Marta I. R. Alves


The ionised Galactic disk

Rod Davies, Clive Dickinson, Richard Davis, Mark Calabretta, Lister Staveley-Smith


Radio recombination lines (RRLs) have been widely used to detect and catalogue HII regions, which trace current formation of massive stars. The diffuse electron gas along the Galactic plane is also of intrinsic interest. Its detection complements the data available from catalogues of HII regions and more importantly it helps to answer open questions on the relationship between the narrow and the thick components of the warm ionised medium, namely the filling factor and the transport of the ionising radiation from the hot stars to the broad distribution.

I use three RRLs at 1.4 GHz from the HI Parkes All-Sky Survey and associated Zone of Avoidance survey to map the diffuse ionised gas within 4 degrees of the Galactic plane, between longitudes of 20 to 44 degrees, at 14.4 arcmin resolution. This is the first fully-sampled map of RRL emission giving the first unambiguous free-free emission map in this region.

The use of the total continuum data from the same survey enables the derivation of the electron temperature for a sample of HII regions, which shows a gradient with Galactocentric radius consistent with previous results. It is found that the diffuse electron gas on the Galactic plane as a similar temperature to that of the individual HII regions. The thickness of the ionised layer is compared with other ISM components.