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Cosmology and Galaxy Evolution with the SKA

Credit: NASA/JPL-Caltech/VLA/MPIA

The Southern Pinwheel galaxy, M83, imaged in ultraviolet light (blue and green) and radio emissions at a wavelength of 21 cm (red). The blue and pink pinwheel in the centre is the main stellar disk of the galaxy, and the red extended arms are composed of the neutral hydrogen gas that fuels star formation. Credit: NASA/JPL-Caltech/VLA/MPIA

By Martin Meyer & Chris Blake

The SKA will provide new insights into how galaxies are assembled over time, from the hydrogen gas that fills the universe to the properties of dark matter and dark energy that dominate the large-scale structure of the cosmos.

To understand the origin and nature of our universe, astronomers study the large-scale distribution of matter and energy and the evolution of this structure over cosmic time. This effort combines cosmology, which aims to understand the large-scale properties of the universe as a whole, and galaxy evolution, which studies how the universe evolved from a relatively smooth and featureless gas into the highly clustered and rich distribution of matter seen in galaxies today.

Galaxies are composed of gas, dust, stars and dark matter, and are subject to many different physical processes. Therefore, understanding galaxy evolution requires diagnostic information from across the electromagnetic spectrum.

Traditionally, studies of galaxy evolution have been based on light from optical and nearby wavelengths, such as infrared and ultraviolet light. However, such observations are blind to one of the most ubiquitous materials in the universe: neutral atomic hydrogen.

Hydrogen dominated the early gas content of the universe, and provides the main fuel for all stars and galaxies. Neutral hydrogen can be observed only via an atomic energy transition that emits a photon with a radio wavelength close to 21 cm. With a lifetime of around 11 million years, this transition is extremely rare for any given atom. However, it can be detected by radio astronomers because there is just so much hydrogen gas out there. A large spiral galaxy such as the Milky Way, for instance, can contain more than a billion times the mass of the Sun in neutral atomic hydrogen. Emissions from this gas have been observed from many thousands of galaxies beyond our own, but only a few hundred galaxies beyond the nearby universe have so far been directly detected due to limitations in the sensitivity of existing telescopes.

This brings us to the importance of the Square Kilometre Array (SKA). By combining unprecedented sensitivity, a wide field of view and the ability to create highly detailed maps of the radio sky, the SKA will provide transformational new capabilities for advancing research in this field and revolutionising our understanding of the universe.

The SKA will be able to observe the hydrogen content of many millions of galaxies across large fractions of the universe’s history. Fully deployed, it will have the sensitivity to see objects with masses similar to the Milky Way across more than 10 billion years of cosmic time. Its wide field of view will enable large volumes of the universe to be efficiently mapped quickly, and deep single-pointing surveys will aim to detect emissions from the most distant galaxies possible.

This combined strategy will provide new insights into a range of important questions. How has the total amount of gas in the universe changed across cosmic time? How has the distribution of this material evolved? How are galaxies refuelled by the material between galaxies and the cosmic web? How do galaxies interact with their environment? How are galaxies impacted by star formation, supernovae and super-massive black hole activity?

A key strength of the SKA in answering such questions will be its ability to make high-resolution 3D maps of neutral hydrogen emissions. These maps are constructed by measuring the spectrum of light emitted from distant galaxies. As light travels across the universe to our telescopes, the expansion of the universe “stretches” its wavelength, an effect known as redshifting. The degree of redshifting of the spectrum, compared with nearby galaxies, is used to determine the distance to the objects.

Using this technique, the SKA will observe how gas is distributed across the universe on the largest scales and how this gas is distributed within galaxies. This will enable astronomers to carry out ground-breaking studies of how the gas content of galaxies relates to the locations and motions of their stars, dust, and other important galactic components.

These methods and capabilities will make the SKA a uniquely powerful tool for understanding the rich distribution of galaxies seen in the universe today. Combined with data from other telescopes around the world, the SKA will give our most complete census yet of all the major ingredients of galaxies, and provide a new understanding of how they have evolved across large fractions of cosmic time.

By constructing large-scale 3D maps of the distribution of matter across the universe, the SKA can also advance answers to some of the most interesting and pressing questions in modern cosmology. Cosmologists still do not understand the fundamental matter and energy contents of the universe, or the physical laws through which they interact. Astronomers have known for many decades that galaxies appear to be dominated by a “cold dark matter” that exerts a measurable gravitational force but does not emit light. More recently a suite of cosmological measurements, in particular observations of supernovae as standard candles, has provided evidence that the universe has entered an unexpected phase of accelerating expansion, propelled by a mysterious repulsive component of gravity known as “dark energy” that apparently constitutes 69% of the current energy density of the universe.

There is currently no theoretical understanding of either dark matter or dark energy. Possible explanations include modifying the laws of gravity on large cosmological scales from the vision put forward by Newton and Einstein, or introducing new fundamental particles or fields into nature, which may be equivalent to (for example) the Higgs field that endows particles with mass. In either case, the apparent existence of dark matter and energy affords astronomers with an exciting possibility to test the fundamental laws of nature.

One of the most powerful methods for performing such tests is to construct a 3D map of the distribution of matter in the universe. This network of “large-scale structure” is sculpted by the gravitational forces that cause galaxies to grow from small clumps in the early universe, by the overall expansion of the universe driven by dark energy, and by the temperature and other properties of the dark matter from which galaxies form. The mathematical patterns within these maps allow the determination of the expansion rate of the universe, the variation of gravity on cosmological scales, and the properties of the constituents of the universe.

A number of optical telescopes, such as the Anglo-Australian Telescope and the Sloan Telescope, have produced measurements of large-scale structure by mapping the positions of millions of galaxies. However, radio astronomers have two significant advantages for building these maps: the abundance of neutral hydrogen in the universe and the large range of redshifts that can be simultaneously probed as the sky is imaged at radio wavelengths.

Such radio observations do not necessarily need to detect individual radio-emitting galaxies. The technique of “intensity mapping”, one of the most exciting to be pursued by the SKA, involves measuring the integrated hydrogen emission within larger “pixels” in the sky out to high redshifts, where each pixel may contain the combined emission of millions of individual galaxies. These methods are being demonstrated by the leading large-aperture radio telescopes of today, such as the Green Bank Telescope. The greatest difficulty with this method is to subtract the emission originating from diffuse synchrotron radiation produced within our own galaxy, which may be thousands of times brighter than the faint cosmological signal. However, the application of modern statistical methods should allow the different patterns to be disentangled.

Observations by the SKA should also allow tests of the theory of “inflation”: the rapid expansion that cosmologists speculate occurred in the first split-second of the lifetime of the universe, converting quantum fluctuations into the “seeds” from which future galaxies developed. Inflation makes an important prediction that matter density ripples at the largest scales of the universe are described by a well-defined statistical model. The detection of even a minute departure from this model would rule out the inflationary explanation and leave cosmologists grasping for new ideas to explain the earliest moments of the universe.

The SKA will provide the ultimate maps of cosmic large-scale structure, including the hydrogen content of many millions of galaxies, spanning a sizable fraction of the observable universe. This dataset will be a powerful resource for discovering the properties of the dark matter, dark energy and inflationary theories that constitute our modern picture of cosmology, but for which we currently possess no compelling explanation. By investigating the structure and evolution of our universe and the galaxies within it, we gain important insights into the origins of our own home in the cosmos and the laws of nature that govern it.

Martin Meyer is a Senior Research Fellow at the International Centre for Radio Astronomy Research, The University of Western Australia. Chris Blake is an Associate Professor at Swinburne University of Technology.