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Astronomy

Searching for the cosmic dawn

The Hubble Space Telescope, teaming up with a 'cosmic lens', has revealed a highly magnified galaxy thought to date back to 500 million years after the Big Bang. The find provides a glimpse of the first stages of galaxy formation. See Letter p.406

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Astronomers have long sought to trace the history of the Universe from its origins to the present day. Our earliest picture comes from the study of the cosmic microwave background radiation, which provides a portrait of the Universe when it was less than 400,000 years old. With not a single star yet formed at that time, the Universe was shrouded in darkness and permeated by newly created hydrogen atoms. The next available detailed picture comes nearly one billion years later and reveals a dramatically different landscape. Not only were galaxies containing billions of stars common, but the hydrogen that once filled most of space had become highly ionized between these galactic systems. In this issue, Zheng and colleagues1 (page 406) help to fill in the intervening time with the discovery of a galaxy that pushes the cosmic frontier back to just 500 million years after the Big Bang. When the galaxy's photons departed nearly 13.2 billion years ago, the Universe was less than 4% of its current age. By studying this early galaxy, Zheng et al. offer insight into when and how the first galaxies assembled, and whether the energetic radiation they produce was responsible for the 'reionization' of intergalactic hydrogen.

Zheng and colleagues' discovery follows in the footsteps of an exciting period that was ushered in by the installation in 2009 of Wide Field Camera 3 on the Hubble Space Telescope, which provided astronomers with a phenomenal leap forward in infrared imaging capability. Because the expansion of the Universe stretches the wavelength of light by a factor of 1 + z, where z is the redshift of an object such as a galaxy, observations in the infrared regime are a key domain in which to discover the earliest galaxies. To identify such systems, astronomers make use of the fact that light with wavelengths shorter than the redshifted hydrogen Lyman-α line limit, which is 0.1216 (1 + z) micrometres, is absorbed by intervening hydrogen clouds. For galaxies within the first 650 million years of cosmic history (redshifts greater than 8), such Lyman-α absorption extinguishes all light at optical wavelengths. So, by searching for galaxies that have a 'break' in their flux between the optical and near-infrared parts of the electromagnetic spectrum, astronomers can identify those galaxies that are most likely to lie at great distances.

But even with the deepest images yet obtained2,3 by Hubble's infrared camera, it has proved extremely difficult to break through to the first 500 million years of cosmic time. By identifying the characteristic Lyman-α absorption described above, researchers have unveiled3 more than 100 galaxies thought to lie between 650 million and 850 million years after the Big Bang, but only one galaxy had been found that could be dated back to 500 million years4.

To combat the difficulties imposed by the faintness of distant galaxies, Zheng et al. used a clever phenomenon called gravitational lensing. This technique relies on the principle that light rays from distant galaxies are bent and often magnified as they pass through the vicinity of massive objects on their way to Earth. By pointing telescopes towards such massive cosmic lenses (Fig. 1), for example a cluster of nearby galaxies, it is possible to detect distant galaxies that are bright enough for detailed study owing to the boost provided by gravitational lensing5,6.

Figure 1: A gravitational lens.
figure1

A. FRUCHTER (STSCI) ET AL./WFPC2/HST/NASA

Clusters of galaxies can act as cosmic lenses, bending and magnifying the light from distant background sources. This image by the Hubble Space Telescope of the massive galaxy cluster Abell 2218 reveals many lensed galaxies, one of which is so distant that its light left it when the Universe was just 800 million years old. Zheng et al.1 studied another galaxy cluster lens, MACS 1149+2223 (see Fig. 2 of the paper), which has revealed a distant galaxy thought to date back to 500 million years after the Big Bang.

Zheng and colleagues have been using Hubble's infrared camera to systematically search for distant magnified galaxies behind some of the most massive nearby galaxy clusters. After analysing 12 galaxy clusters using the Lyman-α absorption technique, their efforts finally yielded success, uncovering a galaxy thought to lie just 500 million years after the Big Bang. The foreground cluster of galaxies magnifies the galaxy's light by a factor of 15, allowing its properties to be dissected in greater detail than if it had been found by conventional methods.

Crucial to the authors' finding were observations conducted with the Spitzer Space Telescope, which probes infrared light from old stars. These observations indicate that the galaxy had a significant component of old stars. The authors estimate that stars had been forming in the galaxy for up to 200 million years, building up a stellar mass 150 million times that of the Sun. If this system is representative of galaxies at this redshift, it would suggest that vigorous star formation was already occurring in galaxies by 300 million to 500 million years after the Big Bang. The energetic radiation emitted by these systems could ionize a significant fraction of intergalactic hydrogen within just 500 million years of the Big Bang, consistent with expectations from measurements of the polarization of the cosmic microwave background radiation7.

Zheng and colleagues' discovery will stimulate further searches for galaxies at this early epoch, and much work remains to be done. Currently, the number of sources that can be dated back to 500 million years after the Big Bang (just two1,4) is too small for reliable measures of their number density to be extracted. Moreover, without spectroscopic observations to complement the images, the galaxies' distances from Earth cannot be determined unambiguously. Some progress on both fronts is expected in the coming years from surveys conducted with the Hubble and Spitzer telescopes, as well as with new infrared spectrographs that have been installed on ground-based telescopes.

Within the next decade, however, the exploration of galaxies in the early Universe will be transformed by the construction of giant ground-based telescopes with apertures of 20–40 metres and by the launch of the James Webb Space Telescope. These powerful facilities will not only dramatically increase the number of galaxies known throughout the first 500 million years, but will also provide the spectroscopic capability necessary to confirm their distances. Through spectroscopy of highly magnified galaxies such as that reported by Zheng et al., these studies can even begin to reveal the galaxies' chemical make-up and the kinematics of the gas they contain, leading to a much-improved understanding of when galaxies emerged and how their radiation contributed to the reionization of hydrogen.

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Correspondence to Daniel Stark.

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Stark, D. Searching for the cosmic dawn. Nature 489, 370–371 (2012). https://doi.org/10.1038/489370a

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