"Rogue" Dwarf Shines New Light on Tiny Galaxies

Scott Norris
for National Geographic News
November 21, 2007

A fast-moving "rogue" on the outskirts of the Andromeda galactic system may provide new insight into the nature of so-called dwarf galaxies and their potential role in galaxy formation.

The newly discovered dwarf, dubbed Andromeda XIV, may also indicate that the large spiral galaxy Andromeda—known to researchers as M31—is far more massive than had been thought.

Andromeda XIV is the latest in a number of recently discovered dwarf galaxies in the galactic local group, which includes our own Milky Way and its nearest spiral neighbor, M31.

Most of the previously known dwarfs in the local group are clearly satellites bound by gravity to the larger galaxies, researchers say.

But Andromeda XIV appears to be moving too fast to be locked in orbit around M31, said team leader Steven Majewski of the University of Virginia in Charlottesville.

For the new dwarf to be a satellite, M31 would have to be much more massive and exert a much stronger gravitational pull than current models suggest, Majewski said.

Perhaps more likely, Andromeda XIV may simply be a new arrival just entering our galactic neighborhood from distant regions of space.

"If Andromeda XIV is unbound to M31 and falling in for the first time, it would show that the assembly of the local group is not yet complete and that we are still picking up new members from outside," Majewski said.

A new arrival would give scientists a unique view of a dwarf galaxy in a relatively "pure" state, uninfluenced by billions of years of gravitational tug-and-pull with a larger neighbor.

Majewski's team—which also includes researchers at the University of California in Santa Cruz and Los Angeles—announced their discovery in a paper published this week in the Astrophysical Journal Letters.

Dark Building Blocks

Dwarf galaxies are so named because they contain only a few billion stars—a relatively miniscule number compared to the hundreds of billions of stars in galaxies like the Milky Way.

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