Dancing Galaxies

http://www.nasa.gov/mission_pages/WISE/multimedia/gallery/pia13454.html

NASA’S Wide-Field Infrared Survey Explorer (WISE), recently took a picture of two galaxies—three, technically, with NGC 3077 on the bottom, but it’s fairly insignificant—and posted it to NASA.gov. Messier 81 is the “typical ‘grand design’” spiral galaxy on the bottom of the image. Messier 82, on the top, is also a spiral galaxy, but we can only see it edge-on, as seen in this image. Until 2005, when astronomers “teased out” evidence of its spiral structure from near-infrared images, Messier 82 was actually classified as an irregular galaxy. From our point of view, it seems to have a long, thin bar shape—hence its common name the Cigar Galaxy.

These two galaxies are called “dancing galaxies” because they brushed by each other a few hundred million years ago—a relatively recent meeting. They brushed close enough to trigger an amazing torrent of star formation in Messier 82, and a fair amount of star formation in Messier 81 as well. Messier 81’s gravitational influence made gas close to the centre of Messier 82 to precipitously compress, causing an explosion of star formation concentrated, obviously, near the core of the galaxy. This is why Messier 82 is currently also a starburst galaxy, because it is currently forming stars at an extraordinarily high rate. And as you can see, there are also yellow/orange areas around Messier 82 extending “up” and “down” in the photograph. These areas are what is known as a “superwind,” a massive amount of gas and dust blowing perpendicular to the plane of Messier 82. It’s caused by the intense radiation from all the newly formed stars. The superwind consists mostly of polycyclic aromatic hydrocarbons, common products of combustion found here on Earth. In this case, it can be literally thought of as the smoke from the cigar.

As for Messier 81, is has more pronounced and well-defined spiraling arms than is usually seen in other isolated spiral galaxies—and they also show areas of compressed gas and dust, which is evidence of star. This has been caused by its gravitational interaction with Messier 82.

This image was made from all four infrared detectors on WISE, overlapped to make one image. Blue and cyan represent wavelengths 3.4 and 4.6 microns, which is typically light from stars. Green and red represent light at 12 and 22 microns, which is primarily emission from warm dust.