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The Spectra of Star Forming Galaxiesat High

We review the spectral properties of the population of star forming galaxies at z 3 discovered using a colour selection technique which targets the Lyman discontinuity at 912 A. The UV luminosity of the typical z 3 galaxy exceeds by more than one order of magnitude that of the most luminous starbursts in the nearby universe, although the maximum star formation intensity (SFR per unit area) is within the limits found in local surveys. We consider in detail the likely magnitude of dust extinction and conclude that published estimates of the volume-averaged star formation rate at high z need to be revised to higher values by a factor of about 3. This correction improves the agreement between the observations and recent theoretical predictions. Ly emission is generally weak, most likely as a result of resonant scattering. The large equivalent widths of the strongest interstellar lines and their systematic blueshift (by up to several hundred km s ?1) relative to the Ly emission line are indicative of highly energetic outtows in the interstellar medium. Pilot observations have detected the redshifted H and O III] emission lines in the K band. The widths of these features imply dynamical masses of 10 10 M for the innermost star forming regions; the total masses involved are likely to exceed 10 12 M .