How do you eat a galaxy? By being an even bigger galaxy. Over the course of the last year, UC Santa Cruz astronomers Alis Deason and Puragra Guha Thakurta and their team of collaborators found that our Milky Way may have a cannibalistic past.
“Cannibalization” occurs when a small galaxy comes into the proximity of a larger galaxy in space. When this happens, the immense gravitational pull of the larger galaxy pulls the smaller into its orbit. Over the course of billions of years, the larger galaxy swallows up the smaller galaxy’s stars, which is why the process is called “cannibalization.”
“If you imagine a smaller galaxy falling into our galaxy, it’s like a series of pendulums,” said Laura Prichard, an undergraduate exchange student from the University of Leeds who helped with the research. “So they’re all swinging through the galaxy, but then they all seem to gather at one point — when they’re stopping, turning around — and then they swing back again.”
This turnaround point is at approximately 100,000 light years away from the center of the Milky Way, which is also believed to be the extent of the smaller galaxy’s orbit. Alis Deason, the lead author of the study, said this relationship can be explained by the stars “remembering” their former orbit — they turn around at this point because it marks the extent of their former galaxy.
“Usually these stars continue to follow the same orbit that the satellite did,” Deason said, “because the orbits of the stars here generally retain memories of their initial conditions.”
For the study, the team used seven years worth of images of the Milky Way’s neighboring galaxy, Andromeda, taken from Guha Thakurta’s and others’ previous studies. Using Andromeda as a reference point, the astronomers measured the relative movement of 13 stars in the Milky Way’s outer halo during this seven-year period.
In the measurements she took from the images, Deason found that the stars’ movements were noticeably odd.
“What we’d expect in theory is that these stars mainly move backwards and forwards, what we call radial motion,” Deason said. “We found that their tangential motion, or sideways motion, was actually about the same as the radial motion. We had expected it to be a lot smaller. We’re kind of puzzled by this and we came up with the idea that perhaps these stars are what we call a shell.”
Like the pendulums in Prichard’s metaphor, this “shell” is the area where the oscillating stars slow down and stop before they assume their former course.
At this radius, the Milky Way’s density of stars sharply declines, a curiosity that may be explained by cannibalism. Guha Thakurta said the stars might still be trapped by the swallowed galaxy’s gravity. Gravitationally, they “remember” their origins. But the stars’ memories run deeper than gravity.
They may also chemically “remember” their former galaxy. Deason said smaller galaxies are metal-poor because they don’t have as many stars forming. Because stars are the element factories in galaxies, less stars or less mass means lighter elements.
“The fact that the star halo is metal-poor is suggestive that a lot of the stars came from smaller galaxies,” Deason said.
While a shell seems likely, it is not the only plausible reason for the astronomers’ observations.
“This is astronomy, so there are always several explanations,” Deason said. “It’s very hard to say we’re 100 percent certain.”
Other plausible explanations include that stars move more tangentially than astronomers previously thought or that these stars formed “in situ.” This means that the stars had formed from gas within the Milky Way, but were then “kicked out,” or ejected from a more central position by an incoming satellite, Deason said.
The evidence from these 13 stars most strongly suggests a cannibalization event, Deason said, but more research is needed to confirm this.
The team hopes to extend their research by applying for access to the Hubble Space Telescope’s image archives. If the access and funding are granted, they will be able to resume data collection in May 2013. Deason said she and her team predict they will find between 500 and 1,000 more shell stars, if their current theory is correct.
“[Further research] really offers the potential to completely map out these sorts of events throughout the galaxy, see how common these sorts of cannibalism events are,” Guha Thakurta said. “Or, we may find that this one that we already found is the only big one, everything else is smooth, even that would be a statement about the history of our galaxy’s eating habits. This finding is an eye-opener for astronomers. The potential is immense.”