New Proof Spiral Galaxies Eat, Digest Dwarfs

A ghostly halo of dust and gas surrounds the spiral galaxy M63, aka the Sunflower galaxy, in a recently released picture taken in 2005.

The image—made using a remote-controlled, privately owned telescope in New Mexico—was captured during a recent survey of spiral galaxies, which found telltale remnants of galactic digestion.

Larger spiral galaxies—each containing hundreds of billions of stars—are thought to grow by "eating" neighboring dwarf galaxies, which contain just a few billion stars.

As the smaller, satellite galaxy is pulled toward the hungry spiral, gravity distorts and shreds the dwarf, so that within a few billion years the smaller galaxy is reduced to wisps and tendrils called tidal streams. Over a few billion more years, the faint streams of stars are assimilated into the spiral.

Tidal streams and other structures linked to galactic feeding frenzies have been seen around our own Milky Way galaxy and its immediate vicinity since 1997.
But the new survey, led by David Martínez-Delgado of the Max Planck Institute for Astronomy, is the first to show that these structures exist around much more distant spirals, backing up this theory of galaxy evolution.

"This process could be also very important in elliptical galaxies," Martínez-Delgado said in an email. "But we are only studying nearby spiral galaxies with a mass similar to our galaxy in the local universe ... [so] that we can understand the formation of the Milky Way."The main body of the Sunflower galaxy sits at the center of a negative version of the picture in the previous frame. The negative reveals the incredibly faint structures of the tidal streams surrounding this galaxy.

The Sunflower lies 30 million light-years from Earth and stretches 60,000 light-years across. Despite its distance, the spiral galaxy is easy to spot in northern skies near the Big Dipper using a modest amateur telescope.

In fact, all the pictures in the new survey were made using private telescopes equipped with commercially available cameras, located at observatories in New Mexico, California, and Australia.

Messy Eater

Dark arms and faint stellar clouds surround the spiral galaxy NGC 7531, as seen in the negative of a picture of the galaxy taken as part of the new survey. According to the study team, the survey suggests that major tidal streams, with masses between 1 and 5 percent of the galaxy's total mass, are common around spiral galaxies.

Our local group of galaxies hosts several tidal streams, including the Arcturus stream, thought to be the remains of a dwarf galaxy consumed by the Milky Way. (Related: "'Rogue' Dwarf Shines New Light on Tiny Galaxies.")

But galaxy growth via consumption isn't the only source of tidal streams: The Magellanic stream, a bridge of stars connecting the Large and Small Magellanic clouds, is thought to have been created when the two Milky Way satellite galaxies had a near collision.



Ebbing With the Tides

An artist's impression shows the likely path of the dwarf galaxy that got ripped apart to form NGC 4651's umbrella-like tidal stream.

Partially hidden by the galaxy's disk in visible images, the negative images of NGC 4651 (not shown) used in the new survey revealed an additional shell of stellar debris around the east side of the galaxy, offering clues to the death throes of the original dwarf.

"Dwarf galaxies are the first systems that formed in the universe, then they merged to form larger systems like the Milky Way," Martínez-Delgado said. "The dwarf companions that we observe today around spiral galaxies—like the Milky Way and Andromeda—are the survivors of this process."