A new study using data from NASA鈥檚 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.
July 27 in .
To reach this conclusion, the researchers combined data from two Hubble surveys: the and the . 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鈥檚 past activity.
鈥淲e need to measure the individual stars because they are the fossil record of the galaxy鈥檚 formation,鈥� said , a research professor of astronomy at the 乱伦社区 and a co-author on the study. 鈥淗ubble 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.鈥�
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鈥檚 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鈥檚 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鈥檚 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.
鈥淚t鈥檚 just like after running a marathon, sometimes you鈥檝e got to take a bit of a breather,鈥� said lead author , a UW graduate student in astronomy.聽
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.
鈥淥ne of the major motivations for this program was to probe potential interactions between M32 and Andromeda鈥檚 disk,鈥� said co-author , 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鈥檚 disk.
鈥淲e can鈥檛 explicitly say that we are seeing a decrease in star formation because of M32. But it鈥檚 right there, and it鈥檚 definitely the most likely suspect,鈥� 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.
鈥淭here鈥檚 a strong scientific value to this archival data,鈥� said co-author , a professor of astronomy and astrophysics at University of California Santa Cruz. 鈥淎ndromeda is important because it鈥檚 a neighbor that is close enough that we can see it in great detail while also getting a global perspective.鈥澛�
An even greater global perspective is likely to come from NASA鈥檚 Nancy Grace Roman Space Telescope after it launches as early as Sunday, Aug. 30. Roman鈥檚 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 will image the entirety of Andromeda鈥檚 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.聽
UW co-authors include , professor emerita of astronomy and , and , former UW graduate students in astronomy.
A complete list of co-authors is .
This research was funded by NASA.
For more information, contact Wainer at tobinw@uw.edu.
This story was adapted from a press release by .