The Andromeda galaxy has been forming fewer and fewer stars for about 500 million years.
An analysis based on observations from the Hubble Space Telescope traces this decline with unprecedented precision. This neighbor of the Milky Way provides a unique opportunity to understand how a large spiral galaxy gradually slows its activity.
The Andromeda Galaxy (M31 galaxy)
Image Wikimedia
500 million years ago, Andromeda produced the equivalent of about one solar mass of new stars each year. This rate had halved 40 million years ago. It now stands at nearly 0.2 solar masses per year, according to the team whose results were published in The Astrophysical Journal.
To reconstruct this recent history, researchers studied the colors and brightness of millions of stars individually observed by Hubble. Massive blue stars live short lives. Their distribution therefore makes it possible to estimate where and when new stars formed over the last few hundred million years.
The decline is not uniform across the galaxy. Much of the recent star formation is concentrated in a ring about 32,000 light-years from the center. This same ring also explains a large part of the measured slowdown, as its activity has decreased sharply in the most recent periods.
The lack of gas does not seem sufficient to explain this evolution. Andromeda still has material capable of fueling new stars. The study's authors instead envisage a gradual return to a calmer state after a previous more active phase. A past interaction may have temporarily compressed the gas and stimulated star birth.
The small satellite galaxy M32 is one suspect. It may have crossed Andromeda's disk and disrupted its gas clouds. However, current data do not allow a direct link to be established. Other internal mechanisms, such as disk dynamics or the gradual dissipation of an old disturbance, also remain possible.
Andromeda is about 2.5 million light-years away. Its proximity allows individual stars to be observed while retaining an overall view of the galaxy. Researchers now want to combine Hubble's archives with ground-based observations. The Nancy Grace Roman Space Telescope should then map the entire disk with a much wider field.