National Geographic News
Galaxy picture: a dwarf galaxy that's merged with another

The dwarf galaxy NGC 4449 is seen surrounded by a faint stellar stream.

Image courtesy NAOJ

Rachel Kaufman

for National Geographic News

Published February 10, 2012

For the first time astronomers have captured highly detailed pictures of a dwarf galaxy consuming an even smaller companion—one so diminutive that at first it looked like nothing more than a smudge.

"It wasn't clear what it was" originally, study co-author Aaron Romanowsky, an astronomer at the University of California, Santa Cruz, said of the mini galaxy.

The small blob of stars was first spied in digitized photographic plates from the Digitized Sky Survey project.

The tiny galaxy went unconfirmed for five years because it's so very faint. The nearby dwarf galaxy NGC 4449, in the constellation Canes Venatici, is about 50 times brighter.

Recently, though, Romanowsky and colleagues decided to follow up by capturing more detailed images of the dwarf galaxy and its neighboring smudge.

An international team—including study leader David Martinez-Delgado of the Max Planck Institute for Astronomy in Heidelberg, Germany, and co-author Jay Gabany, an amateur astronomer—collected data from the Blackbird Observatory in New Mexico and the Subaru Telescope in Hawaii.

"We got this fantastic image where we could actually see the individual stars of the galaxy" that's being swallowed by the larger one, Romanowsky said.

(Related picture: "New ALMA Telescope Peers Into Galaxy Smashup.")

Galaxies Feed "Like Russian Dolls"

Astronomers could tell that one galaxy was "eating" the other because of a so-called stellar stream around NGC 4449.

The pictures clearly showed a halo of scattered stars around the larger galaxy, which is being created as its smaller counterpart gets shredded by the merger.

(Related: "Galaxy 'Crumbs' Found in Milky Way—Proof of Recent Feeding.")

The discovery confirms some aspects of galaxy-formation theory, which says that galaxies of all sizes should consume each other.

"Large galaxies should eat smaller galaxies, and smaller galaxies should eat [even] smaller galaxies—like Russian dolls," Romanowsky said.

The finding also could explain why NGC 4449 is currently an active star-forming region: The smaller galaxy is orbiting the larger one in an elliptical orbit, much like how a comet orbits a star, Romanowsky said.

Although the companion galaxy appears exceptionally small to our eyes, when dark matter enters the picture, the two galaxies are actually much closer in mass, with the fainter dwarf maybe a fifth the mass of the brighter one.

(Also see "Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby?")

"When that thing makes a close pass, it's going to push gravitationally on [the larger galaxy] and get it all riled up," he said.

Such a push about 500 million years ago could have been what started the intense star birth we see today.

The dwarf galaxy-merger paper has been accepted to the Astrophysical Journal Letters.

NGC 4449's companion galaxy was independently discovered by a team led by astronomer Michael Rich of the University of California, Los Angeles, based on data from the Polaris Observatory Association. That study was published in the February 9 issue of the journal Nature.

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