Ultraluminous Infrared Galaxies
Ultraluminous infrared galaxies (ULIRGs) are galaxies which are about 100 to 1,000 times brighter in the infrared than a typical galaxy like our own Milky Way. They are the most luminous galaxies known in the local universe, and radiate more than ninety percent of their light in the infrared. Most of these galaxies are found in merging and interacting galaxy systems. It is thought that their brightness results from galactic collisions, which increase the rate of star formation.
Infrared emission from galaxies comes primarily from three sources: stars, interstellar gas, and dust. The emission from stars peaks in the near infrared (1-3 microns - a micron is short for a micrometer or 0.000001 meters). Emission from atoms and molecules in interstellar gas makes up only a few percent of the infrared output of galaxies. The primary source of infrared radiation beyond 3 microns is thermal emission from dust grains heated by starlight.
The brightest infrared galaxies are usually the ones which have a lot of dust (from star-forming regions for example). Astronomers using the IRAS satellite observed 20,000 galaxies in the infrared. Many of these were starburst galaxies - galaxies which are forming enormous numbers of new stars and are thus extremely bright in the infrared. Further infrared studies of these galaxies may find the cause of this star-forming frenzy. At the left is a 2MASS infrared image of a starburst galaxy.
Sometimes galaxies, each containing billions of stars, collide with each other. These collisions trigger star formation in these galaxies by causing the compression of clouds of gas and dust to the point where they collapse due to their own gravity. Due to a high rate of star formation, colliding gas-rich galaxies radiate very strongly in the infrared. The ISO satellite observed several colliding galaxies in the infrared and detected many areas of intense star formation within them.
Below are nine examples of ultraluminous infrared galaxies. Several of the images show galaxies in collision.
Ultraluminous Infrared Galaxies (Jason Ware)