The Lyman Continuum in Starburst Galaxies Observed with the Hopkins Ultraviolet Telescope

The starburst galaxies IRAS 08339+6517, Mrk 1267, Mrk 66, and Mrk 496 (=NGC 6090) were observed with the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 mission. All four galaxies have radial velocities larger than 5000 km s^-1, permitting the measurement of their intrinsic Lyman-continuum fluxes redward of the H I absorption edge in our Galaxy. The sample was selected on the basis of having the most favorable conditions for the escape of Lyman-continuum photons. Two σ upper limits of F_λ < 7 × 10^-16 ergs s^-1 cm^-2 Å^-1 were obtained for the flux around 900 Å within the rest frame of each galaxy.

A set of theoretical spectral energy distributions has been calculated. We show that there exists a tight correlation between the continuum luminosity at 900 Å and the total number of photons emitted in the Lyman continuum, which is valid for high- and low-metallicity galaxies and essentially all relevant initial mass functions and star formation histories. Comparison with the observed values suggests that, on average, less than 3% of the intrinsic Lyman-continuum photons escape from the program galaxies.

Models for the ionization of the intergalactic medium at high z by young starbursts require a significant fraction of the ionizing radiation to escape from the galaxy. If the four galaxies observed by us have properties similar to young galaxies at redshift z ≈ 3, such galaxies are not likely to provide Lyman-continuum photons for the ionization of the early universe.