Evidence that the z = 3.4 radio galaxy B2 0902+34 may be a protogalaxy

THE monochromatic luminosities of high-redshift ( z > 3) radio galaxies rise steeply between wavelengths of about 2,000 and 5,000 Å, to form a characteristic 'red bump' 1–6 . It is usually assumed that this bump arises from the photospheric emission of red, post-main-sequence stars. For a sufficient number of stars of this type to have evolved, however, these galaxies must be at least 0.4–2 Gyr old; yet z =3 corresponds to only 1.7 Gyr after the Big Bang (assuming a Hubble constant of 50 km s −1 Mpc −1 and that Ω 0 = 1), bringing the larger age estimates uncomfortably close to the beginning of the Universe. Here we show that, at least in the case of the high-redshift radio galaxy B2 0902 + 34, the basic assumption is incorrect: the red bump is caused not by photospheric emission from post-main-sequence stars, but by the presence of bright emission lines from doubly ionized oxygen. Both the spec-trum and the luminosity of the underlying continuum suggest that B2 0902 + 34 is a galaxy observed during its initial burst of star formation.