Astronomers working with the Hubble Space Telescope have studied hundreds of thousands of galaxies, to improve their understanding of how galaxies are born and change over time. These new results have allowed them to construct a model of what the Milky Way would have looked like shortly after it was first formed, between seven and 11 billion years ago.A team of astronomers led by Pieter G. van Dokkum of Yale University and Shannon Patel of Leiden University has shed light on how the Milky Way galaxy formed in its early years. By studying images of more than 100,000 spiral galaxies, at distances of up to 11 billion light years away, they were able to build a clear picture of how galaxies like our own typically develop after their initial formation. They found that these early galaxies contain huge amounts of gas, with relatively few stars and very little dust. New stars are produced at a prodigous rate – some fifteen times faster than they are being produced in the Milky Way today – and over time the spirals bulk out into the form they have now. Spirals start with the same basic structure that they carry in later life: A central bulge, probably containing a supermassive black hole, surrounded by a thin disc, like two fried eggs placed back to back. Elliptical galaxies, by contrast, start out with just a large dense core. These galaxies grow through interactions with other galaxies – they fly close by each other, distort each other gravitationally, and eventually coalesce into a new, larger galaxy. Over time the elliptical becomes large enough to consume smaller galaxies, and so it grows ever larger.
As is becoming common in modern astronomy, the scientists did not need to comission time for observations on the Hubble. Instead, they accessed the data from three previous large-scale surveys that had already been performed in earlier research, and filtered out the objects of interest. The projects used were the 3D-HST survey, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and the Great Observatories Origins Deep Survey. Each survey looked at the sky in a different way, and by combining them the team were able to assemble spectroscopic, infrared and visual imagery of distant galaxies. This information was enough to allow them to identify the age, size, distance and type of each galaxy. They could also measure factors like the rate of star formation, and amounts of dust and gas.
By sampling such a large number of galaxies, astronomers can be sure of catching them at all different ages. “In these observations, we’re capturing most of the evolution of the Milky Way,” explained team member Joel Leja of Yale University. “These deep surveys allow us to see the smaller galaxies. In previous observations we could only see the most luminous galaxies in the distant past, and now we can look at more normal galaxies. Hubble gives us the shapes and colors of these spirals as well as their distances from Earth. We also can measure the rates at which each part of the galaxies grew. All of this is difficult to do from the ground.”
So what does all this tell us about the history of the Milky Way? In the very beginning, a spiral galaxy formed. It was composed mostly of gas, had very few stars, and was quite dim. Dim enough, in fact, that galaxies that far back in time can’t be seen by the Hubble. We will have to wait till the James Webb telescope is built and parked in orbit before we can start looking that deeply into the universe. But eventually it had grown to the point where our story begins – somewhere between seven and eleven billion years ago. At this stage it looks like the bottom panel in the image above – thin, no dust lanes, slightly blue and dotted with thousands of pink star-forming regions as its abundant stocks of gas coalesce into billions of new stars at a rate of about 5 each year. Over time the biggest and hottest of these stars race through their lifecycle and explode into supernovae, seeding heavier elements into the rest of the galaxy. Later on, less massive stars die more gracefully, shedding their outer layers packed with simpler elements like carbon. Together, these outpourings start building up into vast dust clouds, until the galaxy looks as it does today: Thick, bright, opaque, and full of dark patches where dark dust clouds block the light.