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News Release 8 of 48

September 23, 2004 09:00 AM (EDT)

News Release Number: STScI-2004-28

Hubble Approaches the Final Frontier: The Dawn of Galaxies

September 23, 2004: Detailed analyses of mankind's deepest optical view of the universe, the Hubble Ultra Deep Field (HUDF), by several expert teams have at last identified, what may turn out to be, the earliest star-forming galaxies. Astronomers are now debating whether the hottest stars in these early galaxies may have provided enough radiation to "lift a curtain" of cold, primordial hydrogen that cooled after the big bang. This is a problem that has perplexed astronomers over the past decade, and NASA's Hubble Space Telescope has at last glimpsed what could be the "end of the opening act" of galaxy formation. These faint sources illustrate how astronomers can begin to explore when the first galaxies formed and what their properties might be.

But even though Hubble has looked 95 percent of the way back to the beginning of time, astronomers agree that's not far enough.

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Q & A: Understanding the Discovery

  1. 1. What is cosmic reionization, and why is it so important?

  2. As radiation and matter intersected in the young universe, the gas in the universe basically see-sawed from being hot to cold, to hot again. It was extremely hot during the big bang, expanded and cooled, but then was re-heated by the earliest stars. The ultraviolet light from these stars knocked electrons off of hydrogen atoms, therefore ionizing them. This is a watershed in the evolving universe. The light from stars dominated the volume of space by lifting the veil of cold hydrogen.

  3. 2. Why do early galaxies look different than nearby galaxies?

  4. Galaxies grew from small clumps of stars to ever-bigger assemblages. This bottom-up hierarchy is like brooks, feeding streams, which then converge to form great rivers. These galaxies are not small because they are far away, but because they are truly tiny. (Due to relativistic ffects of the expanding universe, the visual effect of shrinking angular size with distance no longer works when objects are farther than halfway across the universe. They actually stay about the same apparent size, if they are not truly physically smaller).

  5. 3. How do we know these galaxies are so far away?

  6. The light from distant objects is stretched to longer wavelengths, or reddened, by the expansion of the universe. However, a nearby object's light can also be reddened by intervening dust, or if the object is intrinsically cool and dim. Astronomers carefully measure the colors of the galaxies and the HUDF observations and match them against predicted colors of normal, but distant galaxies. This calibration is double checked by the spectrograph aboard Hubble that can actually measure the cosmological Doppler shift in receding objects, thereby providing an independent and robust distance measurement.

  7. 4. Why can't Hubble see even farther into the universe?

  8. These objects are so faint that, for a given exposure, Hubble's present cameras can't pick up any more particles of light from the far universe. Also, the light from farther galaxies is stretched into the infrared region of the spectrum, and a different type of telescope is needed to do detailed studies of this wavelength region. That's NASA's James Web Space Telescope, scheduled for launch in 2011.

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Credit: NASA, ESA, R. Windhorst (Arizona State Univ.) and H. Yan (Spitzer Science Center/Caltech); A. Bunker (Cambridge Univ./Univ. of Exeter); G. Illingworth (Univ. of California, Santa Cruz/Lick Observ.); M. Stiavelli (STScI), J. Rhoads (STScI), and the HUDF Team