Sen—The Hubble Space Telescope has revealed the first visual evidence of how our galaxy, the Milky Way, assembled itself into the spiral of stars we see today.

Using Hubble's deep-sky surveys to study the evolution of 400 galaxies similar to the Milky Way and noting their appearance at various stages of development over a time span of 11 billion years, astronomers found the Milky Way likely began as a faint, blue, low-mass object containing lots of gas. Gas is the fuel for star birth and the blue colour is an indicator of rapid star formation.

The Milky Way was also probably a flat disk with a bulge in the middle, both of which grew simultaneously into the pinwheel seen today. The Sun and Earth reside in the disk and the bulge is full of older stars and a supermassive black hole that probably grew along with the galaxy.

Artist's illustration of the Present and Early Milky Way. Image credit: NASA/ESA and Z. Levay (STScI)

"For the first time, we have direct images of what the Milky Way looked like in the past," said study co-leader Pieter G. van Dokkum of Yale University. "Of course, we can't see the Milky Way itself in the past. We selected galaxies billions of light-years away that will evolve into galaxies like the Milky Way. By tracing the Milky Way's siblings, we find that our galaxy built up 90 percent of its stars between 11 billion and 7 billion years ago, which is something that has not been measured directly before."

Hubble's superior resolving power allowed the researchers to study how the structure of the Milky Way changed over time. At the peak of star formation, when the universe was about 4 billion years old, the Milky Way- like galaxies were pumping out about 15 stars a year, while the Milky Way today is creating only one star a year.

Study co-leader Shannon Patel of Leiden University in The Netherlands said "There is no evidence of a bulge without a disk, around which the disk formed later." Team member Erica Nelson, of Yale University, added: "These galaxies show us the whole Milky Way grew at the same time, unlike more massive elliptical galaxies, in which the central bulge forms first."

Composite image of galaxies similar to ours at various stages of construction over 11 billion years. Image credit: NASA, ESA, P. van Dokkum (Yale University), S. Patel (Leiden University) and 3D-HST Team.

The researchers used the 3D-HST survey, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and the Great Observatories Origins Deep Survey. These combined spectroscopy with visible and near-infrared imaging by Hubble's Wide Field Camera 3 and Advanced Camera for Surveys.

The astronomers measured the distances and sizes of the galaxies and calculated the mass of each from its brightness and colours. The survey galaxies are consistent with computer models which show at early stages a majority of the bulges of spiral galaxies were built up at the same time as their corresponding disks.

Team member Joel Leja of Yale University explained. "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 colours 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."