Stellar Motions In Outer Halo Reveal Evidence Of Milky Way Evolution
February 22, 2013

Stellar Motions In Outer Halo Reveal Evidence Of Milky Way Evolution

April Flowers for - Your Universe Online

Astronomers using NASA's Hubble Space Telescope to peer deep into the vast stellar halo that envelopes our galaxy have uncovered tantalizing evidence for the possible existence of a shell of stars that are a relic of cannibalism by our Milky Way.

For the first time ever, Hubble was used to gain precise measurements of the sideways motion of a small sample of stars located far from the galaxy's center. The unusual lateral motion of these stars is circumstantial evidence that they may be the remnants of a shredded galaxy that was gravitationally devastated by the Milky Way billions of years ago. The presence of these stars supports the theory that the Milky Way grew, at least in part, through the accretion of smaller galaxies.

The findings of this study will appear in an upcoming issue of the Astrophysical Journal.

"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," Roeland van der Marel of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, said in a statement.

These stars also offer an opportunity for measuring the "hidden" mass of our galaxy in the form of dark matter - an invisible form of matter that does not emit or reflect radiation. Our Milky Way galaxy "home" — out of the 100 billion galaxies in the Universe — offers the closest and therefore best site for detailed study of the history and architecture of a galaxy.

A research team from the University of California, Santa Cruz (UCSC), led by Alis Deason, worked with van der Marel to identify 13 stars located roughly 80,000 light-years from the galaxy's center. These stars lie in the Milky Way's outer halo, which is filled with ancient stars that date back to the formation of our galaxy.

These stars showed more of a sideways, or tangential, amount of motion than the team expected, which is different from what astronomers know about the halo stars near the Sun. Most halo stars tend to move in radial orbits in which the stars plunge toward the galactic center and travel back out again. If there is an over-density of stars at 80,000 light-years this tangential motion can be explained. Think of cars backing up on an expressway. A shell-like feature, as seen around other galaxies, would form from this traffic jam.

The team identified the outer halo stars out of seven years worth of Hubble telescope observations of the Andromeda Galaxy. The Hubble peered through the Milky Way's halo in order to study the Andromeda stars, more than 20 times farther away than the halo.

For studying the Andromeda galaxy, the halo stars are in the foreground of the images and considered clutter.

For Deason's study, however, they are pure gold as the observations offered a unique opportunity to look at the motion of the Milky Way halo stars.

As each Hubble image contains more than 100,000 stars, finding the halo stars was meticulous work. "We had to somehow find those few stars that actually belonged to the Milky Way halo," van der Marel said. "It was like finding needles in a haystack."

The stars were identified based on their colors, brightnesses, and sideways motions. Because they are so much closer, the halo stars appear to move faster than the stars of the Andromeda system. The halo stars were identified by Sangmo Tony Sohn of STScI, who also measured both the amount and direction of their slight sideways motion.

Although the stars move on the sky only approximately one milliarcsecond a year — equivalent to watching a golf ball on the Moon moving one foot per month — the movement was measured with 5 percent accuracy made possible in visible-light observations because of Hubble's razor-sharp view and instrument consistency.

"Measurements of this accuracy are enabled by a combination of Hubble's sharp view, the many years' worth of observations, and the telescope's stability. Hubble is located in the space environment, and it's free of gravity, wind, atmosphere, and seismic perturbations," van der Marel said.

Inner halo stars have highly radial orbits. The team compared the tangential motion of the outer halo stars with their radial motion and the results were surprising; the two were equal. Normally, computer simulations of galaxy formation show an increasing tendency towards radial motion the farther one moves out into the halo.

The observations from this new study imply the opposite. One plausible explanation is the presence of a shell structure in the Milky Way halo, as such a shell can form by accretion of a satellite galaxy. This is consistent with a theory in which the Milky Way continues to evolve over its lifetime due to such accretion.

The scientists compared their results with Sloan Digital Sky Survey data of halo stars. The Sloan study uncovered a higher density of stars at about the same distance as the 13 outer halo stars that the Hubble group had found. The Triangulum and Andromeda constellations have a similar excess of halo stars as well, beyond which the number of stars plummets.

"What may be happening is that the stars are moving quite slowly because they are at the apocenter, the farthest point in their orbit about the hub of our Milky Way," Deason explained. "The slowdown creates a pileup of stars as they loop around in their path and travel back towards the galaxy. So their in and out or radial motion decreases compared with their sideways or tangential motion."

Astronomers have observed shells of stars in the halos of some galaxies. They predicted that the Milky Way may contain them as well but until now there was limited evidence to support this theory. Because they are dim and spread out against the sky, the halo stars in our own galaxy are hard to find.

"These unexpected results fuel our interest in looking for more stars to confirm that this is really happening," Deason said. "At the moment we have quite a small sample. So we really can make it a lot more robust with getting more fields with Hubble." The current observations of the Andromeda galaxy make for a very small "keyhole view" of the sky.

Putting together a clearer picture of the Milky Way's formation history is the team's goal. It will be possible to calculate an accurate mass for the galaxy by knowing the orbits and motions of many halo stars.

"Until now, what we have been missing is the stars' tangential motion, which is a key component. The tangential motion will allow us to better measure the total mass distribution of the galaxy, which is dominated by dark matter. By studying the mass distribution, we can see whether it follows the same distribution as predicted in theories of structure formation," Deason said.