Discovering the Early Monsters of the Universe
by Lennox Cowie
Over the last decade, studies of galaxy formation have moved to cover all stages of galaxy formation, from the cosmic dawn when galaxies first began to shine, to the present day, when galaxies seem to be approaching the end of their lives. Along with my collaborators, Amy Barger, Wei-Hao Wang, and Jonathan Williams, I have been working toward taking a cosmic census of the galaxies in the young Universe and determining how they evolved into what we see today.
At the end of the 1990s, Amy, David Sanders, and I discovered that when the Universe was about a quarter of its present age, galaxies with individual light outputs equivalent to many thousands of our own Milky Way dominated the Universe. By now these monster galaxies have almost died out, leaving much more numerous smaller galaxies. Intriguingly, by using a new telescope on Mauna Kea, we have recently discovered that these monsters were fired up even earlier than we had previously thought--and may, in fact, have dominated all of the star formation that occurred very early in the Universe's history.
With their enormous bursts of star formation, one might have thought that these extremely luminous galaxies would be fairly easy to see, even from far across the Universe. However, given their late discovery, this was clearly not the case. How were they escaping detection? The reason we had such a hard time finding them is that their violent activity is hidden from view at visual wavelengths. These galaxies were surrounded by huge amounts of dust and gas. Dust absorbs starlight produced within galaxies and reradiates it as a warm glow at longer wavelengths. In fact, the obscuration of these galaxies was so great that it required observations from a new camera--the Submillimeter Common-User Bolometer Array (SCUBA)--on the 15-meter (49-foot) James Clerk Maxwell Telescope (JCMT) on Mauna Kea to reveal that such powerful galaxies even existed! (Submillimeter astronomy studies the sky at wavelengths from about 0.01 to 0.04 inches, which is the short end of the radio part of the electromagnetic spectrum.)
This three-color optical image shows the region of the sky that contains one of the first extremely luminous, distant dusty galaxies discovered with SCUBA observations. The circle shows the accurate location of the galaxy as determined from SMA observations. Note that the position is completely visually blank because of the galaxy's dust content and distance. The two neighboring galaxies in the image are a red elliptical galaxy and a green star-forming galaxy, both of which are much closer to us than the SCUBA galaxy. The inset shows a submillimeter image of the hidden galaxy taken with the SMA. Images by Len Cowie.
SCUBA revolutionized our thinking about galaxy formation and evolution. But SCUBA's low resolution meant that the one thing it could not do was provide us with accurate positions for the galaxies. This made it very difficult to study these galaxies at other wavelengths, which we need to do to ascertain the properties of the galaxies in more detail and to determine their distances. For some of the SCUBA sources, we were able to use higher-resolution observations made at longer radio wavelengths to get around this problem. Spectroscopic follow-up of these sources with the Keck telescopes revealed that they had existed 2-3 billion years after the Big Bang. However, studying the rest of the galaxies found with SCUBA required a new telescope on Mauna Kea, the Submillimeter Array (SMA), which consists of eight 6-meter (20-foot) radio dishes that are used together. Recent observations with the SMA have given us another exciting result: dusty, extremely luminous galaxies teeming with newly formed stars existed less than 1.5 billion years after the Big Bang. These sources are so faint at visual wavelengths that we could never have discovered them without the submillimeter observations. Our discovery of these massive young sources may present yet another serious challenge to our current understanding of galaxy formation and evolution.
We expect even more discoveries at submillimeter wavelengths with future telescopes and instrumentation. The next generation submillimeter camera, SCUBA-2, is scheduled to begin operations on the JCMT on Mauna Kea soon. It will cover large areas of sky up to a thousand times faster than SCUBA, enabling us to detect substantial numbers of galaxies. Follow-up observations with future telescopes, such as the Atacama Large Millimeter Array (ALMA), an international facility with a major U.S. investment that is now being constructed in Chile, will enable us to learn more about this important population. Soon we may know just when the very first monster galaxies appeared.