Tobin Wainer’s Hubble Space Telescope Andromeda M31 study says star formation in Andromeda has slowed for 500 million years and now stands at about one-fifth of the Sun’s mass per year. The study was published Monday in The Astrophysical Journal and reconstructs that change from about 200 million individual stars across two-thirds of the galaxy’s disk.
Wainer, the lead author at the University of Washington, compared the slowdown to a runner taking a breather after a marathon: “It’s just like after running a marathon, sometimes you’ve got to take a bit of a breather,” he said. Ben Williams, an astronomer at the University of Washington and a co-author, said, “We need to measure the individual stars because they are the fossil record of the galaxy’s formation. Hubble is the only telescope that can give you high enough spatial resolution over a large enough area to be able to do that in Andromeda.”
Panchromatic Hubble Andromeda Treasury
The team combined data from the Panchromatic Hubble Andromeda Treasury and the Panchromatic Hubble Andromeda Southern Treasury to map the galaxy in ultra-sharp detail. That map let the researchers trace how star formation changed from about one solar mass per year 500 million years ago to about half a solar mass per year 40 million years ago, before reaching today’s lower level.
Much of the recent star formation has occurred in a ring about 32,000 light-years from Andromeda’s center, and the study says the decline was driven largely by decreasing activity in that ring. Andromeda is about 2.5 million light-years from Earth and comparable in size to the Milky Way, which makes it close enough for Hubble to resolve individual stars across a large part of the disk.
Messier 32 and Andromeda
The study also examined whether the drop in activity was connected to M32, the satellite galaxy separated from Andromeda by about 16,000 lig. Previous research had already shown that Andromeda experienced a dramatic burst of star formation about 2 billion years ago, so the new result adds a long decline after that earlier surge.
The paper leaves one central question open: what specifically caused the decline in star formation across Andromeda over the last 500 million years. For now, the published data give a detailed history of the slowdown, but they do not settle the mechanism behind it.
