The new IR-mapping from WISE reveals striking differences between related galaxies. This new IR-image of the two galaxies M81 and M82 reveals great differences between them

This image from NASA's Wide-Field Infrared Survey Explorer, or WISE, features two stunning galaxies engaged in an intergalactic dance. The galaxies, Messier 81 and Messier 82 (M81 & M82), swept by each other a few hundred million years ago, and will likely continue to twirl around each other multiple times before eventually merging into a single galaxy. The relatively recent encounter triggered a spectacular burst of star formation visible in both galaxies.

M81 at left is a prototypical "grand design" spiral galaxy with its pronounced and well-defined arms spiraling into its core. At the wavelengths WISE sees, these beautiful arms show areas of compressed interstellar gas and dust, which go hand-in-hand with areas of increased star formation. The spiral density waves that create this compression and star formation have been enhanced by the close gravitational interaction with its partner galaxy, Messier 82, causing the arms to appear more prominent than what is typically seen in other isolated spiral galaxies.

M82 (right) is also a spiral galaxy. However, it is seen edge-on from our point of view. It was originally classified as an irregular galaxy, until 2005, when astronomers were able to tease out spiral structure in near-infrared images (similar to wavelengths that WISE sees). Viewed in visible wavelengths, this galaxy appears to have a long thin bar shape, hence its common name, the Cigar Galaxy.

M82 is also a starburst galaxy, meaning it is currently forming stars at an exceptionally high rate. This huge burst of activity was caused by its close encounter with Messier 81, whose gravitational influence caused gas near the center of M82 to rapidly compress. This compression triggered an explosion of star formation, concentrated near the core. The intense radiation from all of the newly formed massive stars creates a galactic "superwind" that is blowing massive amounts of gas and dust out perpendicular to the plane of the galaxy. This ejected material (seen as the orange/yellow areas extending up and down) is made mostly of polycyclic aromatic hydrocarbons, which are common products of combustion here on Earth. It can literally be thought of as the smoke from the cigar.

Photo: M81 & M82 with a DSLR-camera. Courtsey of Spacedog

M81 and M82 are both very bright galaxies and can be seen on a clear, dark night with binoculars in the northern constellation Ursa Major, which contains the Big Dipper. In visible light, Messier 81 is one of the brightest galaxies that can be seen. M82 is not as bright at visible wavelengths, but in infrared light, it is by far the brightest galaxy in the entire night sky.

This image was made from observations by all four infrared detectors aboard WISE. Blue and cyan (blue-green) represent infrared light at wavelengths of 3,4 and 4,6 microns, which is primarily light from stars. Green and red represent light at 12 and 22 microns, which is primarily emission from warm dust.

Source: NASA