Astrophysics

First results from Planck observatory

Early data from the Planck space satellite provide information about dust in distant galaxies, as well as in the Milky Way, and on the properties of gas in some of the largest clusters of galaxies in the Universe.

Astronomers have long known1 that most of the stars in the Universe are born in messy environments containing dusty clouds. Young stars in such dust-enshrouded regions are not visible to optical telescopes; thus, multi-wavelength studies, from the radio to the X-ray regime, are used to better understand how stars form in our Galaxy. But for more distant galaxies, including some of the first galaxies in the Universe, such dusty expanses are essentially invisible across most wavelengths. One exception is the wavelengths in the far-infrared and microwave regimes, which are roughly 1,000 times longer than those of visible light. Stars heat up the dust surrounding them to temperatures of roughly 20 kelvin — much lower than that of the stars themselves, but nevertheless high enough for the dust to radiate microwave and far-infrared light. This warm-dust signature, called the cosmic infrared background, has now been observed2 by a large team of astronomers working with data from the Planck space observatory. The results are part of a series of studies that form a collection of 26 papers, published by the Planck team in Astronomy & Astrophysics (see go.nature.com/au8vap).

The Planck satellite's measurement of the cosmic infrared background2 improves on previous measurements, including data3 obtained by Herschel, a twin observatory to Planck launched by the European Space Agency aboard the same rocket in 2009. The rocket carried them to the Earth–Sun Lagrangian point L2 (1.5 million kilometres from Earth in the opposite direction from the Sun), where the satellites can be stationary relative to both the Sun and Earth, allowing for shielding from the Sun's radiation.

Planck detects microwave light in several wavelength bands in which the warm-dust emission can be observed (Fig. 1). Because the Universe is expanding and the wavelength of light stretches with the expansion, the light that we observe has a longer wavelength than it had at its source. This means that, for the same dust temperature, observing the dust emission at the longer wavelengths corresponds to observing an epoch when the Universe was smaller, and hence younger. By measuring the dust at different wavelengths, Planck can track the emission from star-forming galaxies as a function of cosmic time. Planck's observations2 suggest that most of the emission in the longer-wavelength bands comes from galaxies that formed at a time when the Universe was less than 2 billion years old (the age of the Universe today is approximately 14 billion years).

Figure 1: The cosmic infrared background.
figure1

ESA/PLANCK COLLABORATION

The images show the anisotropies, or irregularities, of the cosmic infrared background in three of the frequency channels (217 gigahertz, 353 GHz and 857 GHz) probed by the Planck observatory2 over a 26° × 26° patch of the sky. The anisotropies are visible as globular structures and correspond to dusty galaxies clumped together on large scales. As we move across frequency channels, different epochs of cosmic time become visible: observations at 217 GHz offer a glimpse of some of the oldest galaxies in the Universe, which formed when the Universe was less than 2 billion years old.

To achieve this measurement, the Planck team performed2 a sophisticated software analysis called component separation. This was required because these wavelength bands contain radiation from many other sources, mostly the Milky Way, but also the cosmic microwave background (CMB, relic radiation from the early Universe glowing at 2.7 K). The strengths of these sources vary differently as a function of wavelength. By combining Planck's nine wavelength bands with additional external measurements, the team was able to separate the cosmic-infrared-background component from the other sources of radiation. The authors found2 a broad agreement in results between different areas in the sky, which had been specially chosen for having low radiation from our Galaxy, suggesting that the component separation was successful.

The emission from the Milky Way is not just a contaminant of the cosmic infrared background; it also contains some surprises of its own. One of these relates to the 'anomalous microwave emission' at centimetre wavelengths. This has been known about for a few years, but its origin has been controversial. In particular, although this radiation has been observed4 to correlate with the emission from small dust grains in the Galaxy, simple models of thermal emission from dust could not explain its wavelength dependence. However, if the dust particles are spinning at high rates, they can radiate at a wavelength that relates to their spinning frequency and size. In this spinning-dust model, the emission occurs over a relatively narrow range of wavelengths that happens to coincide with the longest-wavelength band of the Planck observatory. Planck's observations of emission from the Milky Way provide5 strong support for the spinning-dust model.

Not all of the results from Planck are related to dust radiation. Light propagating through hot gas can be scattered off electrons zooming around these high-temperature regions. The result of this process, named the Sunyaev–Zeldovich (SZ) effect after the two Russian scientists who first proposed6 it, is that longer-wavelength light is shifted to shorter wavelengths. When viewed against the background provided by the CMB radiation, this effect leads to a dark hole at longer wavelengths at the position of a gas clump on the sky. Similarly, it causes a bright peak of light at shorter wavelengths at the same position.

With Planck's many wavelength bands, both of these features can be observed, leading to a convincing detection of the SZ effect. The sources most likely to provide a detectable SZ signal are the most massive galaxy clusters, which contain huge amounts of some of the hottest gas in the Universe. The Planck team found7 nearly 200 cluster candidates with this technique, of which about 20 were previously unknown. Most of these have subsequently been confirmed as real clusters by follow-up studies, including X-ray observations8 with the XMM-Newton satellite. Combined analysis of these data provides detailed information about the gas density and temperature distribution in the clusters, resulting in a better understanding of the processes that led to their formation.

These new results7 demonstrate that it is possible to find clusters of galaxies with the SZ technique even for surveys looking at the entire sky, in contrast to previous SZ detections — by the South Pole Telescope9 and Atacama Cosmology Telescope10 — that searched smaller patches of the sky. Ultimately, the SZ method will allow clusters to be observed at a much larger distance from Earth than is possible with other methods, such as X-ray emission. One exciting application of the SZ approach would be to probe the growth of the largest (and thus rarest) structures at early times. Such observations would provide a measurement of the different components that make up the Universe and of the size of the initial density fluctuations that eventually grew to become galaxies and galaxy clusters.

The early results from Planck demonstrate that the observatory is working flawlessly, and provide a first glimpse of its scientific potential. However, the best is yet to come. The main mission of Planck is to map the CMB radiation and its polarization with unprecedented precision. This measurement will provide a window onto the early Universe and offer clues as to what created the first seeds of structure. Planck may also detect the relic gravity waves from the Big Bang through the observations of CMB polarization. The task is complicated by the relative faintness of the CMB compared with other sources of radiation, such as dust emission, in most of the wavelength bands. Careful separation of components is thus needed to isolate the CMB signal, a task that has proved challenging and is the main reason that these early results do not include any primary CMB data. These CMB results are expected to be announced in early 2013. Given the spectacular instrument performance of Planck shown by its early findings2,5,7, the cosmology community is eagerly awaiting more results.

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Correspondence to Uroš Seljak.

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Seljak, U. First results from Planck observatory. Nature 482, 476–477 (2012). https://doi.org/10.1038/482475a

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