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Session 43 - The Diffuse ISM: Milky Way and Beyond.
Display session, Tuesday, June 11
Great Hall,
Understanding the relationship between the star formation process and the ISM is a key step towards unravelling the complex history of galaxy evolution. A longstanding problem has been the nature and importance of the feedback process by which massive stars deposit energy into the ISM via photoionization, stellar winds and supernovae. We have obtained deep H\alpha, [SII] and [OII] images of the edge--on SBm galaxy NGC 55, located in the nearby Sculptor Group, which reveal a spectacular variety of ionized gas features. NGC 55 is morphologically similar to the LMC, but is slightly more luminous (M_B = -18.6). In addition to its numerous bright HII regions, NGC 55 hosts several large filamentary and bubble--like features extending well above the main body of the galaxy, reaching several kpc off the plane, as well as copious amounts of truly diffuse ionized gas. We will discuss the morphology and large scale distribution of both the discrete star--forming regions and the filamentary/diffuse ionized features, and investigate the possible relationship between them. The current massive star formation rate, as determined from the H\alpha emission line flux, is derived and used to estimate the mechanical luminosity from stellar winds and supernovae which is available to drive the extra--planar ionized features. Emission--line ratios ([SII]/H\alpha, [OII]/H\alpha) constrain the ionization mechanism of the gas and we will particulary focus our attention on those features located far from bright HII regions. If these structures are due to photoionization by Lyman continuum photons which have leaked from discrete HII regions, then a correlation is expected between the ionization state of a given feature and the distance to the nearest HII region: we will discuss the evidence for such an effect from our data.
