An international team of astronomers using NASA’s Hubble Space Telescope has found tantalizing evidence for the existence of a shell of stars that are a relic of cannibalism by our galaxy.
This image shows the disk of the Milky Way Galaxy surrounded by a faint halo of old stars (NASA / ESA / A. Field, STScI)
“Hubble’s unique capabilities are allowing astronomers to uncover clues to the galaxy’s remote past. The more distant regions of the galaxy have evolved more slowly than the inner sections. Objects in the outer regions still bear the signatures of events that happened long ago,” said Dr Roeland van der Marel of the Space Telescope Science Institute in Baltimore, co-author of a paper accepted for publication in the Astrophysical Journal (arXiv.org version, Hubblesite’s version).
The astronomers used Hubble to precisely measure the sideways motions of a small sample of stars located far from the center of the Milky Way. Their unusual lateral motion is circumstantial evidence that the stars may be the remnants of a shredded galaxy that was gravitationally ripped apart by our galaxy billions of years ago. These stars support the idea that the Milky Way grew, in part, through the accretion of smaller galaxies.
The team identified 13 stars located roughly 80,000 light-years from the galaxy’s center. They lie in the Milky Way’s outer halo of ancient stars that date back to the formation of our galaxy.
The team was surprised to find that the stars showed more of a sideways, or tangential, amount of motion than they expected. This movement is different from what astronomers know about the halo stars near the Sun, which move predominantly in radial orbits. Stars in these orbits plunge toward the galactic center and travel back out again. The stars’ tangential motion can be explained if there is an over-density of stars at 80,000 light-years, like cars backing up on an expressway. This traffic jam would form a shell-like feature, as seen around other galaxies.
The astronomers plucked the outer halo stars out of seven years’ worth of archival Hubble observations of the Andromeda Galaxy. In those observations, Hubble peered through the Milky Way’s halo to study the Andromeda stars, which are more than 20 times farther away. The Milky Way’s halo stars were in the foreground and considered as clutter for the study of Andromeda. But to the current study they were pure gold. The observations offered a unique opportunity to look at the motion of Milky Way halo stars.
“We had to somehow find those few stars that actually belonged to the Milky Way halo,” Dr van der Marel said. “It was like finding needles in a haystack.”
The astronomers identified the stars based on their colors, brightnesses, and sideways motions. The halo stars appear to move faster than the Andromeda stars because they are so much closer. They identified the halo stars and measured both the amount and direction of their slight sideways motion. The stars move on the sky only about one milliarcsecond a year, which would be like watching a golf ball on the Moon moving one foot per month. Nonetheless, this was measured with 5 percent precision, made possible in visible-light observations because of Hubble’s razor-sharp view and instrument consistency.
Stars in the inner halo have highly radial orbits. When the team compared the tangential motion of the outer halo stars with their radial motion, they were very surprised to find that the two were equal. Computer simulations of galaxy formation normally show an increasing tendency towards radial motion if one moves further out in the halo.
These observations imply the opposite trend. The existence of a shell structure in the Milky Way halo is one plausible explanation of the findings. Such a shell can form by accretion of a satellite galaxy. This is consistent with a picture in which the Milky Way has undergone continuing evolution over its lifetime due to the accretion of satellite galaxies.
Shells of stars have been seen in the halos of some galaxies, and astronomers predicted that the Milky Way may contain them, too. But until now there was limited evidence for their existence. The halo stars in our galaxy are hard to see because they are dim and spread across the sky.
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Bibliographic information: A. J. Deason et al. 2013. The Velocity Anisotropy of Distant Milky Way Halo Stars from Hubble Space Telescope Proper Motions. ApJ, in press; arXiv: 1302.5111
