single-molecule interaction forces. (P. Hansma et al., Rev. Sci. Instrum., in press.) — Jermey N. A. Matthews

The chemists' familiar lineup of bond types has a new member. Joining ionic, covalent, hydrogen, and van der Waals is an unusual ultralong bond between two similar atoms. The new dimers consist of rubidium atoms in their ground state and other rubidium atoms in highly excited Rydberg states. Tilman Pfau of the University of Stuttgart in Germany and his coworkers made the dimers after first trapping Rb atoms under conditions just shy of what’s needed to make a Bose-Einstein condensate. Holding the dimers together is a weak, wavy potential whose ability to bind atoms under or near BEC conditions was predicted in 2000 by Chris Greene, Alan Dickinson, and Hossein Sadeghpour. Enrico Fermi had identified the potential’s underlying interaction back in 1934, well before BECs made their laboratory debut. Because the potential is so wide and weak, it binds the two atoms only if they’re far apart. The dimers are big indeed. At 80 nm, they’re wider than a ribosome, the 5-megadalton macromolecule that translates our DNA into protein, and longer than the transistor gates in the newest, most powerful microprocessors. Pfau’s group made the simplest, most stable of the dimers that Green and company predicted. Other, more exotic dimers from the same family, dubbed trilobites (see image) and butterflies by Greene, are next on Pfau’s to-do list. (V. Bendkowksy et al., Nature , in press; http://arxiv.org/abs/0809.2961.) — Charles Day

A limiting factor in hydrogen fuel-cell performance is the slowness of the oxygen-reduction reaction that occurs at the cathode. Platinum is an effective catalyst for promoting that reaction, but it is both expensive and scarce. The performance of alternatives based on transition metals such as iron or cobalt has been disappointing to date. Now experimenters led by Jean-Pol Dodelet at Canada’s Institut National de la Recherche Scientifique have found a way to make an Fe-based catalyst that rivals Pt. The current density of a fuel cell made from the new catalyst can equal that of a Pt-based fuel cell at cell voltages ≥ 0.9 V. The method used by Dodelet and coworkers is the culmination of systematic research that delineated the key steps to making the catalyst. One step was to use a ball-milling technique to force iron acetate and another ingredient (PTDA or phenanthroline) into the micropores of carbon-black particles (see the figure). Challenges still lie ahead. Below 0.9 V, the voltage of fuel cells made from the new Fe-based catalyst falls off too rapidly with increasing current density, largely because the diffusion of oxygen into the catalytic sites is not optimized. And the lifetime of the catalysts is limited by the instability of its materials. Still, those working on such alternative catalysts are greatly energized by the new results. (M. Lefèvre et al., Science 324, 71, 2009.) — Barbara Goss Levi

Ultrashort, ultraintense laser pulses undergo competing interactions: The nonlinear Kerr effect self-focuses the beam, while multiphoton ionization generates a plasma that defocuses the beam and prevents it from collapsing. The result is a self-channeled, nondiffracting beam with a tight core, termed a filament, consisting of the intense laser field and the generated plasma (see Physics Today, August 2001, page 17). Filaments emit broadband light in the forward direction and are self-healing, properties that yield a variety of applications, including remote atmospheric sensing and spectroscopy.
Recent work by Pavel Polynkin (University of Arizona), Demetrios Christodoulides (University of Central Florida), and colleagues has put a new twist on the filaments. Unlike earlier studies, which relied on Gaussian or other axially symmetric beam profiles, Polynkin and company used axially asymmetric beams: With a phase modulator, they shaped the transverse profile of their femtosecond pulses into the form of a two-dimensional Airy function. The resulting beams remained diffraction free, but their peak intensities followed a parabolic trajectory reminiscent of projectile motion. (Momentum was still conserved, however, thanks to the momentum of the other parts of the beam.) The figure shows the calculated plasma density that accompanies a 5-mJ Airy beam as its peak traces its parabolic path. The curvature could be controlled experimentally by changing the focal lengths of the lenses used. The forward emission from curved laser filaments could find use as a broadband, wide-angle illumination source for remote sensing and for laser-induced breakdown spectroscopy. (P. Polynkin et al., Science 324, 229, 2009.) — Richard J. Fitzgerald

