On the escape of ionizing radiation from starbursts

Far-ultraviolet spectra obtained with FUSE show that the strong C II λ1036 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 (τ0 ≥ 102). Alternatively, the areal covering factor of opaque material is typically ≥94%. Thus, the fraction of ionizing stellar photons that escape the ISM of each galaxy is small: our conservative estimates typically yield fesc ≤ 6%. Inclusion of extinction due to dust will further decrease fesc. 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 ≥ 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.