A Low Upper Limit to the Lyman Continuum Emission of Two Galaxies at [CLC][ITAL]z[/ITAL][/CLC] ≃ 3

Abstract

Long-exposure, long-slit spectra have been obtained in the UV/optical bands for two galaxies at z=2.96 and 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 frel,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 4 times lower than the previous detection derived from a composite spectrum of 29 Lyman break galaxies at z~=3.4. If this value is typical of the escape fraction of the z~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. Based on observations obtained in service mode at the ESO Very Large Telescope (VLT) for the program 65.O-0631.