The Survey for Ionization in Neutral-Gas Galaxies: III. Diffuse,
Warm Ionized Medium and Escape of Ionizing Radiation
M.S. Oey, G.R. Meurer, S. Yelda, E.J. Furst, S.M. Caballero-Nieves,
D.J. Hanish, E.M. Levesque, D.A. Thilker, G.L. Walth, and the SINGG Team
We use the first data release from the SINGG H-alpha survey of HI-selected
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+/- 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-alpha surface brightness
> 2.5e+39 erg s-1 kpc-2 within the H-alpha half-light radius, do show much
lower fractions of diffuse H-alpha 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 HI 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.
ApJ 661, 801
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