A new Hubble Space Telescope survey shows intriguing patterns of star formation in dwarf galaxies aligned in a plane around the Andromeda Galaxy.
Andromeda Galaxy
G. Zacchi / S&T Online Photo Gallery
Astronomers have used the Hubble Space Telescope to create an unprecedentedly detailed 3D map of the dwarf galaxies buzzing around Andromeda (M31). Their findings hint at a disruptive galactic merger a few billion years ago and could even help astronomers to home in on the true nature of dark matter.
Sitting 2.5 million light-years away, Andromeda is the nearest major galaxy to the Milky Way. Like our own galaxy, Andromeda is surrounded by many smaller, dwarf galaxies. However, the “ecosystem” around Andromeda is far more complex. A team of astronomers, led by Alessandro Savino (University of California at Berkeley), used observations gleaned from more than 1,000 orbits of the Hubble Space Telescope to chart these island worlds like never before. The team’s findings are published in The Astrophysical Journal.
Ground-based view of Andromeda Galaxy, taken by Akira Fujii, with inset close-ups on individual galaxies from Hubble
Ground-based view of Andromeda Galaxy, taken by Akira Fujii, with inset close-ups on individual galaxies from Hubble
"Everything scattered in the Andromeda system is very asymmetric and perturbed,” says team member Daniel Weisz (also University of California, Berkeley). “It does appear that something significant happened not too long ago.”
Despite the chaos, there is some underlying order, as half of Andromeda’s satellite galaxies appear confined to the same plane. “That's weird,” says Weiss. “It was actually a total surprise to find the satellites in that configuration, and we still don't fully understand why they appear that way.”
The brightest of Andromeda’s companions — M32 — might hold clues to the system’s tumultuous history. The dwarf could be the leftovers of a much larger galaxy, one that Andromeda may have cannibalized a few billion years ago. M32 seems to have experienced a burst of star formation around that time. The disruption wrought by this merger could account for the haphazard configuration of Andromeda’s satellites.
Star formation in the other dwarf galaxies around Andromeda is equally confounding. “[It] really continued to much later times, which is not at all what you would expect for these dwarf galaxies," says Savino.
Samantha Penny (University of Portsmouth, UK), who was not involved in the research, agrees this is a standout finding. “Dwarfs in Andromeda kept forming stars for longer than those belonging to the Milky Way: This is not expected from theory and computer simulations,” she says.
“No one knows what to make of that so far,” adds Savino.
This animation begins with a view of the Andromeda Galaxy, zooming in through a scattering of foreground stars. The animation crosses 2.5 million light-years to reach the Andromeda, adorned by 36 dwarf galaxies. A Hubble Space Telescope was made to plot the galaxies' 3D locations relative to Andromed. The video then circles around a model of the Andromeda system based on Hubble data.
NASA / ESA / Christian Nieves (STScI) / Alessandro Savino (UC Berkeley); Acknowledgment: Joseph DePasquale (STScI) / Frank Summers (STScI) / Robert Gendler
For Ivan Baldry (Liverpool John Moores University, UK), who was also not involved in the research, observations like these could help us to answer deeper cosmological questions. “Dwarf galaxies are important in understanding dark matter as they formed in mini dark matter halos,” he says.
Martin Rey (University of Bath, UK), again not involved in the research, concurs. “The real story here is about dark matter,” he says. The number of dwarf galaxies astronomers observe is at odds with the number predicted to exist by conventional dark matter models. “Understanding whether this conflict comes from the physics of stars or the physics of dark matter — which of those theories is the broken one — is where the field is at right now,” Rey says.
“If you can constrain how stars form and evolve in dwarf galaxies through a survey like this then you are directly informing what we can infer about dark matter — that's the bigger picture of the massive effort that this team has done.”
There will be more to come. In another five years, data from Hubble and the James Webb Space Telescope will help astronomers turn back time, rewinding the cosmic clock billions of years into the past to fully chart the story of nearest neighbor.