Hubble shows star formation in Andromeda galaxy is winding down

A new study using data from NASA’s Hubble Space Telescope finds that star formation in the nearby Andromeda galaxy has undergone a 500-million-year decline, with an even steeper drop in the last 40 million years. Andromeda, a spiral galaxy comparable in size to our Milky Way, is close enough to be seen with the unaided eye from areas with dark skies. Located about 2.5 million light-years from Earth—practically our cosmic backyard—Andromeda offers an opportunity for astronomers to examine its stellar populations in detail, leading to a better understanding of the past of galaxies like our own.

The results were published Monday in The Astrophysical Journal.

Reading Andromeda’s stellar record

To reach this conclusion, the researchers combined data from two Hubble surveys: the Panchromatic Hubble Andromeda Treasury and the Panchromatic Hubble Andromeda Southern Treasury. Together, these two surveys mapped two-thirds of Andromeda’s disk in ultra-sharp detail. In total, the team measured about 200 million individual stars across the galaxy, giving them a detailed picture of Andromeda’s past activity.

“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,” said Ben Williams, an astronomer at the University of Washington and a co-author of the study.

Massive stars are bluer and short-lived, while less massive stars are redder and longer-lived. As a result, areas that have experienced recent star formation tend to have a larger fraction of blue stars, while areas with less recent star formation typically have a redder population. The team divided the Andromeda images into thousands of squares, each spanning 300 light-years on a side, and determined the history of star formation within each parcel to gain a comprehensive view of the galaxy’s past.

Previous research showed the Andromeda galaxy experienced a dramatic burst of star formation about 2 billion years ago, likely due to a past interaction or merger with another galaxy. Since then, star formation has steadily declined.

A sharper recent slowdown

Astronomers measure the rate of star formation in terms of the mass, or amount of gas and dust, converted into stars per year. The researchers calculated that 500 million years ago, Andromeda formed stars at a rate of about one solar mass per year. However, the formation rate dropped to about half that by 40 million years ago. The current rate has plummeted even farther, to about one-fifth of the mass of our sun per year.

The team also examined whether that decline was consistent across the galaxy or concentrated in certain areas. They found that much of the recent star formation has occurred in a star-forming ring located about 32,000 light-years from the galaxy’s center. As a result, much of the decline they measured is driven by decreasing activity within that ring.

Rather than being the result of reduced material from which new stars can form, the decline is more likely to be a natural winding down from its previous, more active state.

“It’s just like after running a marathon, sometimes you’ve got to take a bit of a breather,” said Tobin Wainer, lead author at the University of Washington.

M32 emerges as a suspect

The team also investigated whether there was any connection between the decrease in activity in Andromeda and its proximity to the satellite galaxy M32 (Messier 32). M32 is separated from Andromeda by about 16,000 light-years in the plane of the sky; however, its 3D location in space is uncertain. As a result, astronomers are unsure if or when it might have interacted with Andromeda in the past.

“One of the major motivations for this program was to probe potential interactions between M32 and Andromeda’s disk,” said Zhuo Chen, a co-author at the University of Washington.

Survey data from the Panchromatic Hubble Andromeda Southern Treasury allowed the team to study the history of star formation in Andromeda near M32. They found that this area showed signs of decreased star formation compared with other regions. The timing of this decrease, which the study finds began roughly 60 million years ago, could help constrain when M32 interacted with Andromeda’s disk.

“We can’t explicitly say that we are seeing a decrease in star formation because of M32. But it’s right there, and it’s definitely the most likely suspect,” said Wainer.

Roman will widen the view

The team plans to continue analyzing the Hubble data and combine it with data from ground-based observatories to gain additional insights into the history of Andromeda.

“There’s a strong scientific value to this archival data. Andromeda is important because it’s a neighbor that is close enough that we can see it in great detail while also getting a global perspective,” said Raja GuhaThakurta, a co-author at the University of California, Santa Cruz.

An even greater global perspective is likely to come from NASA’s Nancy Grace Roman Space Telescope after it launches as early as Sunday, Aug. 30. Roman’s gigantic field of view can cover at least 100 times the area of Hubble at near-infrared wavelengths in a single observation. A newly approved Roman observing program will image the entirety of Andromeda’s disk and areas of its surrounding halo, allowing astronomers to measure hundreds of millions of stars and enabling groundbreaking new science.

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Lisa Lock

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Robert Egan

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