An HST Search for Lyman Continuum Emission From Galaxies at 1 . 1 < z < 1 . 4

If enough of their Lyman limit continuum escapes, star-forming galaxies could be significant contributors to the cosmic background of ionizing photons. To investigate this possibility, we obtained the first deep imaging in the far ultraviolet of eleven bright blue galaxies at intermediate redshift (1.1 < z < 1.4) with the STIS FUV MAMA detector on the Hubble Space Telescope. No Lyman continuum emission was detected. Sensitive, model-independent, upper limits of typically < 2×10 −19 ergs cm −2 sec −1Å −1 were obtained for the ionizing flux escaping from these normal galaxies. This corresponds to lower limits on the observed ratio of 1500 to 700Å flux of 150 up to 1000. Based on a wide range of stellar synthesis models, this suggests that less than 6%, down to less than 1%, of the available ionizing flux emitted by hot stars is escaping these galaxies. The magnitude of this spectral break at the Lyman limit confirms that the basic premise of " Lyman break " searches for galaxies at high redshift can also be applied at intermediate redshifts. This implies that the integrated contribution of galaxies to the UV cosmic background at z ∼ 1.2 is less than 15%, and may be less than 2%. Blueward of the Lyman limit lies a wavelength region of great observational uncertainty and astrophysical significance. These far-UV photons control ionization of the interstellar and intergalactic media today, and at all times since recombination (Madau et al. 1999).