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"Fossil" Galaxies May Have Helped End Dark Ages
01.23.06
An international team of astronomers used NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to make the first direct measurement of the leakage of ionizing radiation from a dwarf galaxy undergoing a burst of star formation. The result will help astronomers determine whether the first stars or some other object ended the cosmic dark age.
Image right: The left hand panel shows a visible light image of Haro 11 acquired at the European Southern Observatories in Chile. The right hand panel shows a false-color composite image of the central part of the galaxy acquired with NASA's Hubble Space Telescope. Red areas represent visible light captured by the Wide Field Planetary Camera 2 on board Hubble. Green areas are ultraviolet light, from the Advanced Camera for Surveys (ACS) on Hubble. Blue areas, also photographed by the ACS, represent emission from hydrogen gas. The ultraviolet light traces hot, young stars; the visible light traces older, cooler stars; while the line emission from hydrogen traces the interaction of energetic radiation with the gas in the galaxy. Credit: The Astrophysical Journal (Kunth et al. 2003, ApJ, Vol. 597, Page 266)
Ionized atoms have an unbalanced electric charge. In electrically balanced (neutral) atoms, positive electric charge in the center (nucleus) is balanced by surrounding negatively charged electrons. Radiation from stars and other celestial objects ionizes gas between stars and galaxies by removing electrons from the atoms that comprise the gas. The ionization history of the Universe therefore reveals when the first luminous objects formed and when the first stars began to shine.
Evidence suggests that the Universe was created about 13.7 billion years ago as a result of a rapid expansion of space called the Big Bang. The infant Universe was too hot for light to shine. Heat is just atoms in motion, and the heat of creation smashed atoms together with such force that they were completely ionized -- they were broken up into electrons and atomic nuclei, which scatter light like fog.
About 380,000 years after the Big Bang, the Universe cooled enough to allow the electrons to combine with atomic nuclei, and a gas of electrically neutral atoms formed. This gas was transparent, so at this point, the initial flash of energy created during the Big Bang was released, and we detect it today as the cosmic microwave background radiation. However, after the creation flash, the lights went out, because there were no stars or any other bright objects -- they had not yet formed. This long night is known as the cosmic dark ages.
Image left: Artist's concept of the first stars in the Universe turning on. Wilkinson Microwave Anisotropy Probe (WMAP) data reveals that this era occurred 200 million years after the Big Bang, much earlier that many scientists had suspected. Credit: NASA/WMAP Science Team Print resolution copy (10 meg jpg image)
Astronomers are unsure if the first stars or some other object ended the dark ages, but FUSE observations of "Haro 11", a nearby dwarf starburst galaxy in the southern constellation of Sculptor, provide a clue. "This is the latest example where the FUSE observation of a relatively nearby object holds important ramifications for cosmological questions," said Dr. George Sonneborn, NASA/FUSE Project Scientist at NASA's Goddard Space Flight Center, Greenbelt, Md.
Dwarf galaxies are small, very faint galaxies containing a large fraction of gas and relatively few stars. Some astronomers think these little galaxies are relics from the early Universe. According to one model of galaxy formation, many of these smaller galaxies came together to build up larger ones. If true, dwarf galaxies can be thought of as "fossils" that have managed to survive -- without significant changes -- from that earlier period to the present. In this scenario, these smaller galaxies also would have been relatively more common and important in the early Universe than they are now.
The team, led by Dr. Nils Bergvall of the Astronomical Observatory in Uppsala, Sweden, analyzed the FUSE data on Haro 11 and discovered that between 4 percent and 10 percent of the ionizing radiation produced by the hot stars in Haro 11 is able to escape into intergalactic space. If this result is typical of other dwarf starburst galaxies, including the counterparts of these objects in the early Universe, this would indicate that such objects could have made a substantial contribution to the re-ionization of the Universe after it first cooled and recombined into neutral atoms after the Big Bang.
"Although other groups have looked for this effect, this is the first solid detection of ionizing radiation escaping from a starburst region and into the intergalactic medium," Bergvall said. The result will be published in the European journal Astronomy and Astrophysics.
The present Universe is mostly ionized, and astronomers generally agree that this re-ionization occurred between 12.5 and 13 billion years ago, when the first large-scale structures (galaxies, galaxy clusters) were forming. The details of the re-ionization are unclear, but are of intense interest to astronomers studying the dark age of the Universe.
According to the team, likely contributors of ionizing radiation in the early Universe include the intense radiation generated as matter fell into black holes that formed what we now see as the quasars, and the leakage of radiation from regions of early star formation. But until now, there was no direct evidence indicating star formation regions could have made a contribution.
The FUSE project is a NASA Explorer mission developed in cooperation with the French and Canadian space agencies by the Johns Hopkins University, Baltimore, Md., the University of Colorado, Boulder, and the University of California, Berkeley. The mission is operated out of Johns Hopkins University's Homewood campus in Baltimore. NASA Goddard manages the program for NASA's Science Mission Directorate.
RELATED LINKS:
How it all began: an introduction to Cosmology
More about NASA's FUSE mission
More about NASA's WMAP mission
NASA Goddard Space Flight Center