Intriguing patterns of star motion and composition in the Milky Way hint at the presence of a remnant of a smaller galaxy consumed by the Milky Way billions of years ago, according to early results of a new star survey. This galaxy would have been the Milky Way's last significant "meal."
At a presentation at the annual meeting of the American Astronomical Society, held last week in San Diego, astronomers emphasized that they can't yet claim success in their search for historic relics of galaxies merging into the Milky Way. But something tantalizingly "silverish" has cropped up early in their needle-in-a-haystack hunt. They have scheduled observations that will expand their data set, which already contains information on 1,500 sunlike stars, to a planned total of 10,000 sunlike stars. What they find could have far-reaching ramifications for theories about the origins of the galaxy and other large-scale structures.
"The Milky Way is a fairly large galaxy, and we believe it was formed by the merging of a number of smaller galaxies," says Rosemary Wyse, a professor of physics and astronomy in the Krieger School of Arts and Sciences. "What's not clear is what merged when, and this is very significant in trying to constrain the theories of how galaxies evolve."
Wyse is an investigator on the 2dF Old Stellar Population Survey, an attempt to map out structural details of parts of the Milky Way galaxy. Wyse and her co-investigators will study the composition, distance and movement of 10,000 stars like the sun, concentrating most of their observations on sunlike stars in the outer regions of the galaxy, away from the thin disk that is the dominant structural feature of the Milky Way.
"The Milky Way is a disk galaxy, which means it has a disk of gas and stars, and these are moving in approximately circular orbits around the center of the galaxy, and essentially all the stars in the disk are moving together," Wyse says. "There is also a component called the thick disk, because it's puffed up to a higher scale height than the rest of the disk."
This "puffing up" effect is most likely a result of the entry of a fairly massive satellite galaxy into the Milky Way, Wyse says. Astronomers like Wyse believe the merger pumped orbital energy from the smaller galaxy into the Milky Way, leading to the increase in height. Wyse and her collaborators previously gathered evidence showing that the thick disk's formation, thought to be the last significant galaxy merger in the Milky Way's history, probably occurred around 10 billion years ago. This would mean the most significant galactic mergers to affect the Milky Way's development were over by the time the universe was merely less than a third of what astronomers believe is its present age--a significant constraint on galaxy formation theories.
Such a colossal act of consumption would have been a messy process that should have left behind telltale remnants. Wyse and her 2dF survey co-investigators, Gerry Gilmore of the Cambridge Institute of Astronomy and John Norris and Ken Freeman of the Mount Stromlo and Siding Spring Observatory in Australia, have designed the 2dF survey to investigate and reveal definitive evidence of this last great merger and other mergers.
"Just as the tidal interactions between the moon and the Earth cause distortions, there's going to be tidal interactions as a satellite galaxy comes into the Milky Way, and those can be severe enough to actually tear mass off the outer parts of the satellite galaxy," Wyse says. "Those stars should be left behind on an orbit that is similar to the orbit of the satellite galaxy at that time."
In addition to galactic remnants still orbiting the fringes of the Milky Way, stars and gas from satellite galaxies that entered the Milky Way may have retained distinct patterns of movement or unusual composition. Under the detailed scrutiny of the 2dF survey, such distinct characteristics should stand out against the typical patterns of movement and composition found in the Milky Way. Researchers may then be able to use the remnant to learn more about the galactic merger that produced it, including when the merger took place, and the initial orbit and size of the satellite galaxy that merged into the Milky Way.
Use of the characteristics of groups of stars to constrain theories about galaxy formation and evolution was first pioneered by researchers Olin Eggen, Donald Lynden-Bell and Allan Sandage in 1962. At the time, their results suggested a fairly simple and rapid evolution of galactic structure. More recent results have shown the process to be more complex and provoked new questions.
"The favorite theory of galaxy formation right now is that based on a universe that's dominated by cold, dark matter," Wyse says. Cold, dark matter can't directly be detected by astronomers, who instead infer its existence by the motions of stars and galaxies.
"Cold, dark matter predicts that big galaxies started off as small ones which merged together. But it also predicts that lots of the small galaxies survived this process. And we do not see nearly as many small ones left as are predicted to survive," Wyse says.
The Milky Way, for example, currently has approximately 10 satellite galaxies but, depending on the details of the cold, dark matter theory used, should have anywhere from 30 to 100 or more satellite galaxies.
Learning more about the Milky Way's merging history, Wyse notes, will help advance the debate on such important questions of cosmology.
Astronomers perform their observations for the survey at the Anglo-Australian Observatory near Coonabarabran, New South Wales, Australia. The telescope's 2 degree-field spectrograph, a complex, powerful instrument, makes the 2dF survey possible.
"The 2dF sets us up," Wyse says. "It allows you to get spectra of 400 objects at once. It used to be you had to do this star by star, which was physically tedious and took a long time. But now we can do hundreds at one time."
The 2dF spectrograph is supported by the Anglo-Australian Observatory, which is funded by the British and Australian governments.