Charting cosmic history

Somewhat unexpectedly, the Hubble Space Telescope (HST) proved capable of capturing the light from extremely distant galaxies, seen at the brink of cosmic time. One of the main goals of its successor, the James Webb Space Telescope (JWST), is to better understand the nature of the first generation of stars and black holes forming inside these infant galaxies. Thanks to its vastly improved sensitivity at near-infrared wavelengths compared to previous telescopes, as well as the unique spectroscopic capabilities of the NIRSpec instrument, we are now able to explore previously uncharted cosmic territory.

JWST now routinely discovers galaxies so distant, that the light we are seeing traces back to 13 billion years ago, when the Universe was less than 5% of its current age. Like vast mountain ranges, the cosmic landscape at the time was slowly beginning to be shaped by gravity. These primordial structures were still veiled in a dense 'fog' of cold, neutral clouds. Early galaxies, too, are blanketed by this pervasive fog in a way that completely blocks their extreme ultraviolet (EUV) light, similar to how a sophisticated pair of sunglasses blocks out the Sun's harmful UV radiation.

However, the steadily growing population of galaxies gradually heated the opaque, neutral gas into a transparent plasma—in other words, their light started clearing the fog—in a process called reionisation.

Several JWST surveys have been dedicated to studying galaxies within the so-called reionisation era, where their redshifts—a measure of cosmic distance, quantifying how much the light has been stretched due to the ongoing expansion of the Universe—take up extreme values of 6 or higher. One such survey, the JWST Advanced Deep Extragalactic Survey (JADES), already revealed several hundreds of these, including a select few beyond the magical redshift z = 10 barrier. Most recently, one galaxy found in JADES that was suspected of having a mind-boggling redshift of z = 13 had its distance confirmed, officially earning it the code name JADES-GS-z13-1.

Mysteries of the early Universe

At this distance, the light from the galaxy captured by JWST has been travelling over 13.4 billion years. That is the vast majority of the current age of the Universe (13.8 billion years), meaning the signal dates back to when the cosmos was 'only' 330 million years old. GS-z13-1 measures less than 230 light years across—several hundred times smaller than our own galaxy, the Milky Way—yet is forming stars at approximately the same rate, about one per year.

Remarkably, this galaxy also shows a clear, telltale signature that can only be seen once the surrounding fog of neutral gas has completely lifted, suggesting that this galaxy has made an unexpectedly early start to reionisation. In fact, directly seeing the so-called Lyman-α radiation coming from hydrogen atoms was so surprising that when first noticed by the JADES collaboration, it was met with a healthy dose of skepticism.

After all, although JWST has been discovering a larger number of galaxies with redshifts greater than 10 than we had expected, it has been firmly established that the reionisation process should not be completed until 600 million years later. Moreover, not even a hint of Lyman-α has been seen in other newly discovered galaxies at the redshift frontier—let alone anything comparable to the exceptional strength displayed in GS-z13-1. Why would it be any different?

An early start to reionisation?

The answer may lie in the fact that the locations where most galaxies form, and hence where the fog is first lifted, are distributed very unevenly. Rather than reionisation occurring in one big flash, the fog likely clears quickly around peaks of the cosmic landscape where the first galaxies are born, while lingering in the shadowy valleys. It is probable that GS-z13-1 is in the right place, at the right time.

Nevertheless, even if it is located in one of the first small fog-less patches, the sheer luminosity of Lyman-α emission we see requires a flood of energetic light to rush from the galaxy. Most likely, such powerful radiation cannot be explained as coming from regular stars. The powerful bursts of light may be a sign of extraordinarily massive, hot stars. Alternatively, we might not be seeing starlight, but a supermassive black hole actively devouring large amounts of gas.

While both pose intriguing possibilities, the former may bring us one step closer to what is widely considered the holy grail in extragalactic astronomy: to look back far enough in time to directly see the birth of the very first generation of stars. Made from pure hydrogen and helium, these stars are thought to reach enormous masses and shine exceedingly brilliantly.

Further observations of GS-z13-1 and other similar galaxies will be necessary to put this and alternative scenarios to the test. Whatever is concealed inside GS-z13-1, however, it already sheds an unforeseen light on how the first galaxies formed and influenced their surroundings. It will only be a matter of time until JWST discovers a new clue to this fascinating puzzle.

Links to further reading

Publications

• Publication (open access)
• News & Views article (by Michele Trenti)

Press releases

• European Space Agency (ESA)
• Space Telescope Science Institute (STScI)
• NASA
• Cosmic Dawn Center (DAWN)
• University of Cambridge

Other links

• JADES website (with interactive image viewer)