To the surprise of many astronomers, Andromeda has been dethroned as the heftiest galaxy in the Local Group, our immediate neighborhood in the universe, which contains some 30 galaxies. Instead, a pair of astronomers argue, our own Milky Way, which so far ranked number two, is the heavyweight champion.
Earlier estimates put the mass of Andromeda at about twice that of the Milky Way, because it is larger and brighter and contains twice as many globular star clusters, spherical collections of several hundred thousand stars. But astronomer Wyn Evans of Oxford University in the United Kingdom says this is "poor evidence," because those studies focused on the visible disk of the galaxy and didn't include its dark, spherical halo. Because the disk is more massive than its counterpart in the Milky Way, astronomers argued that Andromeda's total mass, including the halo, would also be larger.
Together with Mark Wilkinson of Cambridge University, Evans analyzed velocity measurements of 37 objects that orbit Andromeda at large distances, such as small satellite galaxies, outlying globular clusters, and planetary nebulae. Because they're far away from the galaxy's center, the speeds of these objects are determined by the galaxy's total mass--including the dark matter in the halo. From these speeds, it appears that the total mass of Andromeda is only half the total mass of the Milky Way, the duo reports in a paper slated for publication in the Monthly Notices of the Royal Astronomical Society. Apparently, Andromeda's halo is much less massive than the Milky Way's.
But other astronomers aren't convinced. "I don't believe it yet," says Sidney van den Bergh of the Dominion Astrophysical Observatory in Victoria, Canada. He says two of the satellite galaxies used by Evans and Wilkinson are not really part of the Andromeda group; leaving them out could make a big difference, he says.
Evans concedes that "our error bars are pretty large." But he says new data further support the findings. For instance, the velocities of five faint dwarf galaxies discovered in 1998 and 1999 also point to a skimpier Andromeda. A final answer, however, will have to come from measuring the speed of more objects in Andromeda's halo, and measuring them more accurately, Evans says; NASA's Space Interferometry Mission and the European Space Agency's Global Astrometric Interferometer for Astrophysics, slated for launch in 2006 and 2009 respectively, will likely settle the matter.