The Oldest Known Galaxy

[sheeny (Al)]

I found these in this morning's Nature Alert,

Al.

Editor's Summary

14 September 2006
The oldest known galaxy

The galaxy described on page 186 may be, for the moment, the most distant and hence oldest galaxy known. Large samples of galaxies have been found at redshifts of z6, but detections at earlier times tend to be uncertain and unreliable. But this 'new' old galaxy has a spectroscopic redshift of z=6.96, corresponding to just 750 million years after the Big Bang; and a Lyman- emission line in its spectrum suggests that active star formation was under way when the Universe was only about 6% of its present age. This galaxy was detected during a survey using the Subaru Suprime-Cam on the summit of Mauna Kea. Looking at the galaxy population as a whole, the same survey produced a number density of galaxies at z7 that is only 18-36% that at z=6.6. A separate search for galaxies at at z7–8 using data from the Hubble Space Telescope yielded (conservatively) only one candidate galaxy, where 10 would be expected if there were no evolution in the galaxy population between z7 and z6. The simplest explanation for this is that the Universe is just too young to have built up many luminous galaxies at z7–8 by hierarchical merging of small galaxies.
News and Views: Astronomy: Dawn after the dark age

The latest surveys provide evidence for one, maybe two, galaxies farther back in cosmic time than ever detected before. But does the fact that we don't see more mean these are the very first galaxies to be formed?
Richard McMahon

Letter

Nature 443, 186-188(14 September 2006) | doi:10.1038/nature05104; Received 3 April 2006; Accepted 18 July 2006
A galaxy at a redshift z = 6.96

Masanori Iye1,2,3, Kazuaki Ota2, Nobunari Kashikawa1, Hisanori Furusawa4, Tetsuya Hashimoto2, Takashi Hattori4, Yuichi Matsuda5, Tomoki Morokuma6, Masami Ouchi7 and Kazuhiro Shimasaku2

When galaxy formation started in the history of the Universe remains unclear. Studies of the cosmic microwave background indicate that the Universe, after initial cooling (following the Big Bang), was reheated and reionized by hot stars in newborn galaxies at a redshift in the range 6 < z < 14 (ref. 1). Though several candidate galaxies at redshift z > 7 have been identified photometrically2,3, galaxies with spectroscopically confirmed redshifts have been confined to z < 6.6 (refs 4–8). Here we report a spectroscopic redshift of z = 6.96 (corresponding to just 750 Myr after the Big Bang) for a galaxy whose spectrum clearly shows Lyman- emission at 9,682 Å, indicating active star formation at a rate of 10M yr-1, where M is the mass of the Sun. This demonstrates that galaxy formation was under way when the Universe was only 6 per cent of its present age. The number density of galaxies at z 7 seems to be only 18–36 per cent of the density at z = 6.6.

Letter

Nature 443, 189-192(14 September 2006) | doi:10.1038/nature05156; Received 6 April 2006; Accepted 1 August 2006
Rapid evolution of the most luminous galaxies during the first 900 million years

Rychard J. Bouwens1 and Garth D. Illingworth1

The first 900 million years (Myr) to redshift z 6 (the first seven per cent of the age of the Universe) remains largely unexplored for the formation of galaxies. Large samples of galaxies have been found at z 6 (refs 1–4) but detections at earlier times are uncertain and unreliable. It is not at all clear how galaxies built up from the first stars when the Universe was about 300 Myr old (z 12–15) to z 6, just 600 Myr later. Here we report the results of a search for galaxies at z 7–8, about 700 Myr after the Big Bang, using the deepest near-infrared and optical images ever taken. Under conservative selection criteria we find only one candidate galaxy at z 7–8, where ten would be expected if there were no evolution in the galaxy population between z 7–8 and z 6. Using less conservative criteria, there are four candidates, where 17 would be expected with no evolution. This demonstrates that very luminous galaxies are quite rare 700 Myr after the Big Bang. The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z 7–8 by the hierarchical merging of small galaxies.