Andromeda Galaxy Found Much Larger Than Previously Thought

Andromeda galaxy offers astronomers more and more surprises; this time - an enormous halo of stars extending far beyond the swirling disk of the famous galaxy, the closest large galactic neighbor to our Milky Way, suggesting an Andromeda five times bigger than previously thought. "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.

The research team uses observations at the Kitt Peak National Observatory in Arizona and the W. M. Keck Observatory in Hawaii. The astronomers detected a sparse population of red giant stars, smoothly distributed around the galaxy out to a distance of at least 500,000 light-years from the center.

The stars are bound to the galaxy by gravity even at that distance. "These stars probably represent Andromeda's stellar halo, a distinct structural component of the galaxy that has eluded astronomers for over 20 years," Guhathakurta said. "The researchers have found evidence that stars in the halo are chemically anemic compared with stars in the inner parts of the galaxy," said Jasonjot Kalirai, a postdoctoral fellow at UCSC.

"The halo stars are "metal-poor," meaning they contain smaller amounts of the heavier elements, a finding that is consistent with theoretical models of galaxy formation", Kalirai said.

Andromeda (M31) is a large spiral galaxy which appears very similar to our own Milky Way. Astronomers find it difficult to investigate the overall structure of the Milky Way from Earth, but Andromeda offers a global view of a typical spiral galaxy that is close enough for astronomers to observe individual stars within it (at about 2.5 million light-years from Earth).

Andromeda is also the biggest galaxy in the "Local Group," along the Milky Way and other 30 galaxies. "The suburbs of M31 and the Milky Way are so extended that they nearly overlap in space, despite the great distance between these two galaxies. If the whole of M31 were bright enough to be visible to the naked eye, it would appear to be huge, larger in apparent size than the Big Dipper", said R. Michael Rich of UCLA.

Spiral galaxies possess 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 center, whereas the density of the halo stars falls off more gradually. "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, while the halo extends five times farther than that", said Guhathakurta.

"We now believe that previous groups have been mistakenly identifying the outer parts of the Andromeda bulge as its halo.".

The team could detect the halo by developing a technique for distinguishing halo stars in Andromeda from the more numerous foreground stars in the Milky Way. "A foreground star with low luminosity and a luminous star that is much farther away can be hard to tell apart because they appear to be equally bright from our perspective," Guhathakurta said.

The new technique combined five diagnostic criteria, using photometry (brightness measurements) and spectroscopy (which separates starlight into a spectrum of different wavelengths), including radial velocity and parameters based on differences in surface gravity between red giants and dwarf stars. "There are assuredly other kinds of stars in Andromeda's halo, but they are just too faint for us to get spectra of them."

"Galaxy formation theories tell us that halos are pristine--the oldest component of the galaxy--but this is based almost entirely on studies of our own galaxy. A detailed study of this newly discovered Andromeda halo will allow us to test whether these theories apply more generally to galaxies other than the Milky Way," said Guhathakurta.