Astronomers have found an enormous halo of stars bound to the Andromeda galaxy and extending far beyond the swirling disc seen in images of the famous galaxy, our nearest large galactic neighbour.
The discovery, which was reported to the American Astronomical Society at a meeting in Seattle, suggests that Andromeda is as much as five times larger than astronomers had previously thought.
"I am absolutely astounded by how big this halo is. As we looked farther and farther out, we kept finding stars that look like halo stars," said Puragra (Raja) Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz, who will present the findings at the meeting.
Guhathakurta and his collaborators at UCSC, UCLA, and the University of Virginia are conducting an ongoing study of Andromeda's stellar halo, using observations at the Kitt Peak National Observatory in Arizona and the W M Keck Observatory in Hawaii.
Their new findings are based on data gathered using the four-meter Mayall Telescope at Kitt Peak and the DEIMOS spectrograph on the 10-meter Keck II Telescope in Hawaii.
According to the researchers, these stars probably represent Andromeda's stellar halo, a distinct structural component of the galaxy that has eluded astronomers for over 20 years.
Following their discovery, the researchers also found evidence that the stars in the halo are chemically anaemic compared with stars in the inner parts of the galaxy.
Andromeda is a large spiral galaxy very similar to our own Milky Way. While it is difficult for astronomers to study the overall structure of the Milky Way from Earth's vantage point within it, the Andromeda offers a global view of a classic spiral galaxy that is close enough for astronomers to observe individual stars within it.
The Andromeda is about 2.5 million light-years from Earth and is the largest galaxy in the "Local Group," which also includes the Milky Way and about 30 smaller galaxies.
Spiral galaxies typically have three main components: a flattened disk, a bright central bulge with a dense concentration of stars, and an extended spherical halo of sparsely distributed stars.
The concentration of stars in the central bulge decreases exponentially with increasing distance from the centre, whereas the density of the halo stars falls off more gradually (as an inverse power of the radius).
In Andromeda, the disk has a radius of about 100,000 light-years. Outside the plane of the disk, stars plausibly belonging to the central bulge can be found as far out as 100,000 light-years from the center of the galaxy, said Guhathakurta.
The group's ongoing investigation of Andromeda's halo promises to shed new light on the question of how large galaxies formed, he added.