Astronomy Picture of the Day 12-11-03

Explanation: From planet Earth, we view this strongly interacting pair of galaxies, cataloged as Arp 81, as they were only about 100 million years after their mutual closest approach. The havoc wreaked by gravity during their ominous encounter is detailed in this color composite image from the Hubble Space Telescope, showing twisted streams of gas and dust, a chaos of massive star formation, and a tidal tail stretching for 200 thousand light-years or so as it sweeps behind the cosmic wreckage. Also known as NGC 6622 (left) and NGC 6621, the galaxies are roughly equal in size but are destined to merge into one large galaxy in the distant future, making repeated approaches until they finally coalesce. Located in the constellation Draco, the galaxies are 280 million light-years away. The dark vertical band which seems to run through NGC 6621's location is a camera artifact.

Interacting galaxies are quite common - here's Arp 295:

Galaxy shows key evidence for furious star formation
NATIONAL RADIO ASTRONOMY OBSERVATORY NEWS RELEASE
Posted: December 10, 2003

Astronomers have discovered a key signpost of rapid star formation in a galaxy 11 billion light-years from Earth, seen as it was when the Universe was only 20 percent of its current age. Using the National Science Foundation's Very Large Array (VLA) radio telescope, the scientists found a huge quantity of dense interstellar gas -- the environment required for active star formation -- at the greatest distance yet detected.

A furious spawning of the equivalent of 1,000 Suns per year in a distant galaxy dubbed the Cloverleaf may be typical of galaxies in the early Universe, the scientists say.

VLA image (green) of radio emission from HCN gas, superimposed on Hubble Space Telescope image of the Cloverleaf galaxy. The four images of the Cloverleaf are the result of gravitational lensing. Credit: NRAO/AUI/NSF, STScI

"This is a rate of star formation more than 300 times greater than that in our own Milky Way and similar spiral galaxies, and our discovery may provide important information about the formation and evolution of galaxies throughout the Universe," said Philip Solomon, of Stony Brook University in New York.

While the raw material for star formation has been found in galaxies at even greater distances, the Cloverleaf is by far the most distant galaxy showing this essential signature of star formation. That essential signature comes in the form of a specific frequency of radio waves emitted by molecules of the gas hydrogen cyanide (HCN).

"If you see HCN, you are seeing gas with the high density required to form stars," said Paul Vanden Bout of the National Radio Astronomy Observatory (NRAO).

Solomon and Vanden Bout worked with Chris Carilli of NRAO and Michel Guelin of the Institute for Millimeter Astronomy in France. They reported their results in the December 11 issue of the scientific journal Nature.

In galaxies like the Milky Way, dense gas traced by HCN but composed mainly of hydrogen molecules is always associated with regions of active star formation. What is different about the Cloverleaf is the huge quantity of dense gas along with very powerful infrared radiation from the star formation. Ten billion times the mass of the Sun is contained in dense, star-forming gas clouds.

"At the rate this galaxy is seen to be forming stars, that dense gas will be used up in only about 10 million years," Solomon said.

In addition to giving astronomers a fascinating glimpse of a huge burst of star formation in the early Universe, the new information about the Cloverleaf helps answer a longstanding question about bright galaxies of that era. Many distant galaxies have super-massive black holes at their cores, and those black holes power "central engines" that produce bright emission. Astronomers have wondered specifically about those distant galaxies that emit large amounts of infrared light, galaxies like the Cloverleaf which has a black hole and central engine.

"Is this bright infrared light caused by the black-hole-powered core of the galaxy or by a huge burst of star formation? That has been the question. Now we know that, in at least one case, much of the infrared light is produced by intense star formation," Carilli said.

The rapid star formation, called a starburst, and the black hole are both generating the bright infrared light in the Cloverleaf. The starburst is a major event in the formation and evolution of this galaxy.

"This detection of HCN gives us a unique new window through which we can study star formation in the early Universe," Carilli said.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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