On the escape of ionizing radiation from starbursts

Far-ultraviolet spectra obtained with FUSE show that the strong C II lambda 1036 interstellar absorption line is essentially black in five of the UV-brightest local starburst galaxies. Because the opacity of the neutral ISM below the Lyman edge will be significantly larger than in the C ii line, these data provide strong constraints on the escape of ionizing radiation from these starbursts. Interpreted as a uniform, absorbing slab, the implied optical depth at the Lyman edge is huge (tau (0) greater than or equal to 10(2)). Alternatively, the areal covering factor of opaque material is typically greater than or equal to 94%. Thus, the fraction of ionizing stellar photons that escape the ISM of each galaxy is small: our conservative estimates typically yield f(esc) less than or equal to 6%. Inclusion of extinction due to dust will further decrease f(esc). An analogous analysis of the rest-UV spectrum of the star-forming galaxy MS 1512-cB58 at z = 2.7 leads to similar constraints on fesc. These new results agree with the constraints provided by direct observations below the Lyman edge in a few other local starbursts. However, they differ from the recently reported properties of star-forming galaxies at z greater than or equal to 3. We assess the idea that the strong galactic winds seen in many powerful starbursts clear channels through their neutral ISM. We show empirically that such outflows may be a necessary-but not sufficient-part of the process of creating a relatively porous ISM. We note that observations will soon document the cosmic evolution in the contribution of star-forming galaxies to the metagalactic ionizing background, with important implications for the evolution of the IGM.