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 angstrom, t o 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 cor responds to only 1.7 Gyr after the Big Bang (assuming a Hubble constan t of 50 km s-1 Mpc-1 and that OMEGA0 = 1), bringing the larger age est imates uncomfortably close to the beginning of the Universe. Here we s how that, at least in the case of the high-redshift radio galaxy B2 09 02 + 34, the basic assumption is incorrect: the red bump is caused not by photospheric emission from post-main-sequence stars, but by the pr esence of bright emission lines from doubly ionized oxygen. Both the s pectrum and the luminosity of the underlying continuum suggest that B2 0902 + 34 is a galaxy observed during its initial burst of star forma tion.