5. Concluding remarks
The O VI column densities derived from our ORFEUS II data are far too large to agree with the predictions of a photoionized model. These column densities clearly favour the hot halo concept as described by Spitzer (1956) and by Shapiro & Field (1976). The asymptotic column density of O VI in Fig. 3 of cm-2 can be compared with that of C IV and N V (Savage et al. 1997) of and , respectiveley. If the O VI , N V and C IV were to coexist in space (the similar absorption velocities of O VI and C IV point to that) and ignoring the abundance of the other ionic stages of these elements, the equivalent gas column density is N(H) cm-2 (based on the solar abundances -3.4 dex for C, -4.1 dex for N and -3.2 dex for O). Such identical equivalent column densities can be understood in a simple exponential pressure model, the small scaleheight for O VI from previous measurements could not. However, without knowledge of the real gas distribution in the halo it is not possible to relate our findings with the consequences of the interplay of ionisation and cooling in the halo.
Due to its very high ionisation potential O VI remains the most likeley tracer of hot gas outflow from the Galaxy. A direct measure of this outflow is not possible at the present time and will be subject of further research and the scientific goal for future Far Ultraviolet missions like ORFEUS III or FUSE.
© European Southern Observatory (ESO) 1998
Online publication: September 8, 1998