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The First Galaxies in the Universe

Credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit

Credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit

By Stuart Wyithe

By measuring the spatial distribution of cosmic hydrogen, the SKA will help to answer some of the biggest missing pieces in our knowledge of the universe’s history, including when the first galaxies formed and how they influenced the universe around them.

As part of the fundamental quest to define our existence, astronomers seek to understand the origin and evolution of our universe. A great deal of progress towards this goal has been made in the past decade as astronomers have measured the mass, expansion rate and age of the universe with unprecedented precision. At the same time, enormous surveys of galaxies over volumes that span the far reaches of cosmic distance and time have increased our understanding of how galaxies live and die.

However, our knowledge of the first generations of galaxies remains limited. We do not know how the first galaxies formed. We do not know what they looked like, or how big they were. Indeed, we do not even know when galaxies first appeared in our universe.

Fortunately, astronomy has a great advantage when it comes to studying the distant past. Because it takes light a finite amount of time to travel from the first galaxies to our telescopes, astronomers study the distant universe not as it exists today but rather as it looked at a much earlier time. Thus, by looking out to ever-larger distances, astronomers are able to construct a fossil record of the history of our universe. Uncovering this fossil record is a key goal driving the development of the SKA.

Currently, our best theory is that the universe began with the Big Bang, which left small ripples of density in otherwise smooth space. Just after the Big Bang the universe was filled with very hot gas, mostly hydrogen separated into its constituent proton and electron. This formed an impenetrable “fog” beyond which astronomers cannot observe directly.

As the initially very hot universe expanded and cooled, the gas of protons and electrons “recombined” to form atomic hydrogen, and this allowed light to travel freely through the universe for the first time (Fig. 1). This light is observed today as the Cosmic Microwave Background. It is the most distant light we can observe, coming from a time only 380,000 years after the Big Bang. This light extends our fossil record of the universe to more than 99.997% of its history, and encodes information about seeds of density that grew under gravity to form the first galaxies.

How long after the Big Bang did the first galaxies form? The most distant galaxies known have been discovered using the Hubble Space Telescope. These early galaxies existed less than 500 million years after the Big Bang, at a time when the universe was less than 5% of its current age.

However, astronomers don’t believe that these were the original galaxies because they were already comparable in size to massive galaxies in the nearby universe. Instead, these massive galaxies must have grown from even earlier, smaller galaxies. Unfortunately these smaller, more distant galaxies are expected to be so faint that they may not even be observable with the James Webb Space Telescope, Hubble’s imminent successor.

Rather than observing the ultra-faint first galaxies directly, astronomers plan to study how these galaxies affected the surrounding universe during the first 5–10% of the universe’s history around 0.5–1.0 billion years after the Big Bang. During this time, most of the normal matter in the universe existed as atomic hydrogen in the space between the first galaxies.

However, when the first stars formed inside these early galaxies, the ultraviolet part of the starlight they emitted reionised this intergalactic atomic hydrogen, separating it back into its constituent proton and electron. This reionisation era is known as the Age of Enlightenment that followed the end of the universe’s Dark Ages.

This reionisation of cosmic hydrogen increased the temperature of intergalactic gas throughout the cosmos from a few tens of degrees Kelvin to more than 10,000°K, in the process stripping electrons from the nuclei of most atoms in the universe. This event was the last major “phase change” of the universe, and the only major part of the universe’s thermal history, or change in temperature with time, that is yet to be measured.

Under the right conditions, atomic hydrogen emits radio waves at a wavelength close to 21 cm. Astronomers have used this 21 cm radiation for more than six decades to study the structure of our Milky Way galaxy, as well as to look for new galaxies. With the SKA, however, these 21 cm radio waves will instead be used to observe atomic hydrogen gas in the space between galaxies.

However, when studying reionisation, these radio waves will be detected by the SKA at a much longer wavelength than the 21 cm at which it was emitted. This is because the wavelength of the radiation increased as it travelled through the expanding universe, a phenomenon referred to as redshifting. Moreover, the size of the wavelength redshift increases as hydrogen gas is observed further in the distant past.

