A 3.5-Gyr-old galaxy at redshift 1.55

• DOI: 10.1038/381581a0
• Publication Date: 2003-08-12T02:04:33Z
• Authors: James Dunlop; John Peacock; Hyron Spinrad; Arjun Dey; Raul Jimenez; Daniel Stern; Rogier Windhorst
• Keywords: 0103 physical sciences; 01 natural sciences

ONE of the most direct methods of constraining the epoch at which the first galaxies formed—and thereby to constrain the age of the Universe—is to identify and date the oldest galaxies at high redshift. But most distant galaxies have been identified on the basis of their abnormal brightness in some spectral region1–4; such selection criteria are biased towards objects with pronounced nuclear activity or young star-forming systems, in which the spectral signature of older stellar populations will be concealed. Here we report the discovery of a weak and extremely red radio galaxy (53W091) at z = 1.55, and present spectroscopic evidence that its red colour results from a population of old stars. Comparing our spectral data with models of the evolution of stellar populations, we estimate that we are observing this galaxy at least 3.5 Gyr after star-formation activity ceased. This implies an extremely high formation redshift (z > 4) for 53W091 and, by inference, other elliptical galaxies. Moreover, the age of 53W091 is greater than the predicted age of the Universe at z = 1.55, under the assumption of a standard Einstein–de Sitter cosmology (for any Hubble constant greater than 50kms−1 Mpc−1), indicating that this cosmological model can be formally excluded.