Numerous "Tramp" Stars Adrift in Intergalactic Space Could Await Discovery

So-called tramp stars, flung from their galaxies in past gravitational interactions, could exist in great numbers outside the Milky Way Galaxy


Take a look up in the night sky. Whether on a mountaintop, largely unfazed by light pollution, or wedged in the heart of a muddied-sky metropolis, thank your lucky stars that you can see anything at all. A new study investigating the disruptive effects of galaxies merging or tugging on each other shows that there should be numerous stars thrown from their habitual confines during such interactions and into intergalactic space. From a planet encircling one of those lonely stars, the heavens would make for pretty dull viewing.

"If you were on an Earth orbiting a sunlike star that was halfway between us and the Andromeda Galaxy, or halfway between us and the Virgo Cluster, you would look out in the night sky, and you would actually see with your naked eye zero stars," says study co-author Michael Shara, an astrophysicist at the American Museum of Natural History in New York City. "You would see a few distant, blurry, fuzzy patches, and those would be the galaxies." Without a moon to reflect light, nighttime out there would be almost pitch-black.

Columbia University graduate student Maureen Teyssier, along with Shara and Columbia astronomy professor Kathryn Johnston, unpack in the December 10 issue of Astrophysical Journal Letters some of the mechanisms by which stars can be ejected from their home galaxies to become tramps—wandering or even escaped stars. Wanderers roam deep into space but remain loosely bound to a galaxy, whereas escapees are thrown clear to travel through space on their own.

In the prevailing, so-called hierarchical view of galactic growth, galaxies merge over time to form ever-larger galactic bodies. Evidence for this theory has been seen in simulations and in scraps of dwarf galaxies littering the edges of the neighboring Andromeda Galaxy, for instance. According to the new research, the violent gravitational interactions between a parent galaxy and an infalling satellite galaxy can liberate stars from the satellite and send them flying off in significant numbers. The majority of wandering and escaped stars appear to come from large satellite galaxies. "If you whack two big guys together, it's like two big trucks smashing into each other," Shara says. "There's lots and lots of debris and fragments flying in all directions."

By the study authors' conservative estimate, at least 0.05 percent of stars should be tramps—a number that, when applied to the Milky Way's estimated hundreds of billions of stars, provides a sizable population. "I think that's a very hard lower limit," Shara says of the 0.05 percent figure. "My guts tell me it's much, much higher than that."

James Bullock, a cosmologist at the University of California, Irvine, who did not contribute to the study but who helped develop simulations used in it, has studied what residues of galaxy formation should look like. He says that wandering and escaped stars have been considered before but that Teyssier, Johnston and Shara provide a new, ground-up look at how they originate. The authors, he says, "realize that this should probably be ubiquitous in the formation of the galaxies, and moreover, what makes it extremely interesting now is that we're approaching a time period where we might actually be able to detect these stars." Projects such as the Large Synoptic Survey Telescope, planned for development in Chile, and Pan-STARRS, now under construction in Hawaii, should be sensitive enough to pick out lone stars well outside the Milky Way.

Phenomena that make distant stars especially identifiable, such as outbursts known as novae, should be observable in large numbers to these next-generation survey telescopes. "It's the detection of these variable stars that is going to let us, for the first time, get a really solid handle on what the density of stars between the galaxies is," Shara says. And detecting a population of tramp stars would at the very least provide a "consistency check" of the hierarchical merger view of galactic growth, Bullock says.

With the benefit of their orbital characteristics, such remnants might even provide clues to the kinds of mergers that formed our Milky Way. "I don't think any one star is going to answer the question," Shara says. "But statistically, when we get thousands of such stars...I think we may have a pretty good archaeological record as to what the past few billion years of dynamical interactions have been."

Shara says he has been interested in tramp stars for a long time but that they do not gather a lot of attention—something he hopes to change with this new research. "I regard this as a kind of running up the flag," he says. "We're calling attention to these things."

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