A Hubble Space Telescope Search for Lyman Continuum Emission from Galaxies at 1.1 < z < 1.4

Abstract

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 11 bright blue galaxies at intermediate redshift (1.1<z<1.4) with the Space Telescope Imaging Spectrograph Far-Ultraviolet Multianode Microchannel Array detector on the Hubble Space Telescope. No Lyman continuum emission was detected. Sensitive, model-independent, upper limits of typically less than 10^-19 ergs cm^-2 s^-1 Å^-1 were obtained for the ionizing flux escaping from these normal galaxies. This corresponds to lower limits on the observed ratio of 1500-700 Å flux of 150 up to 1000. On the basis of 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%.