It is springtime in the Northern hemisphere. Countless buds that have been waiting patiently on the stems and branches of trees and shrubs are now blossoming into life. The cosmic equivalent of this season is the time between a few hundred million and a billion years after the Big Bang. This is when the first stars and galaxies ignited, spewing light into the dark universe.
It is a time in the history of the universe that we are desperate to chart, because it represents part of the cosmological story that we have yet to understand. Now astronomers have detected oxygen in a galaxy further away than ever before – and it existed just 500m years after the Big Bang. The results, published in Nature, are hugely important as they provide new insights into when the first stars formed.
The period of this “cosmic dawn” is important not only because this is when the first galaxies were born, but a crucial cosmic transition also took place. In this process, atoms in the electrically neutral intergalactic medium – a wide sea of hydrogen gas surrounding galaxies – were bombarded with ultraviolet radiation escaping from the first galaxies. This stripped away electrons from atoms and made the gas charged, or “ionised”.
The event, called the Epoch of Reionisation, is still mysterious. We’d like to know – or better yet, see – when this process started. Part of that quest involves finding the most distant galaxies.
When we look out into the universe we detect light that has taken some appreciable time to traverse the gulf that separates us from other stars and galaxies. The light from the screen you are reading this on has taken about a third of a nanosecond to reach your eyes. Light from the nearest star beyond our sun takes four years to reach us. Amazingly, light from the galaxy at the centre of the new study, called MACS1149-JD1, has taken 13 billion years to be detected here on Earth. That means we see MACS1149-JD1 as it was 13 billion years in the past, around 500m years after the Big Bang.
Using a telescope called the Atacama Large Millimetre/sub-millimetre Array (ALMA), the scientists detected a strong signal (an emission line) within the distant galaxy. Just as a prism disperses the light of the sun into a rainbow spectrum, we can disperse the light of distant galaxies, too. This is called spectroscopy. Emission lines are bright spikes in the spectra of galaxies that originate from different elements that can each release light of a very specific energy.
This particular emission line came from ionised oxygen gas. Its presence tells us that the galaxy was forming stars at the time, because the energy required to ionise it must have come from massive, hot, young stars.
If we measured the same type of gas here on Earth, we would detect it at a wavelength of 0.088 millimetres. But other galaxies are receding away from us due to cosmic expansion, and this causes the light they emit to increase in wavelength during the time it takes for the photons to reach us. The more distant a galaxy is, the larger the increase in wavelength.
This is called redshift, and it ultimately tells us the ratio between the size of the universe when the light was first emitted and the size of the universe today. The oxygen emission line observed in MACS1149-JD1 is actually detected at 0.88 millimetres – its wavelength has been stretched by a factor of 10. This means that at the time the light was emitted, the universe was a factor of 10 times smaller than it is today, and just four per cent of its present age.
In this way, the ability to detect emission lines in distant galaxies allows us to pinpoint at what stage in cosmic history we are seeing them. But of course, the most distant galaxies are also the faintest – you need ever more powerful telescopes if you want to peer back further.
ALMA (consisting of 66 individual telescopes working together) is an incredibly powerful telescope – it is revolutionising our view of the early universe. Not only is it providing exquisite sensitivity, but operates in part of the electromagnetic spectrum that gives access to a wide range of emission lines.
To help matters, the team also exploited a natural telescope: a massive cluster of galaxies. Light from MACS1149-JD1 has had to pass through this intervening cluster on its journey to ALMA. This is so massive that it significantly warps spacetime, meaning that the light is “bent” in a process called gravitational lensing. Gravitational lensing amplifies the brightness of MACS1149-JD1, making it a little easier to see.
Indirect glimpse of first stars
MACS1149-JD1 is not the most distant galaxy on record, but what this new study adds to our understanding is an insight into the history of the formation of the galaxy. This happened hundreds of millions of years before the current observation, and much further back than even the most distant galaxy known.
In fact, the presence of oxygen in the galaxy tells us that star formation must have been going on for some time in MACS1149-JD1. That’s because oxygen can only be formed within stars in a process called stellar nucleosynthesis. But what we don’t know is when those stars first ignited.
By combining data from the Hubble Space Telescope, the European Southern Observatory’s Very Large Telescope and the Spitzer Space Telescope, the authors made a model of the “stellar population” within MACS1149-JD1. This allowed them to estimate the mixture of stars that give rise to the emission from the galaxy observed in certain bands of the electromagnetic spectrum.
The model involves estimating the “star formation history” of the galaxy, describing the rate of production of stars in the past. The modelling suggests that, in order to produce the observed emission, stars must have started forming just 250m years after the Big Bang, when the universe was just two per cent of its present age. In other words, MACS1149-JD1 was already a fairly well established galaxy, even at this early time.
This is a huge scientific accomplishment as it is currently impossible to observe galaxies that existed 250m years after the Big Bang. However, the new James Webb Space Telescope, which is due for launch in 2020, may be able to do so.
But until then, thanks to the new study, we now have a way of indirectly studying when stars first formed in ancient galaxies like MACS1149-JD1. In effect, by observing the blossom, astronomers have estimated when the bud first opened.