Shortly after the Big Bang, the Universe was completely dark. Stars and galaxies, which provide the Universe with light, had not yet formed, and the Universe consisted of a primordial soup of neutral hydrogen and helium atoms and invisible ‘dark matter’. During these cosmic dark ages, which lasted for several hundred million years, the first stars and galaxies emerged. Unfortunately, observations of this era are challenging because dark-age galaxies are exceptionally faint1. Writing in Nature, Willis et al.2 provide a glimpse of what happened during the dark ages by doing some galactic archaeology. By measuring the ages of stars in one of the most distant clusters of galaxies known, the authors located galaxies that formed stars in the dark ages, close to the earliest possible time that stars could emerge.

Read the paper: Spectroscopic confirmation of a mature galaxy cluster at a redshift of 2

A galaxy cluster is a group of thousands of galaxies that orbit each other at speeds3 of about 1,000 kilometres per second. They are prevented from flying apart by the gravitational pull of the accompanying dark matter, which has the equivalent total mass of about one hundred trillion Suns4. Astronomers use these clusters as laboratories for many experiments in astrophysics, such as measuring the composition of the Universe, testing theories of gravity and determining how galaxies form. Willis et al. used one of the most distant clusters known to study when the most massive galaxies in the Universe began to produce stars.

Although nearby clusters, such as the Coma cluster, are easier to observe than those farther away, we cannot measure their ages precisely because the galaxies are extremely old. It is difficult to differentiate between, for example, a galaxy that is 7 billion years old and one that is 13 billion years old5. Therefore, to obtain a precise date for when clusters first formed their stars, Willis and colleagues used NASA’s Hubble Space Telescope to look at one of the most distant clusters they could find.

Because light travels at a finite speed, the most distant clusters we can see are also those in the earliest stages of the Universe that we can see. The light from the cluster examined by Willis et al. has been travelling for 10.4 billion years before it reaches Earth, which means that we are looking at a cluster as it was just 3.3 billion years after the Big Bang. Consequently, this cluster acts as a keyhole through which we can peer into the early Universe (Fig. 1).

Figure 1 | Chronology of the Universe. After the Big Bang, the Universe consisted of a cosmic soup of radiation and matter. About 400,000 years later, it entered an era known as the cosmic dark ages in which it was devoid of light. The first stars and galaxies began to emerge a few hundred million years later, and gradually provided the Universe with light. Willis et al.2 report that star formation in a distant cluster of galaxies began roughly 370 million years after the Big Bang. The light that we see from this galaxy cluster was emitted when the Universe was about 3.3 billion years old. The cluster is likely to have become one of the largest structures in the present-day Universe, comparable in mass to the Coma cluster.Image credits: Willis and colleagues’ galaxy cluster: N. A. Hatch; Coma cluster: Russ Carroll, Rob Gendler, Bob Franke/Dan Zowada Memorial Observatory, Wayne State Univ.

Willis and colleagues found that the cluster contains several galaxies that have similar red colours. The colour of a galaxy can be used to estimate its age because younger stars are bluer than their older, redder counterparts. As a result, galaxies that have red colours formed their stars a long time ago5. By comparing the colours of the cluster galaxies with those of models, the authors estimated that the stars of these galaxies started to emerge when the Universe was only 370 million years old. This epoch is when we expect the first stars to have formed in the cosmic dark ages6.

Evidence of ancient Milky Way merger

One particularly intriguing point is that Willis et al. identified at least 19 galaxies in the cluster that have similar colours, which means that the galaxies have similar ages. At the time when these galaxies formed their stars, they would have been well spread out, so it is a conundrum as to why they all began producing stars at approximately the same time. Were they influenced by their environment? Alternatively, did the star formation in one galaxy somehow trigger a chain reaction, leading to star formation in nearby gas clouds? We do not currently have the answer, but what is clear from the authors’ work is that these distant clusters are full of the oldest galaxies in the Universe.

In my opinion, Willis and colleagues’ age estimates are the best ones possible, given the limited data that the authors have from the Hubble telescope. However, determining ages from the colours of galaxies is a relatively crude method that is subject to large uncertainties. For example, a young galaxy that contains a lot of astronomical dust can have the same colour as an old galaxy containing little dust. Therefore, although the authors’ results are tantalizing, they should be treated with caution until NASA’s James Webb Space Telescope (JWST) is launched in the next few years.

The JWST will measure spectra of the light emitted by these galaxies. A comparison of the spectra with models will be a much more accurate way to determine the ages of the stars than using the colours of galaxies. Furthermore, because it is easier to measure the ages of earlier galaxies than those of more recent ones5, it makes sense to target galaxies in the progenitors of these galaxy clusters in the early Universe. Willis and colleagues’ results make a strong case for these distant clusters being some of the first targets that the JWST should observe.