Luminous Infrared Galaxies
A false-color photograph of interacting galaxies. This picture consists of infrared images taken with IRAC (the SAO-led camera on NASA's Spitzer Space Telescope; red color), and visible images from NASA's Hubble Space Telescope (blue/green colors). These two galaxies began their collision about 40 million years ago: as they tug at each other, the disrupted gas prompts new stars to form. Eventually the galaxies will meld into one.
Our Milky Way home galaxy, like most galaxies, shines brightly in visible light because its stars are hot and emit at optical wavelengths. But twenty years ago the Infrared Astronomy Satellite, IRAS, discovered that the universe contained many fabulously luminous galaxies -- some more than a thousand times brighter than our galaxy -- that are practically invisible at optical wavelengths. The reason is that their infrared light comes not from stars, but from dust that is heated by starlight (although only up to frosty temperatures of about 70 kelvin, about 200 degrees below zero Celsius). At these temperatures very little optical light is emitted, while the dust absorbs most of the visible stellar radiation. IRAS galaxies are luminous because they contain so much of this dust. Astronomers are quite sure that the energy to heat the dust to even these temperatures comes from giant bursts of star formation that are hidden from optical view by the dust itself, but they do not know what triggers these bursts, nor whether our Milky Way might someday erupt in a similar way.
Two mechanisms have been suggested for triggering massive starbursts in a galaxy: collisions with another galaxy, and/or processes associated with a massive black hole at the galaxy's center. Most luminous infrared galaxies are so far away, however, that telescopes have been unable to sort out which of these two mechanisms is correct, or more precisely, when one mechanism predominates over the other. Five CfA astronomers, Sukanya Chakrabarti, Thomas Cox, Lars Hernquist, Philip Hopkins, and Brant Robertson, and a colleague, have performed detailed computer simulations of interacting galaxies using a code that they developed over several years. In a new paper out this month, they use their results to calculate the luminosity of these interacting systems, to uncover how that luminosity evolves with time as a galaxy ages, and to determine the relative contributions of starburst activity and nuclear activity to the infrared emission.
Their comprehensive calculations offer persuasive answers to several key problems. They find, contrary to previous thinking, that the most critical element in determining the temperature and luminosity of a galaxy is not the illumination from the source (either a starburst or the nucleus). Instead, it is injection of winds or jets into the galaxy, or other disruptive activity from the starburst or nucleus, that is responsible because this feedback stimulates the gas in the galaxy to produce more stars. They also find that the black hole nucleus produces warmer dust because it tends to deposit its energy in a much shorter time frame (only tens of millions of years versus hundreds of millions of years for starbursts). The new results not only shed important light on these cosmic beacons, they point to continued productive use of computer simulations in the study of how galaxies are born and age.