2001/05/01 by T. Heckman, T. M. Heckman, K. R. Sembach +5
Engineering · Physics and Astronomy · #Astronomy and Astrophysical Research #Background radiation #Cosmic dust #Cosmic ray #Extinction (optical mineralogy) #Galaxies: Formation, Evolution, Phenomena #Galaxy #Interstellar medium #Ionizing radiation #Opacity #Optical depth #Space Technology and Applications #astro-ph
paper · pdf · doi:10.1086/322475
17 pages; ApJ, in press
arxiv created 2001/05/01 · openalex publication_date 2001/09/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
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 (τ 0 ≥ 10 2 ). 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 f esc ≤ 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 f esc . 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.