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 survey of H iYselected galaxies to study the quantitative behavior of the diffuse, warm ionized medium ( WIM ) across the range of properties represented by these 109 galaxies. The mean fraction f WIM of diffuse ionized gas in this sample is 0:59 AE 0:19, slightly higher than found in previous samples. Since lower surface brightness galaxies tend to have higher f WIM , we believe that most of this difference is due to selection effects favoring large, optically bright, nearby galaxies with high star formation rates. As found in previous studies, there is no appreciable correlation with Hubble type or total star formation rate. However, we find that starburst galaxies, defined here by an H surface brightness >2:5 ; 10 39 erg s À1 kpc À2 within the H half-light radius, do show much lower fractions of diffuse H emission. The cause apparently is not dominated by a lower fraction of field OB stars. However, it is qualitatively consistent with an expected escape of ionizing radiation above a threshold star formation rate, predicted from our model in which the ISM is shredded by pressure-driven supernova feedback. The H i gas fractions in the starburst galaxies are also lower, suggesting that the starbursts are consuming and ionizing all the gas, and thus promoting regions of density-bounded ionization. If true, these effects imply that some amount of Lyman continuum radiation is escaping from most starburst galaxies, and that WIM properties and outflows from mechanical feedback are likely to be pressure-driven. However, in view of previous studies showing that the escape fraction of ionizing radiation is generally low, it is likely that other factors also drive the low fractions of diffuse ionized gas in starbursts.

Evidence of environmental dependencies of Type Ia supernovae from the Nearby Supernova Factory indicated by local Hα

“…It also corresponds to the mean log(Σ Hα ) of the warm interstellar medium (WIM) distribution reported by Oey et al (2007). The source of the WIM is controversial.…”

“…This sample was split in two at the median value log Σ Hα = 38.35, which happens to correspond to the mean WIM Hα surface brightness of galaxies (Oey et al 2007) and to the mean Hα sensitivity level at our highest redshift. SNe with low Σ Hα were then designated as "locally passive" (Ia ), while those with high Σ Hα were designated as "locally star-forming" (Iaα).…”

The dust SED of dwarf galaxies

“…Oey et al (2007) show that starburst galaxies present a low diffuse Hα emission consistent with an expected escape fraction of ionizing photons of 25% from the galaxy.…”

Models of diffuse Hα in the interstellar medium: the relative contributions from in situ ionization and dust scattering

“…Observations of ionised gas in other galaxies indicate that the Hα emission from the DIG is approximately equal to that from traditional Hii regions (e.g., Ferguson et al 1996, Zurita, Rozas & Beckman 2000, Thilker et al 2002, Oey et al 2007). We therefore calculate the Hα luminosity of the DIG in the photoionisation simulations and set the Hα luminosity from the point sources equal to this.…”