About ten years ago, astronomers using new submillimeter wavelength facilities discovered the existence of a new class of very distant galaxies. These objects are located so far away that their light has been traveling towards us for over ten billion years - more than 70% of the lifetime of the universe. Although today they are old, we see them as they were only a few billion years after they formed, when they were relatively young.
These galaxies were undetected in the visible but emit strongly at submillimeter wavelengths because they have an abundance of warm dust. What heats the dust is still controversial - probably either massive star formation, or an active black hole at the galactic nucleus, or perhaps both. Our Milky Way galaxy, or at least the region where the sun resides, probably formed between seven and ten billion years ago, and so understanding these remote systems can also help us understand our own origins.
Fortunately, the Infrared Array Camera on the Spitzer Space Telescope (IRAC; SAO astronomer Giovanni Fazio is the PI of the IRAC team) is sensitive enough to have detected these submillimeter galaxies in the infrared. The IRAC images have led to a breakthrough because of IRAC's spatial precision, which is much higher than that of the submillimeter instruments. Since numerous distant galaxies can appear crowded together in the sky, IRAC's resolution enables scientists to identify which galaxies are the unique submillimeter ones by measuring their infrared emission and infrared color - the submillimeter galaxies are very red. But astronomers still have not been able to figure out what heats the dust. Conventional wisdom holds that the infrared colors are unable to sort out whether star formation or black hole activity dominates the heating.
Three SAO astronomers, Matt Ashby, Giovanni Fazio, and their student Josh Younger, and a team of eleven other scientists, have analyzed a set of forty-seven relatively well studied submillimeter galaxies and compared them to a larger sample of other kinds of galaxies. All forty-seven were detected by IRAC with a signal high enough to determine their colors. By comparing the data with theoretical models of galaxy evolution, the team reaches the remarkable conclusion that IRAC infrared observations can indeed distinguish the two groups in about 80% of the cases. The result implies that the two mechanisms (star formation or an active nuclear black hole) are typically not both simultaneously at work in these galaxies. That means that as galaxies evolve, transitions between the two stages of life must occur relatively quick in cosmic terms, less than about a few hundred million years. Not least, the team concludes that the new Spitzer "Warm Mission," which begins next spring with only IRAC and two of its detectors working, should be able to make many more such determinations with relative ease.