Deep Field Survey Shows Oldest Galaxies Yet
Astronomers continue to find mature galaxies at higher and higher redshifts. The latest record, reported in Nature,1 is z=6.96, interpreted to mean the galaxy was present 700 million years after the big bang (usually dated at 13.7 billion years ago). A survey of distant galaxies from the Hubble Ultra Deep Field (HUDF), also reported in Nature,2 found 500 galaxies at z=6 (assumed 900 million years after the big bang) but only one candidate at z=7 to z=8 (700 million years). They interpreted this to mean, “The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z approximately ~7-8 by the hierarchical merging of small galaxies.” Are we, therefore, peering into the “dark ages” before the dawn of galaxies? Richard McMahon, commenting on these studies in the same issue of Nature,3 noted that the HUDF study was restricted to an extremely narrow portion of the sky, so better instruments and more observations will be required; “Only then will the presence, or absence, of further galaxies be able to tell us whether we really are homing in on the era of reionization.”
This story was noted by Space.com, National Geographic; a press release is available on the Hubble Space Telescope site. The European Space Agency press release includes pictures of the Hubble Ultra Deep Field survey, including a dramatic animation zooming in from a wide star field to the very narrow spot where one of the distant galaxies was discovered. “The findings also show that these dwarf galaxies were producing stars at a furious rate,” it says, “about ten times faster than is happening now in nearby galaxies.” The Hubble press release states, “Astronomers had not seen even one galaxy that existed when the Universe was a billion years old, so finding 500 in a Hubble survey is a significant leap forward for cosmologists.” See also the 10/14/2005 and 04/06/2005 entries.
1Iye, Ota et al., “A galaxy at a redshift z = 6.96,” Nature 443, 186-188(14 September 2006) | doi:10.1038/nature05104.
2Bouwens and Illingworth, “Rapid evolution of the most luminous galaxies during the first 900 million years,” Nature 443, 189-192(14 September 2006) | doi:10.1038/nature05156.
3Richard McMahon, “Astronomy: Dawn after the dark age,” Nature 443, 151-152(14 September 2006) | doi:10.1038/443151a.
It must be remembered that things like starbirth rates and galaxy formation timelines are inferred from theory. The observations show only colors, luminosities and chemical compositions of objects, along with other properties of spectra. The shift of certain absorption lines from their normal wavelength positions is interpreted as universal expansion, except by the maverick astronomers (12/06/2004). Even assuming the standard big bang scenario, though, it is a huge lumpiness problem for cosmologists to find galaxies already formed and producing stars at prodigious rates so near the beginning. The sample sizes have been very limited so far. Already, though, it is very surprising to think that whole galaxies could appear so soon out of a sea of particles by natural causes (05/11/2006). Watch for new records to be set in future surveys.