Discovering our cosmic origins: From a garden in Herefordshire to the Hubble Space Telescope:
When I was a teenager I saw the most distant object I will even see with the naked eye, while standing in my pyjamas in my Aunties garden in Herefordshire. It was the Christmas holidays, the ideal time to snuggle by the fire, or alternately, step outside and enjoy the dark skies visible in the depths of the countryside. What I was looking at wasn’t a star, it wasn’t even a cluster of stars, it was over 100 billion stars, collected together in our nearest neighbour, the Andromeda galaxy. It was only a fuzzy white blob, ironically it looked similar to the images that I would be studying 15 years later, but it caught my imagination.
Nowdays, I would dismiss the Andromeda galaxy as too “local”, and instead peer beyond it to study the most distant galaxies observed.
It is approaching 100 years since the field of galaxy evolution really began. Edwin Hubble amongst others, found that the so-called nebulae that astronomers had been studying were not in-fact within our own galaxy the Milky Way, but far far distant. This ground-braking discovery was one of the most profound in astronomy. If you thought you felt small before, prepare yourself, because now in addition to our own Sun being utterly insignificant within our own Galaxy, the Milky Way was only one of billions of galaxies in the observable Universe.
Now, on a miserable looking Friday morning, I can be looking at an image of half a million galaxies, and I can easily download countless other images that cover the sky in a patchwork. The goal of observing so many galaxies is not just stamp-collecting, although you can easily while away an afternoon looking though them. These images show our cosmic origins. They show that no two galaxies are the same. Spiral arms, train-wreck mergers and even relativistic jets are all visible and hot topics of research. And only by studying these galaxies can we come to understand how we got here, how our own galaxy was formed and continues to evolve to this day.
The first galaxies:
The Hubble Space Telescope is the most powerful set of eyes astronomers' have (until 2018, more later. Using the 2.4m (8 ft) mirror in low-Earth orbit it is possible to detect some of the very first galaxies to exist in our Universe. These galaxies were very different to our own. For a start, none of the elements that make up you or I were in existence.
Using some of the most sensitive images humans have ever made of the night sky, I, like tens of other astronomers around the World, are studying these early galaxies. Because of the expanding Universe and the vast scales involved, the light from some of these galaxies is only reaching our telescopes today after a 13.5 billion year journey. This effect (which leads to the light being shifted to the red, or redshifted) allows astronomers to look back in time and studying the build-up of galaxies throughout the full history of the Universe. Unsurprisingly given the huge scales involved, the most distant objects are extremely faint and challenging to study. Unfortunately this makes the images not quite as inspiring as is customary from Hubble, but you have to use your imagination.
Imagine a dark Universe, empty aside from the dark matter and diffuse Hydrogen and Helium that pervades it. In the darkness something is stirring. The first stars are being born. Sophisticated computer simulations predicted that these stars are monsters. Because of the lack of any interesting elements in the primordial gas, these first stars are thought to be hundreds or thousands of times the mass of our own Sun. Such humungous stars produce huge quantities of energy in their short million year lifetimes, before they explode, spewing heavier elements out into the vicinity or possibly even collapsing directly into a black hole.
The current observational limit in astronomy is a galaxy at a redshift of 8.6. This unremarkable number actually means something quite remarkable. It means that we are observing some of the first galaxies in existence, as they were only 500 million years after the Big Bang.
It also means that the light from the galaxy has been stretched by a factor of 1.0 + z = 9.6, and so the UltraViolet photons pumped out by the stars are now so red as to be beyond the reaches of the human eye. What do we know about the galaxies? We know that they are enriched, observations have shown Carbon and Oxygen are present. We know that they rapidly forming stars, from 10 to 100 times the rate in the Milky Way. All of this information is crucial for understanding everything that comes afterwards, i.e. the creation of our galaxy, of our Sun, of us. It points to a picture of early enrichment in the Universe, where the pristine H and He was quickly infused with heavier elements from these violent supernova events in the first few hundred million years.
While the study of galaxies in the first billion years has flourished in the last 20 years, mainly due to the spectacular performance of the Hubble Space Telescope, there are many open questions and unexpected discoveries lying beyond it’s capabilities.
In 2018 the James Webb Space Telescope will launch and start a 5 year mission that will build on the legacy of Hubble. One of the most exciting aspects of the JWST in comparison to Hubble is that it will observe much further to the red end of the spectrum, reaching into the infrared wavelengths. It is also a significantly larger mirror, and it is these two factors combined that will make it so powerful for discovering the first galaxies. These galaxies are both faint, requiring the very best instruments to detect them, and their light is strongly red-shifted, meaning that infrared telescopes are essential to find them.
As well as detecting even earlier galaxies, which will reveal the first 100 million years of star formation, the JWST could potentially discover some of the most energetic phenomenon in nature. Nobody knows what the first stellar explosions will look like, however with JWST these extreme systems will be in reach of our telescopes for the first time. The supernovae may even act as beacons, signally the locations of the first collections of stars that otherwise would be beyond the limits of Hubble and even JWST. If just one of these supernova is discovered with the JWST it will be a remarkable feat of science and engineering. These explosions will provide dramatic signposts pointing to the sites where the first Carbon atoms were created, and hence will truly reveal our cosmic origins.
Dr Rebecca Bowler - Nicholas Kurti Junior Research Fellow, Brasenose College and Hintze Fellow in the Centre for Astrophysical Surveys, Department of Physics