The researchers combined previous extensive observations with the Hubble Space Telescope with their own infrared images of the same region of sky to generate one of the deepest “core samples” of galaxies across the universe ever obtained. The UCLA astronomers employed a technique, called photometric redshifts, that uses visible and infrared images of galaxies to give depth perspective to their needle-like view across the universe, and sorted the galaxies out by their distances.
“We have not only pretty pictures, but also information that tells the distances of these galaxies, how old they are and how many stars they have formed,” said UCLA graduate student James Colbert.
“These galaxies are scattered like crumbs on a table, and we believe that many of them will grow up to be something like our Milky Way, with a hundred billion suns,” said Matthew Malkan, UCLA professor of astronomy and a member of the research team. “We will never have a chance to see the Milky Way when it was young, but we can see these galaxies when they were young, even when they were forming.”
The ultra-long-exposure cosmic pictures were obtained at six wavelengths, including essential new infrared data.
“The combination of optical and infrared data is important because much of the birth and early evolution of galaxies in the young universe turn out to be hidden inside thick clouds of interstellar dust,” Colbert said. “These dust grains absorb the visible light, but allow infrared light to escape. We need the infrared to see through the thick fog of dust, and to make distance and mass estimates of extremely faint galaxies that are much more accurate.”
Infrared light is invisible to the human eye, but can be felt as heat radiation. Because it has a longer wavelength than visible light, it is less likely to be absorbed.
“Before combining these six detailed images, the pictures were like looking down a long, crowded street,” said UCLA research astronomer Michael Rich, who organized the 20-night infrared observing campaign. “Now you can see close-ups of the individuals and determine their ages, heights and weights, who is closer to us, and who is farther away.”
The galaxies are in the Hercules constellation, and the astronomers call their images the Hercules Deep Field. Deep-field images allow the study of even the faintest objects in the universe, Rich said. The team of astronomers also includes Jay Frogel, professor of astronomy at Ohio State, and graduate student Samir Salim at Ohio State.
Many of the galaxies in the Hercules deep field are so far away that their light has been traveling to Earth for much of the history of the universe. Their light left these galaxies long before our solar system even formed. These galaxies also look very different depending on whether they are observed in visible or infrared light.
One such example is illustrated in the second photograph the astronomers presented. This shows how a galaxy located more than nine billion light-years from Earth appears in each of the six wavelengths. It is a faint flattened ring in visible light, but appears as a bright sphere when viewed in the infrared. This galaxy has about 100 billion stars, about as many as in the Milky Way, and may look similar to a young version of our own galaxy.
The astronomers are pursuing detailed follow-up studies on a focused sample of 100 Hercules deep-field galaxies, using the Keck observatory in Hawaii. The team will make the complete Hercules deep-field data set available to astronomers worldwide.
The presence of a rare radio galaxy and surrounding galaxy cluster, seen when the universe was only 15 percent of its current age, led astronomers Rogier Windhorst, at Arizona State, and Bill Keel, at the University of Alabama, to use the Hubble Space Telescope to scrutinize the field initially. The new infrared data were obtained over 20 nights of observation at the MDM telescope on Kitt Peak, in Arizona. The MDM telescope is run jointly by Dartmouth, Columbia, Michigan State and Ohio State universities. The research was funded by NASA through the Space Telescope Science Institute.