Humans take the starry night for granted. But Earth’s night sky hasn’t always sparkled: In the distant past, during the infancy of the Milky Way, it was a much darker place. Now scientists have pictures of what it looked like and how it’s changed.
“For the first time we have images of what the Milky Way looked like in the past,” said Pieter G. van Dokkum, chair of Yale University’s Astronomy department and leader of a project to reconstruct the galaxy’s history with data from NASA’s Hubble Space Telescope. “By looking at hundreds of other distant but similar galaxies, we’ve traced dramatic changes in our own. We’ve captured most of the Milky Way’s evolution.”
Milky Way star formation was explosive in the period between 11 billion and 7 billion years ago, the astronomers found: Nearly 90% of its stars formed then. At peak formation, about 9 billion years ago, it was generating about 15 stars a year, they said.
Van Dokkum’s team traced the history of Milky Way star formation by studying about 400 galaxies, selected from a sample of more than 100,000 galaxies observed by Hubble covering about 11 billion years of cosmic history. These galaxies are expected to evolve as the Milky Way did.
At first the Milky Way, like other galaxies, had lots of gas but no stars. Gravitational forces compressed the gases to extreme densities, leading to the birth of stars.
By the time Earth formed, 4.5 billion years ago, the Milky Way’s rate of star birth had slowed dramatically, to the rate of about three per year.
“The show was mostly over,” van Dokkum said.
The team relied on three large Hubble galaxy programs: the Great Observatories Origins Deep Survey (GOODS), the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), and the 3D-HST survey.
“A lot of what we accomplished required the excellent vision of the Hubble Space Telescope,” said Joel Leja, a graduate student in astronomy at Yale and member of the research team. “The Hubble images help us see the Milky Way ancestors all the way to their early youth. To get the first baby pictures, we need the infrared eyes of NASA’s James Webb Space Telescope, scheduled to launch in 2018.”
The authors reported their research in a paper in The Astrophysical Journal Letters titled “The Assembly of Milky Way-like Galaxies Since z ˜ 2.5.” A related paper is in press.