NASA's Hubble Space Telescope has provided the first visual evidence showing how our home galaxy, the Milky Way, assembled itself into the majestic pinwheel of stars we see today.
Perusing Hubble's deep-sky surveys, astronomers traced 400 galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years.
"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 in New Haven, Conn. "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."
The Hubble telescope's superb resolving power allowed the researchers to study how the structure of the Milky Way changed over time. A scale model of the Milky Way can be imagined by envisioning a fried egg. The egg white is the disk, where the Sun and Earth reside. The yoke represents the central bulge of older stars, home to a supermassive black hole that must have also grown along with the galaxy.
The Hubble images suggest that our galaxy's flat disk and central bulge grew simultaneously into the majestic spiral galaxy of today. "You can see that these galaxies are fluffy and spread out," said study co-leader Shannon Patel, of Leiden University, the Netherlands. "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 that the whole Milky Way grew at the same time, unlike more massive elliptical galaxies, in which the central bulge forms first."
The survey reveals that billions of years ago, the Milky Way was likely a faint, blue, low-mass object containing lots of gas, the fuel for star birth. The blue colors of the Milky Way ancestors are a signpost of rapid star formation. At the peak of star birth, when the universe was about 4 billion years old, the Milky Way-like galaxies were pumping out about 15 stars a year. By comparison, our galaxy today is creating only one star a year.
To identify the far-flung galaxies and study them in detail, the research team used three of the largest Hubble programs, the 3D-HST survey, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey or CANDELS, and the Great Observatories Origins Deep Survey or GOODS. These surveys of the distant universe combined spectroscopy with visible and near-infrared imaging by Hubble's Wide Field Camera 3 and Advanced Camera for Surveys. The research team's analysis involved measuring the galaxies' distances and sizes. The astronomers calculated the mass of each galaxy from its brightness and colors. They selected the galaxies in their census from a catalog they compiled of over 100,000 galaxies. The survey galaxies are consistent with computer models, which show that the bulges, and presumably the black holes, of spiral galaxies at early stages were largely built up at the same time as the disks.
"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." Exploring these galaxies back to their infancy will take the infrared eyes of NASA's James Webb Space Telescope, scheduled to launch in 2018.
The Hubble images also reinforce the idea that major mergers between spiral galaxies were not important in building them up. Computer simulations have shown that mergers would have destroyed the disks. Instead, this census reveals that spirals grew through star formation. This galaxy-formation scenario is different from the way massive elliptical galaxies develop.
"These observations show that there are at least two galaxy-formation tracks," van Dokkum said. "Massive ellipticals form a very dense core early in the universe, including a black hole, presumably, and the rest of the galaxy slowly accretes around it, fueled by mergers with other galaxies. But from our survey we find that galaxies like our Milky Way show a different, more uniform path of growing into the majestic spirals we see today."
The team's results appeared on July 10, 2013, in The Astrophysical Journal Letters. A second paper appears in the Nov. 11 online edition of The Astrophysical Journal.
Pieter van Dokkum
Yale University, New Haven, Conn.
The Observatories of the Carnegie Institution for Science, Pasadena, Calif.