The Galactic Universe

by Dr. Sarah Reeves

The Milky Way above the Pinnacles, WA. Image: Jingyi Zhang

The Milky Way galaxy is about 100,000 light years across and contains an estimated 100-400 billion stars. It’s also our home galaxy. When you look up at the sky from a dark location, you’ll see a stripe across the sky, which is what people commonly refer to as the ‘Milky Way’. Our galaxy is what’s known as a spiral galaxy (a barred-spiral to be precise), with the stars arranged in a spiral-shaped disc, or pancake, of stars. The Sun, the Earth, and our entire Solar System are but one indistinguishable speck in that pancake of stars. We reside in one of the spiral arms of the galaxy about two-thirds of the way out towards one edge. So when you look up at the sky and see that bright stripe of stars, you’re actually looking through the disc of our galaxy at the stars that make up this grand spiral shape.

Model of the Milky Way, indicating the position of the Sun within its spiral arms. Image: NASA/JPL-Caltech/ESO/R. Hurt.

Images from powerful ground and space-based telescopes like the Very Large Telescope (VLT) in Chile or the Hubble Space Telescope (HST) reveal that galaxies can take on a variety of different shapes and sizes. Most galaxies fall broadly into one of two morphologies: either spiral or elliptical. Some spirals (like the Milky Way) have a bar across the centre from which the spiral arms emerge; others do not. Elliptical galaxies, by contrast, hold their stars in one big ellipsoid (or football shape). There are also ‘irregular’ galaxies which have no discernible shape. Elliptical galaxies contain mostly old, red stars, and are no longer actively forming new stars (giving them the nickname ‘red and dead’), while spiral galaxies contain lots of hot, young stars (which are blue in colour). These galaxies range in size from a fraction of the size of the Milky Way (dwarf galaxies, such as the Magellanic Clouds, which can be seen with the naked eye from the Southern Hemisphere), all the way up to monsters like IC1101 (at roughly 20 times the size of our galaxy).

Different galaxy morphologies: barred spiral (top left), spiral (bottom left), and elliptical (right). Image: HST/NASA/ESA/S. Reeves.

One of the biggest questions in astronomy is: how do all these galaxies form and grow? The current understanding is that galaxies grow gradually by a process of collision, as gravity slowly pulls matter together. In this way, galaxies like our Milky Way are built up through a series of mergers over many billions of years. And while studying the stars in these galaxies can tell us a lot about galaxy evolution, there’s actually much more to these giants than meets the eye. In addition to the stars we see, there are two other key components that make a galaxy: gas and dust. The gas (hydrogen gas) is important because it provides the raw material from which the stars form, and is what they burn to produce light throughout their lifetimes. It’s also what fuels the supermassive blackholes believed to lie at the centre of most, if not all, galaxies. The dust meanwhile, provides a site on which this gas can condense in the process of star-formation. The dust both scatters and reddens the light from stars, accounting for the dark patches that run through the Milky Way, but can be detected in infra-red (IR) light with special IR telescopes.

The Parkes Radio Telescope (‘The Dish’). Image: CSIRO.

And what about the hydrogen gas? How do we see and study it? Well, just like the dust, the hydrogen gas emits light at other wavelengths, beyond the visible spectrum (i.e. the colours of light that our eyes can see). In this case, the gas emits radio waves, which can be picked up with sensitive radio telescopes, like Parkes (which many will recognise from the Sam Neill movie ‘The Dish’, about the Apollo 11 Moon Landing). These radio telescopes allow astronomers to produce images, showing what our eyes would see if we could detect radio waves. And when we do, we see something quite astounding. Permeating through galaxies (especially spiral galaxies) and extending far beyond where the stars end is a gas disc, typically about twice the size of the stellar disc. Even more astonishing are the tidal tails and bridges (streams of gas stretching out behind or between galaxies) revealed by radio telescopes. These tails and bridges reveal evidence of past or ongoing galaxy interactions, providing astronomers with a unique view into the process of galaxy evolution.

Comparison of the M81 group of galaxies as seen through an optical (conventional) telescope (left) and a radio telescope (right). Image: Yun et al., 1994.

Because of the time it takes light to travel across the Universe, the more distant we look, the further back in time we are seeing. Until recently though, radio telescopes have only been capable of studying the gas in relatively nearby galaxies, meaning we are missing critical information to help us understand galaxy evolution. But next-generation radio telescopes will allow us to map out the evolution of gas in galaxies up to a redshift of z = 1.0, revealing what galaxies were like when the Universe was just half its current age. The Australian Square Kilometre Array Prototype (ASKAP) is one such telescope. Located in remote Western Australia, far from the interference of radio signals from TV transmissions, microwave ovens and mobile phones, this telescope will give astronomers an unprecedented view of these galaxies. ASKAP is just beginning early science observations and has already made exciting discoveries, including the detection of previously unknown galaxies billions of light years away. As ASKAP gets up to full power over the next couple of years, the data and discoveries will begin to flood in, providing fantastic new insights into the hidden gas in galaxies, and helping astronomers to understand how galaxies grow and evolve.

The ASKAP radio telescope, which comprises 36 small dishes, joined together to create a far more powerful telescope. Image: CSIRO.