Andrew Cardwell

Generation of Warps by Accretion Flows

We present ample evidence for the presence of continuing gaseous infall to the Galactic plane, and then show how interaction of this gas with the disc gas yields a warp of the observed amplitude and direction. The mechanism works for galaxies in general, predicting a small proportion of U-shaped warps, as observed.

Hα Luminosity Function Of H II Regions: Preliminary Calibration of a Powerful Standard Candle

We propose the use of the H II region luminosity function (LF) as a refined powerful standard candle for extragalactic distances. We have found evidence of a change in the properties of H II regions in spirals at a fixed luminosity in Hα which we term the Stromgren luminosity (LHα = 3.39 1038 erg s-1). The key piece of evidence is the presence in the Hα H II region LF of a change in gradient and a local sharp peak. The invariance and high luminosity of the change measured in the LF was proposed in previous studies as a possible secondary standard candle, but it is calibrated here for the first time using the LFs of NGC 925, NGC 4535 and M100, whose Cepheid distances have been determined in the HST Extragalactic Distance Scale Key Project. Using this method we derive a value of H 0 of 73±7 km s-1 Mpc-1.

A Scenario for the Ionization of the Diffuse ISM in Galaxies and the IGM in Clusters

The diffuse Hα emitted from the discs of spirals is of order 50% of their total Hα emission (Ferguson et al., 1996; Zurita et al., 2000), requiring strong energy input. Energy of stellar winds or SNe falls short by two orders of magnitude, white dwarf Lyc by one order. Sciama’s (e.g. 1990) decaying neutrino theory offers an explanation, but clear evidence against it exists (Bowyer et al., 1999). OB stars are probable candidates, but are embedded in H II regions which absorb their Lyc flux; any escaping flux must travel up to kpc in H I to ionize the diffuse gas (DIG). We have evidence that a major fraction of the Lyc escapes, and have modelled its propagation through the DIG.