Astronomers have long thought this sort of intergalactic violence could be the normal way large galaxies grow. The Milky Way and the Andromeda galaxy, the two closest and best-known examples of spiral galaxies, are both known cannibals.

When a dwarf galaxy approaches a large spiral galaxy, the bigger galaxy’s extra gravitational oomph strips gas, stars and dark matter from its hapless victim. Over a few billion years, the smaller galaxy is stretched like taffy into long strips or tendrils of stars.

“Within the hierarchical framework for galaxy formation, minor merging and tidal interactions are expected to shape all large galaxies to the present day,” write David Martínez-Delgado of the Max Planck Institute for Astronomy in Germany and colleagues in a new paper to be published in the October issue of the Astronomical Journal.

The Sagittarius stream around the Milky Way and the Great Southern stream around Andromeda are “archetype fossil records of satellite galaxy mergers,” the authors add. But there aren’t enough large galaxies in the Milky Way’s immediate neighborhood, called the “Local Group” of galaxies, to quantify how common these galactic bullies really are. In the new study, Martínez-Delgado and colleagues observed eight spiral galaxies up to 50 million light-years away, well beyond the local group, to hunt for the galactic equivalents of bloodstains and fingerprints.

The images revealed six new star stream candidates that look like nothing astronomers have seen before. Some of the galaxies, like NGC 4651 (above) — also known as the Umbrella Galaxy — display enormous arcs of stars that resemble an open umbrella. This galaxy’s stellar streams extend up to 50,000 light-years away from its center. The dwarf galaxy corpse had been detected by earlier observations, but had never been interpreted as a tidal stream.

The other galaxies in the survey showed a striking diversity in the shapes of their stellar streams, the authors write, including fuzzy clouds, great circles surrounding the larger galaxy and long spikes and plumes extending thousands of light-years from the galactic center.

“Each halo displays a unique and very complex pattern of stellar debris caused by different defunct companions,” the researchers write.

The variety of shapes neatly matches the shapes predicted by computer simulations of galaxy formation, suggesting that current theories of how galaxies grow by swallowing their neighbors are on the right track.

All the observations were taken using backyard telescopes owned by amateur astronomers: 20-inch telescopes at Blackbird Observatory in New Mexico and Rancho del Sol in California, a 14.5-inch telescope in Moorook, South Australia and a 6-inch telescope at New Mexico Skies.

Image: 1) R. Jay Gabany (Blackbird Obs.)/D. Martínez-Delgado (MPIA and IAC) et al. 2) D. Martínez-Delgado (MPIA). Left column from top to bottom: M 63, NGC 4651, NGC 7531, NGC 5866. Right column from top to bottom: NGC 1084, NGC 4651, NGC 3521, NGC 1055.

See Also:

• Supernova Wind Solves Galaxy Formation Mystery
• Cascade of Star Formation Captured in Omega Nebula
• Sharpest Image Yet of Massive Galaxy Collision
• Hubble Goes Deep, Finds Farthest Galaxies Yet
• Hubble Unravels Odd Galaxy’s History

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Authors: Lisa Grossman

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