Pictures: New Proof Spiral Galaxies Eat, Digest Dwarfs

Sunflower's Hidden Petals

Image courtesy R. Jay GaBany and D. Martínez-Delgado, MPIA

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. (Related: "Robotic Observatory Built on Remote Antarctic Summit.")

Published September 9, 2010

Messy Eater

Image courtesy D. Martínez-Delgado, MPIA

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. (See a picture of the Magellanic clouds over a Brazilian rain forest.)

Published September 9, 2010

Ring Around the Galaxy

Illustration courtesy Jon Lomberg

A small satellite galaxy, seen as an orange blob, gets stretched into a ribbon of stars as it's pulled apart by a larger spiral galaxy in an artist's impression.

Previously, astronomers had used detailed computer models to show how tidal streams and other signatures of galaxy digestion might look around spiral galaxies. The new survey has shown that all the predicted features actually exist in the universe—strong evidence that current models of galaxy evolution are on the right track, the study authors say.

"Cosmological models predict that a galaxy like the Milky Way is formed by the accretion of 100 to 200 dwarf galaxies," Martínez-Delgado said.

Published September 9, 2010

Umbrella Organization

Image courtesy R. Jay GaBany and D. Martínez-Delgado, MPIA

An unusual umbrella-like structure—the remains of a torn-up dwarf galaxy—is just visible in a picture of the spiral galaxy NGC 4651, about 35 million light-years from Earth.

The umbrella is the brightest tidal stream detected during the new survey. In fact, the structure was first spotted by astronomers in 1959, but this is the first time it's been interpreted as the remains of a chewed-up dwarf galaxy.

(See pictures of unusual cosmic shapes created by galaxy mergers.)

Published September 9, 2010

Ebbing With the Tides

Image courtesy R. Jay GaBany and D. Martínez-Delgado, MPIA

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."

(Also see "'Green Pea' Pictures: New Galaxy Class Discovered.")

Published September 9, 2010

Eye of the Storm

Image courtesy D. Martínez-Delgado, MPIA

The spiral galaxy NGC 1084 sits at the eye of a negative version of the same picture, which reveals the tidal streams likely left over from galaxy gluttony.

A previous study of this spiral galaxy showed that it experienced a series of short bursts of star formation over the past 40 million years or so. Astronomers think these bursts could be linked to a merger with a gas-rich dwarf galaxy. (Related: "Earth Likely to Relocate in Galactic Collision.")

Surveying galaxies by combining their positive and negative images presents researchers with a new technique for studying distant galaxy growth via mergers, according to the study authors.

A survey of even more galaxies is already underway, and the team plans to use the data to see whether current models can predict the relative frequency of different structures seen around spiral galaxies thought to have consumed their smaller neighbors.

The new galaxy-survey paper has been accepted for publication in the Astronomical Journal, and a preprint version is now online at the arXiv.org website.

Published September 9, 2010