On the other hand, a hydrogen atom can no longer emit 21 cm radiation when it is reionised into its constituent proton and electron. This important fact implies that astronomers will be able to use the wavelength below which 21 cm radiation is no longer observed by the SKA to determine the time at which the atomic hydrogen was reionised by the first galaxies.

The UV starlight from the first galaxies is predicted to ionise the most nearby regions of intergalactic hydrogen first, producing ionised bubbles around the first galaxies to appear in different regions of the early universe. Thus any clustering of these galaxies, as well as whether the starlight was mostly emitted in the UV or X-ray parts of the electromagnetic spectrum, modify the way in which the reionisation progressed.

By measuring the radiation at a range of wavelengths, the SKA will be able use the redshifting of the 21cm radio waves to make a 3D map of reionisation. Astronomers will use the SKA to measure the size, arrangement and distribution of the ionisation bubbles to infer the properties of the earliest galaxies, even though those galaxies are too faint to be detected!

In this endeavour, modern supercomputer simulations will play a critical role in linking the properties of galaxies with the observations from the SKA. The challenge is daunting: the first galaxies are predicted to have masses of about ten million times the mass of the Sun, and sizes of tens or hundreds of light-years. For comparison, the sizes of the hot ionised regions created by these galaxies are predicted to be tens of millions of light-years across.

To model these large and small scales simultaneously, modern supercomputer simulations model as many as 100 billion particles to describe both the physics of galaxies and the reionisation (Fig. 2). These universes-in-a-computer will provide the link between the properties of galaxies we cannot see and the SKA-derived maps of the hydrogen that surrounds them.

In preparation for the SKA, early experiments to study re­ionisation are already being conducted using the Murchison Widefield Array (MWA) and the LOw Frequency Array for Radio-astronomy (LOFAR). While these telescopes will play a critical role in developing techniques that will be applied to the SKA, they are neither large nor sensitive enough to make an image of the atomic hydrogen during reionisation.

Instead, MWA and LOFAR aim to measure fluctuations in the radio-brightness of different parts of the sky. These fluctuations exist because the atomic hydrogen during the reionisation era was not uniform, and hence the first galaxies that completed reionisation in their local neighbourhood of the universe were distributed in clusters. A measurement of radio-brightness fluctuations would reveal information about the timing of reionisation, and even perhaps about the properties of the early galaxies.

Such detailed studies will require the sensitivity of the SKA, which is designed to be large enough to image radio waves from the atomic hydrogen that surrounded the first galaxies. In doing so, the SKA will build up a picture of where and how the inter-galactic atomic hydrogen was distributed, as well as the sort of radiation – UV or X-ray – that was produced by the first stars.

For example, simulations predict that massive galaxies will tend to be found near other massive galaxies, leading to very large ionised regions in our simulations. Conversely, smaller galaxies will tend to be more randomly distributed in the universe and thus produce many smaller ionised regions. Therefore, by looking at the size of ionised hydrogen regions, the SKA will be able to measure the masses of those early galaxies responsible for re­ionisation (Fig. 3). This is the theory, but the real universe may yield some surprises!

One fascinating feature of our current theories is that once the first galaxies formed, the starlight produced is thought to prevent subsequent galaxies from forming, either from nearby gas and/or gas that is located at great distances away. This phenomena is called feedback, and the effect depends strongly on the type of radiation produced.

The UV and X-ray starlight from the first galaxies can either heat gas close to the galaxy, stopping more galaxies from forming nearby, or it can destroy the molecular hydrogen that is crucial to the formation of the first galaxies in vast regions of the early universe. By measuring how the galaxies were distributed in space, the SKA will shed light on this and other fundamental questions about how the first galaxies formed and evolved.

The formation of the first galaxies represents a very large gap in our understanding of the universe’s history. Until now, astronomers have not had the necessary observational tools to fill in this gap. However, the SKA promises a new observational opportunity to probe the reionisation era.

In partnership with the next generation of great observatories, including the James-Webb Space Telescope and the Giant Magellan Telescope, the SKA will provide astronomers with information about some of the biggest missing pieces in our knowledge of the history of the universe, including the conditions in which the first galaxies formed, when the first galaxies formed, and how they influenced the universe around them.

Stuart Wyithe is an Australian Research Council Laureate Fellow, and Professor in the School of Physics at The University of Melbourne.