RESEARCH NEWS & VIEWS Resistance: Global Report on Surveillance (2014). 3. Ling, L. L. et al. Nature 517, 455–459 (2015). 4. Brown, E. D. Can. J. Microbiol. 59, 153–156 (2013). 5. Lewis, K. Nature Rev. Drug Discov. 12, 371–387 (2013). 6. Payne, D. J., Gwynn, M. N., Holmes, D. J. &

Pompliano, D. L. Nature Rev. Drug Discov. 6, 29–40 (2007). 7. Silver, L. L. Clin. Microbiol. Rev. 24, 71–109 (2011). 8. Wright, G. D. Can. J. Microbiol. 60, 147–154 (2014).
 9. Rinke, C. et al. Nature 499, 431–437 (2013). 10. Nichols, D. et al. Appl. Environ. Microbiol 76,

AST RONO MY

Cosmic fog and smog It emerges that most of the elements heavier than helium are not found in galaxies, where they can be mixed into future stars and planets. Instead, these elements largely reside far from galaxies in ionized gas and dust particles. M O L LY S . P E E P L E S

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ll of the elements on which life is based (carbon, nitrogen, oxygen, iron and so on) are formed in stars and in the explosive stellar deaths known as supernovae. Therefore, it seems reasonable to expect that they will be found where the stars and supernovae are located — in galaxies. However, studies by Shull et al.1 and Peek et al.2 have now revealed that the vast majority of these ‘metals’, as astronomers call all elements not produced in the Big Bang, reside far from the galaxies in which they were born, in the form of both ionized gas and complex molecules. The realization in 1957 that stars are crucibles for almost all the elements of the periodic table3 was immediately coupled with the idea

that metals could be used as tracers of star formation and of the flows of gas into, within and out of galaxies (Fig. 1) — much in the way that marked banknotes are used to trace the flows of cash within an economy. Studying metals in galaxies is relatively easy: stars shine brightly, and the gas and dust in galaxies are dense enough to be studied from the light they emit. Yet, as stars age and die, they expel freshly produced material from galaxies, into the circumgalactic medium (the diffuse gas roughly 20 times more extended than the galaxy itself) and perhaps even farther, into the intergalactic medium (the extremely diffuse gas between galaxies). This tenuous gas outside galaxies, however, is so thin that astronomers can observe it only through its effects on the light of bright distant sources passing through

Background light source

Intergalactic medium (diffuse gas between galaxies)

Circumgalactic medium (diffuse gas near galaxy)

Galaxy

Outflows

Observer

Figure 1 | Observing the diffuse gas outside galaxies.  Stars and supernovae in galaxies are the sites of creation of ‘metals’, but the energy and momentum from dying stars throw these elements (outflows) into the close vicinity of galaxies (the circumgalactic medium) and farther away (into the intergalactic medium). By studying how the light from bright background sources is affected by the diffuse gas outside galaxies, Shull et al.1 and Peek et al.2 have shown that most metals exist far from galaxies, in ionized gas and small, dusty particulates. 4 4 4 | N AT U R E | VO L 5 1 7 | 2 2 JA N UA RY 2 0 1 5

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2445–2450 (2010). 11. Marshall, C. G., Lessard, I. A., Park, I. & Wright, G. D. Antimicrob. Agents Chemother. 42, 2215–2220 (1998). This article was published online on 7 January 2015.

it (Fig. 1). These observations and subsequent interpretations are not easy, because the light passing through the intergalactic medium is affected only minutely by it. Most of intergalactic space is at densities and temperatures such that the gas consists primarily of ions that most strongly absorb light at ultraviolet (UV) wavelengths. Because Earth’s atmosphere (thankfully) absorbs most UV photons, this means that a space telescope must be used to see this gas in the local, modern-day Universe. Using their outstanding compilation from last year4 of archival UV spectra of bright background sources observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, Shull et al. investigated the carbon, nitrogen, oxygen and silicon littering the pervasive intergalactic medium. These data are mainly sensitive to the intergalactic cosmic densities of metals in certain ionized states (for example, C2+ and C3+ but not C4+; and O5+ but not O6+). Because the true intergalactic density of a given element is the sum of its densities in different ionic states, a tricky part of Shull and colleagues’ analysis was correcting these ionic densities for such unseen ionic states to determine to what extent metals have ‘polluted’ intergalactic space. Comparing their newly measured cosmic densities with the expected amounts of metals produced by the stars formed throughout all of cosmic history, Shull et al. conclude that 10±5% of all of the metals produced through cosmic time are found in the intergalactic medium, with the rest either still being in galaxies or relatively close by, in the circumgalactic medium. If Shull and colleagues’ approach to determining the abundances of intergalactic metals using their effects on bright background sources was akin to using a lighthouse to detect cosmic fog, then Peek et al. looked at the cosmic smog. They used the Sloan Digital Sky Survey5 to systematically measure the way in which small intergalactic dust particles (such as graphite, silicate and soot — the same metals that Shull et al. observed, but in a solid state) slightly redden background sources by extraordinarily small amounts; this effect is akin to how small particulates in Earth’s atmosphere make the Sun look redder than normal at sunset. By studying how more than 140,000 background galaxies with otherwise similar colours slightly redden depending on the location of foreground galaxies on the sky, Peek et al. could beat down statistical uncertainties to measure the profiles of dust in the vicinities of the foreground galaxies. They then used these profiles to calculate the total