The venoms from spiders, scorpions, some marine snails, and certain other animals immobilize victims by blocking ion channels that control nerve cells. The bioactive molecules in the venoms are incredibly diverse—cone snails alone produce more than 50 000 distinct peptide venoms—and researchers hope to mine them for potential pharmaceuticals that, say, kill pain or unlock diseased ion channels. Knowing the amino acid sequences would help in that effort. Typically, researchers turn to mass spectrometry, in which the peptides are fragmented and the amino acid sequence deduced, usually in combination with searching a protein database. Unfortunately, the organisms do not have sequenced genomes, so the amino acid sequence has to be determined from mass spectrometry alone. Such de novo sequencing has been hampered by an inability to produce sufficient fragmentation. Now, Beatrix Ueberheide, David Fenyö, and Brian Chait of the Rockefeller University and Paul Alewood of the University of Queensland have devised a method that solves that problem. They realized that a simple chemical trick—the conversion of cysteine, an abundant amino acid in peptide venoms, to a lysine-like charged residue—would put the molecules in a highly positively charged state. They could then be more efficiently fragmented using a technique known as electron transfer dissociation and give rise to a rich mass spectrum. As proof of principle, the team reconstructed the complete sequence for 31 distinct peptide toxins using just 7% of the venom from the gland of a single cone snail. (B. M. Ueberheide et al., Proc. Natl. Acad. Sci. USA , in press.) — R. Mark Wilson

Before they form snowflakes and other hexagonal crystals, water molecules nucleate in smaller configurations. Determining the structure of those precursors—even in the outwardly simple case of water on a clean metal surface—is an area of ongoing interest and controversy. For example, at submonolayer coverage on a copper (110) surface, water molecules form chains that can grow to many tens of nanometers in length but are just 1 nm wide. The chains’ structure has been a mystery, since no arrangement of water molecules into hexagonal units entirely explains the experimental data. Now, Andrew Hodgson and colleagues of the University of Liverpool in the UK have collaborated with Angelos Michaelides’ group at University College London to find the structure. Michaelides and postdoc Javier Carrasco ran calculations on some 50 possible chain structures; they found that the most energetically stable one also gave the best fit to the Liverpool group’s high-resolution scanning tunneling microscopy images (as shown in the top panel) and vibrational spectra. That structure (bottom panel) is an arrangement of pentagons, not hexagons. The water molecules shown in red and yellow are perpendicular to the Cu surface—the hydrogen atoms pointing up are responsible for the bright spots in the STM images, and the ones pointing down (not visible in the figure) interact with the Cu atoms. The researchers suggest that nonhexagon arrangements might be involved at other water–metal interfaces where the structure of water is unknown. (J. Carrasco et al., Nat. Mater., doi:10.1038/nmat2403.) — Johanna Miller

In the standard model of particle physics, the predicted and much-sought Higgs boson (H) remains the principal missing link. The theory attributes the nonzero masses of the quarks, leptons, and weak vector bosons to their interaction with the H's quantum field. Searches at CERN's Large Electron–Positron collider have put a lower limit of 114 GeV (about 120 proton masses) on the H mass, and theoretical analysis of a variety of well-measured particle-physics parameters suggests an upper mass limit of about 185 GeV. Now a significant bite has been taken out of the interval 114–185 GeV by a new analysis of Higgs-search data accumulated in nine years of running at Fermilab's 2-TeV Tevatron proton-antiproton collider. The analysis, a combined undertaking of the large CDF and D0 detector collaborations at the collider, concluded with a confidence limit of 95% that the H mass does not lie between 160 and 170 GeV—presuming that its production and decay properties are those predicted by the standard model (see the figure). The combined data set comes from 10¹⁵ proton–antiproton collisions, but the teams had to limit their searches to events that produced a W or Z weak vector boson. Only a few percent of H-producing collisions are expected also to produce a W or Z. But the decays of those very heavy particles are spectacular enough to provide a discernable signal under the haystack of routine events that would otherwise hopelessly obscure the tiny fraction of events that create an H. The two collaborations expect to accumulate a lot more data before CERN's new 14-TeV Large Hadron Collider joins the Higgs search early next year. (CDF and D0 collaborations, http://arxiv.org/abs/0903.4001v1.) — Bertram Schwarzschild