Star formation in the Andromeda Galaxy (M31) has declined by roughly 80 percent over the past 500 million years, with the most dramatic drop occurring in the last 40 million years, according to a new study using archival data from the Hubble Space Telescope.
The findings, published in The Astrophysical Journal, are based on an analysis of roughly 200 million individual stars across two-thirds of Andromeda’s disk. The researchers divided Hubble images into thousands of squares, each 300 light-years on a side, and reconstructed the star formation history for each parcel.
Ben Williams of the University of Washington, a co-author of the study, said individual stars needed to be measured because they are the fossil record of the galaxy’s formation. Hubble is the only telescope that can provide sufficient spatial resolution over a large enough area to do that in Andromeda, he said.
The data came from two Hubble survey programs: the Panchromatic Hubble Andromeda Treasury (PHAT) and its southern counterpart, PHAST. Together they mapped the star formation history across the galaxy’s disk with unprecedented detail.
Five hundred million years ago, Andromeda was forming roughly one solar mass of stars per year. That rate has since fallen to about 0.2 solar masses per year today. The decline is concentrated in a ring roughly 32,000 light-years from the galaxy’s centre, which has been the most active star-forming region in recent epochs.
Tobin Wainer of the University of Washington, lead author of the study, compared the slowdown to recovering after running a marathon.
The timing of the decline points to a possible interaction with Andromeda’s small satellite galaxy M32, which lies roughly 16,000 light-years away in the plane of the sky. The PHAST data reveal decreased star formation in regions near M32 beginning roughly 60 million years ago, a time frame that could help constrain when the two galaxies interacted.
Wainer said it was not possible to explicitly say that the decrease in star formation was because of M32, but that M32 was the most likely suspect.
Andromeda at 2.5 million light-years is the nearest large spiral galaxy to the Milky Way, making it a unique laboratory for studying galaxy evolution in detail. Future observations with NASA’s Nancy Grace Roman Space Telescope, scheduled for launch as early as August 30, have a field of view at least 100 times larger than Hubble at near-infrared wavelengths and will image the entirety of Andromeda’s disk and surrounding halo.
Source: NASA/Hubble