Andromeda: a gift that keeps giving

The lovely Andromeda galaxy appeared as a warm fuzzy blob to the ancients. To modern astronomers millennia later, it appeared as an excellent opportunity to better understand the universe. In the latter regard, our nearest galactic neighbor is a gift that keeps on giving.

Caltech postdoctoral researcher Scott Chapman and Rodrigo Ibata of the Observatoire Astronomique de Strasbourg in France have led a team of astronomers in mapping the detailed motions of stars on Andromeda’s outskirts. Their recent observations with the Keck Telescopes show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself. This means that the spiral disk of Andromeda is three times larger in diameter than previously estimated.

At the annual summer meeting of the American Astronomical Society on May 30, Chapman outlined evidence of a vast, extended stellar disk that makes the galaxy more than 220,000 light-years in diameter. Previously, astronomers thought Andromeda was about 70,000 to 80,000 light-years across. Andromeda itself is about 2 million light-years from Earth.

The new dimensional measure is based on the motions of about 3,000 of the stars some distance from the disk that were once thought to be merely the “halo” of stars in the region and not part of the disk itself. By taking very careful measurements of the “radial velocities,” the researchers were able to determine precisely how each star was moving in relation to the galaxy.

The results showed that the outlying stars are sitting in the plane of the Andromeda disk itself and, moreover, are moving at a velocity that shows them to be in orbit around the center of the galaxy. In essence, this means that the disk is vastly larger than previously known.

Further, the researchers have determined that the nature of the “inhomogeneous rotating disk”—in other words, the clumpy and blobby outer fringes of the disk—shows that Andromeda must be the result of satellite galaxies long ago slamming together. If that were not the case, the stars would be more evenly spaced. Ibata says, “This giant disk discovery will be very hard to reconcile with computer simulations of forming galaxies. You just don’t get giant rotating disks from the accretion of small galaxy fragments.”

The current results, which are the subject of two papers already available and a third yet to be published, are made possible by technological advances in astrophysics. In this case, the Keck/DEIMOS multi-object spectrograph on the Keck II Telescope possesses the mirror size and light-gathering capacity to image stars that are very faint, as well as the spectrographic sensitivity to obtain highly accurate radial velocities.

The extended stellar disk has previously gone undetected because stars in the region could not be known to be a part of the disk until their motions were calculated. In addition, the inhomogeneous “fuzz” that makes up the extended disk does not look like a disk, but appears to be a fragmented, messy halo built up from many previous galaxies crashing into Andromeda, and it was assumed that stars in this region would be going every which way.

“Finding all these stars in an orderly rotation was the last explanation anyone would think of,” says Chapman. He adds that further work will determine whether the extended disk is merely a quirk of the Andromeda galaxy, or is perhaps typical of other galaxies.

This article is adapted from a press release by Caltech Media Relations. For more information, visit www.astro.caltech.edu/~schapman/m31.html or pr.caltech.edu/media/Press_Releases/PR12703.html.
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