The Survey for Ionization in Neutral Gas Galaxies. III. Diffuse, Warm Ionized Medium and Escape of Ionizing Radiation

Abstract

We use the first data release from the SINGG H-alpha survey of HI-selected\ngalaxies to study the quantitative behavior of the diffuse, warm ionized medium\n(WIM) across the range of properties represented by these 109 galaxies. The\nmean fraction f_WIM of diffuse ionized gas in this sample is 0.59+/- 0.19,\nslightly higher than found in previous samples. Since lower surface-brightness\ngalaxies tend to have higher f_WIM, we believe that most of this difference is\ndue to selection effects favoring large, optically-bright, nearby galaxies with\nhigh star-formation rates. As found in previous studies, there is no\nappreciable correlation with Hubble type or total star-formation rate. However,\nwe find that starburst galaxies, defined here by an H-alpha surface brightness\n> 2.5x 10^39 erg s^-1 kpc^-2 within the H-alpha half-light radius, do show much\nlower fractions of diffuse H-alpha emission. The cause apparently is not\ndominated by a lower fraction of field OB stars. However, it is qualitatively\nconsistent with an expected escape of ionizing radiation above a threshold\nstar-formation rate, predicted from our model in which the ISM is shredded by\npressure-driven supernova feedback. The HI gas fractions in the starburst\ngalaxies are also lower, suggesting that the starbursts are consuming and\nionizing all the gas, and thus promoting regions of density-bounded ionization.\nIf true, these effects imply that some amount of Lyman continuum radiation is\nescaping from most starburst galaxies, and that WIM properties and outflows\nfrom mechanical feedback are likely to be pressure-driven. However, in view of\nprevious studies showing that the escape fraction of ionizing radiation is\ngenerally low, it is likely that other factors also drive the low fractions of\ndiffuse ionized gas in starbursts.\n