NEWS & VIEWS RESEARCH mass of dust in the circumgalactic medium of foreground galaxies, concluding that, amazingly, more of these carbon- and silicon-rich molecules reside, not in the galaxies, but outside them. The combined results of these two studies point to a tantalizing conclusion: most of the elements constituting life are not found in galaxies, where they can be incorporated into future stars and planetary systems; instead, they are predominantly distributed thousands to millions of light years away from galaxies. Although it has been known, or strongly suspected, for decades that galaxies do not contain most of the metals that they have produced, it has been only in recent years that astronomers have been able to systematically locate, quantify and characterize this material outside galaxies. Both studies, however, highlight how little astronomers know — and thus how much remains to be discovered — about the detailed

physical conditions and hence the fates of the Universe’s metals. Shull and co-workers have produced an estimate of the inter­galactic density of metals that is for a time when the Universe is much more mature than the earlier epochs at which similar previous measurements6,7 were made. However, it is not a trivial task to connect these newly detected metals to the individual galaxies in which they were born. Although previous studies8 have indicated that there is a significant reservoir of intergalactic dust, Peek and colleagues’ analysis exquisitely shows how a galaxy’s dusty environment depends on the galaxy itself. Large surveys will be required to obtain similar measurements at earlier cosmic epochs, to decipher when galaxies expelled this dust. It is astounding that most of the potential building blocks of life are found so far from their birthplaces, and that so much of it can survive the trip out of galaxies as complex molecules. As our understanding of intergalactic

PAL A EO CLIMATE

Monsoon matters A simplified global climate model that keeps track of water as it moves through Earth’s water cycle throws fresh light on how the Asian summer monsoon has varied during the past 150,000 years. BRONWEN KONECKY

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he Asian summer monsoon (ASM) is the predominant source of rainyseason precipitation in tropical Asia, and is known to vary in response to global climate changes1. However, different geological archives from South and East Asia, such as cave deposits and marine sediments, provide conflicting accounts of how the ASM responds to basic climate forcings (Fig. 1) — for example, varying concentrations of greenhouse gases, the growth and collapse of ice sheets and changes in Earth’s orbit around the Sun. Writing in Nature Communications, Caley et al.2 use a simplified model to simulate Earth’s climate during the past 150,000 years, and find that this conflict may originate from our interpretation of climate signals recorded in stalagmites from Chinese caves. On timescales of thousands to millions of years, changes in the tilt of Earth relative to the plane of its orbit, the shape of the orbit and its slow wobble about its axis (or ‘precession’) regulate and distribute incoming solar radiation (insolation) that fuels the ASM. The monsoon is thought to intensify during minima in precession, approximately every 23,000 years, when Earth is closest to the Sun during Northern Hemisphere summer (unlike today, when this occurs in winter). Further strengthening

of the ASM occurs in part as a result of the growth and collapse of ice sheets covering northern Eurasia, which alter key wind patterns and shift the amount and location of East Asian rainfall. Greenhouse gases also influence the ASM by increasing the build-up

ASIA Tianmen

Sanbao

Hulu

Dongge

I N D I A N O C E A N

AUSTRALIA

Figure 1 | The Indo-Asian monsoon system.  The shaded branched arrow denotes prevailing monsoonal wind flow. Red dots represent locations of cave stalagmites, which provide records of past climate. Caley et al.2 question whether these stalagmites truly reflect variations in the Asian summer monsoon, in contrast to previous interpretations.

space in the local Universe increases, so does what this vast expanse of rarefied gas tells us about galaxies — and about just how rare and precious our own place in the cosmos is. ■ Molly S. Peeples is at the Space Telescope Science Institute, Baltimore, Maryland 21218, USA. e-mail: [email protected] 1. Shull, J. M. et al. Astrophys. J. 796, 49 (2014). 2. Peek, J. E. G., Ménard, B. & Corrales, L. Preprint at http://arxiv.org/abs/1411.3333 (2014). 3. Burbidge, E. M., Burbidge, G. R., Fowler, W. A. & Hoyle, F. Rev. Mod. Phys. 29, 547–650 (1957). 4. Danforth, C. W. et al. Preprint at http://arxiv.org/ abs/1402.2655 (2014). 5. Abazajian, K. N. et al. Astrophys. J. Suppl. 182, 543–558 (2009). 6. Cooksey, K. L. et al. Astrophys. J. 708, 868–878 (2010). 7. Cooksey, K. L. et al. Astrophys J. 762, 37–52 (2013). 8. Ménard, B. et al. Mon. Not. R. Astron. Soc. 405, 1205–1039 (2010).

of moisture in the atmosphere that is available to feed monsoonal precipitation. But how can we tell what caused ASM variations in the geological past? When decoding Earth’s climate history, timing is everything. The timing of semi-regular changes in rainfall relative to that of insolation, ice sheets and greenhouse-gas concentrations tells us which mechanisms played what part, and when. In many geological archives, ASM maxima seem to result from a combination of three factors: orbitally driven heating of the Asian landmass; release of energy in the southern Indian Ocean, where monsoon moisture originates; and minima in global ice volume. By contrast, stalagmite archives from South and East Asian caves — arguably the best-dated, highestresolution, most continuous records of Asian palaeoclimate recovered so far — suggest that local insolation is the dominant factor, with changes in global ice volume being secondary. This apparent contradiction in timing has long challenged scientists researching the under­ lying controls on the ASM. Like many rainfall proxies from the tropics, cave stalagmite records are based on the ratios of stable oxygen and hydrogen isotopes in water. The abundances of heavy but rare 18O and 2H relative to their lighter, more common counterparts (expressed as δ18O and δ2H, respectively) carry an imprint of that water’s history as it evaporated from the ocean, moved through the water cycle and eventually fell as rain or snow. When water gets incorporated into the calcite of cave stalagmites, it carries with it the isotopic signature that it had when it fell as precipitation. Precipitation δ18O is a proven tracer of hydrological and atmospheric circulation processes3. However, studies4,5 have shown that interpreting precipitation δ18O is not always straightforward, because it integrates many

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Astronomy: Cosmic fog and smog.

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