HI in galaxy outskirts and the extragalactic ionizing flux

Astronomy and Astrophysics – Astronomy

Scientific paper

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Background Radiation, Extraterrestrial Radiation, H I Regions, Ionizing Radiation, Spiral Galaxies, Absorption Spectra, Cosmic Gases, Ionized Gases, Radio Sources (Astronomy), Star Formation

Scientific paper

The lack of starforming regions makes the ionizing background radiation extremely important for the physical conditions of the gas in the outskirts of spiral galaxies. Combining HI observations with models of the interaction between the ionizing radiation and the gas in these regions, the spectrum of the extragalactic background radiation below 0.2 keV can be constrained. The 21 cm absorption in the spectra of 60 background radio sources which lie close to the optical disk of spiral galaxies in the plane of the sky were sought. Measurements were made for high velocity clouds in the direction of 63 bright continuum sources. Results can be summarized as follows: the spin temperature is never so low as to be comparable with the microwave background radiation temperature. This implies that the brightness temperature is directly proportional to the column density and that there is no appreciable HI hidden in emission by extreme subthermal effects; absorption is never observed for column densities NHI less than or equal to 2 times 10 to the 19th power/sq cm. For NHI greater than 2 times 10 to the 19th power/sq cm absorption is sometimes seen but only over a narrow velocity range; there is a transitional column density Nl such that appreciable emission is seen for NHI greater than Nl but very little is seen for NHI less then Nl.

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