A new study using data from NASA’s Hubble Space Telescope found 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 is close enough for astronomers to examine its stellar populations in detail and learn about the past of galaxies like our own.
The results published July 27 in The Astrophysical Journal.
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 the disk of Andromeda 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,” said Ben Williams, a research professor of astronomy at the University of Washington and a co-author on the study. “Hubble is the only telescope that can give you high enough spatial resolution in the blue part of the spectrum over a large enough area to be able to do that in Andromeda.”
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, spanning 300 light-years on each 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 that time, star formation has been steadily declining.
This movie, created from an analysis of data from NASA’s Hubble Space Telescope, demonstrates how the rate of star formation in the Andromeda galaxy has declined over the past 500 million years. Brighter colors indicate where star formation was highest during a given time. Credit: Visualization: Tobin Wainer (UW); Image Processing: Joseph DePasquale (STScI); Video: NASA, ESA, STScI, Gregory Bacon (STScI)
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 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 measure is driven by decreasing activity within that ring.
The decline is 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 lead author Tobin Wainer, a UW graduate student in astronomy.
The team also investigated whether there was any connection between the decrease of activity in Andromeda and its proximity to the satellite galaxy Messier 32, or M32. The M32 galaxy 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 co-author Zhuo Chen, a UW postdoctoral scholar in astronomy.
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 this study finds began roughly 60 million years ago, could help constrain when the M32 galaxy 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,” Wainer said.
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,” said co-author Raja GuhaThakurta, a professor of astronomy and astrophysics at University of California Santa Cruz. “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.”
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 as much area as 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.
Hubble is a project of international cooperation between NASA and the European Space Agency.
UW co-authors include Julianne Dalcanton, a professor of astronomy and Meredith Durbin, Debby Tran and Margaret Lazzarini, former UW graduate students in astronomy.
A complete list of co-authors is included with the paper.
This research was funded by NASA.
For more information, contact Wainer at tobinw@uw.edu.
This story was adapted from a press release by NASA.