Astronomers using NASA's Hubble Space Telescope have solved a 20-year-old mystery by showing that a class of galaxy once thought to be rare is actually the most common type of galaxy in the universe.
Analyzing some of the deepest images ever taken of the heavens, the astronomers conclude that small irregular objects called "blue dwarfs" were more numerous several billion years ago, outnumbering the spiral galaxies like our Milky Way, and giant elliptical galaxies as well. This means the blue dwarfs are a more important constituent of the universe and figure more prominently in the evolution of galaxies than previously thought, researchers say.
The discovery was made by the international Medium Deep Survey team, led by Richard Griffiths of the Johns Hopkins University, Baltimore, MD, and extended by a deeper survey with Hubble Space Telescope by a team led by Rogier Windhorst of Arizona State University, Tempe, AZ.
"The new results have overturned the conventional picture of a universe dominated by giant grand-design spiral systems and elliptical galaxies," said Griffiths. "Instead, we're going to have to come up with a new way of understanding the distorted galaxies we see in huge numbers, which seemed to have formed later than the giant galaxies."
However, they say it is not clear whether these small irregular systems are indeed the building blocks of galaxies like the Milky Way, or have simply faded into obscurity. "Most of these faint objects are extremely blue in color, a strong indication that they are undergoing a brief, rapid burst of star formation," said Windhorst, who along with William Keel of the University of Alabama, Birmingham, AL, conducted a separate survey of remote galaxies.
These faint galaxies were randomly imaged as part of a key Hubble Space Telescope project, called the "Medium Deep Survey." The survey uses Hubble's Wide Field and Planetary Camera 2 (WFPC2) to search for unexpected objects in uncharted areas of sky. This highly efficient and cost-effective survey is conducted in "parallel mode" where the WFPC2 takes detailed pictures while a "primary" instrument, such as a spectrograph, collects data from a predetermined celestial target.
For the past 17 months, Griffiths and co-investigators from the United States (Richard Green, John Huchra, Garth Illingworth, David Koo, Kavan Ratnatunga, Tony Tyson, Rogier Windhorst) and the United Kingdom (Richard Ellis, Gerry Gilmore), have studied more than 50 random snapshots containing high resolution information for a total of tens of thousands of galaxies.
"We were immediately struck by the large numbers of irregular and peculiar galaxies in these HST random images," said Griffiths.
Another deeper Hubble image has further extended these exciting results. The image was obtained by Windhorst and Keel, and analyzed by Simon Driver of Arizona State University, Windhorst, and associates.
"At last, Hubble has allowed crystal clear images of these extremely faint objects, and we find that our universe is dominated by distorted systems of stars," said Driver. "At the faintest limits more than half the galaxies seen are such systems."
"We all know that the (clear) sky during the day is blue - due to scattered sunlight - but if your eyes had much more sensitivity, they would also see a very dim blue glow in the sky at night caused by myriads of faint blue galaxies, the mysterious nature of which was unknown until we imaged them in detail with Hubble," said Windhorst.
The researchers are now measuring the distances to these galaxies using the new generation giant telescopes on Earth.
BACKGROUND INFORMATION: "THE FAINT BLUE GALAXY MYSTERY" – A CENSUS OF GALAXIES
In the 1920s, when Edwin Hubble first discovered that the universe contained countless "island universes" called galaxies, astronomers believed that giant spiral galaxies, such as our own Milky Way and nearby Andromeda, as well as luminous elliptical galaxies, such as Messier 87 in the Virgo cluster, dominated the vast volume of space.
However, since the 1970s astronomers have been perplexed by the enormous numbers of "faint blue galaxies" seen in the deepest images gathered by the world's largest telescopes. Several Hubble Medium Deep Survey team members including David Koo of Lick Observatory, Richard Ellis of the Institute of Astronomy at Cambridge University, England, and Tony Tyson of AT&T Bell Laboratories, have grappled with this problem over the last two decades. In recent years, some hints to the nature of the faint blue galaxies also have come from ground-based surveys of nearby galaxies, including work done by John Huchra and colleagues at the Center for Astrophysics in Cambridge, MA. Due to distortion by Earth's atmosphere, these mysterious objects have remained murky blobs seen against the background glow of the ground-based night sky.
"The clue to further progress lies in extensive spectroscopic studies of these remarkable galaxies using ground-based telescopes," concluded Ellis. Ellis, Karl Glazebrook (Institute of Astronomy at Cambridge University, England) and colleagues have pioneered the construction of multiple object spectrographs precisely for this purpose on Britain's large telescopes. "Thus far we can say that many of these peculiar systems are being seen during an unusually active stage of star formation. Finding the cause of this activity is the remaining puzzle."
International Teams Pursue the Faint Blue Galaxy Mystery
Initial results and images from Wide Field and Planetary Camera 2 were published in the Astrophysical Journal Letters at the end of last year by Griffiths and co-investigators, and Duncan Forbes and co-investigators (Lick Observatory). The present results (Simon Driver of Arizona State University, Windhorst and Griffiths) are scheduled to be published in the Astrophysical Journal this summer. These results were independently and simultaneously found by Glazebrook (Glazebrook, Ellis, Santiago and Griffiths published in the Monthly Notices of the Royal Astronomical Society on July 15, 1995). Both Driver and Glazebrook based their results directly on the extensive Medium Deep Survey database built by Kavan Ratnatunga and colleagues at Johns Hopkins University in Baltimore, MD. The new results from the deeper survey are scheduled for publication in the Astronomical Journal Letters in August by Driver, et al.
The excellent new data have absolutely confirmed the initial hints in data taken before the HST servicing mission in December 1993, that the faint population was intrinsically small systems (papers in the Astrophysical Journal and Letters by Myungshin Im, et al., and by Stefano Casertano, et al. of the Johns Hopkins team, the latter to be published this summer).
The Medium Deep Survey is led by R. Griffiths at the Johns Hopkins University in the United States, in collaboration with eight other co- investigators in the USA and the United Kingdom, including R. S. Ellis and G. F. Gilmore (University of Cambridge), R. F. Green (National Optical Astronomical Observatories), J. P. Huchra (Harvard-Smithsonian Center for Astrophysics), G. D.Illingworth and D. C. Koo (Lick Observatory, University of California at Santa Cruz), K. Ratnatunga (Johns Hopkins University), A. J. Tyson (AT&T Bell Laboratories, New Jersey), and R. A. Windhorst (Arizona State University).
