A low upper limit to the Lyman continuum emission of two galaxies at z ≃ 3

Long-exposure, long-slit spectra have been obtained in the UV/optical bands for two galaxies at and z = 2.96 z = 3.32 to investigate the fraction of ionizing UV photons escaping from high-redshift galaxies. The two targets are among the brightest galaxies discovered by Steidel and collaborators, and they have different properties in terms of Lyalpha emission and dust reddening. No significant Lyman continuum emission has been detected. The noise level in the spectra implies an upper limit of f(rel,esc) = 3f(900)/f(1500) < 16% for the relative escape fraction of ionizing photons, after correction for absorption by the intervening intergalactic medium. This upper limit is z similar or equal to 3.4 times lower than the previous detection derived from a composite spectrum of 29 Lyman break galaxies at z similar or equal to 3.4. If this value is typical of the escape fraction of z similar to 3 galaxies, and is added to the expected contribution of the QSO population, the derived UV background is in good agreement with the one derived by the proximity effect.