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Astron. Astrophys. 338, L1-L4 (1998)

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4. The spatial distribution of O VI

Our data allow to investigate the distribution of O VI in the galactic halo. The absorption equivalent widths have been calculated from the result of the fits. Assuming that absorption is optically thin, the column density can be calculated from


The column densities for all targets are given in Table 1.

If we assume a hydrostatic Galactic halo (corona) of the type postulated by Spitzer (1956), we have an exponential density distribution of O VI , described by the equation


with the O VI midplane density [FORMULA] and scale height [FORMULA]. The projected column density is then given by


Fig. 4 shows the result we found when fitting equation (3) to our column densities.

Best values for the parameters [FORMULA] and [FORMULA] resulting from our N(O VI )[FORMULA] fit are [FORMULA] cm-3 and [FORMULA] kpc. Including the O VI column densities from the literature, we find the data point for 2C 273 to lie just above our 1[FORMULA] upper limit and those for PKS 2155-304 and NGC 346 just below our 1[FORMULA] lower limit. Therefore, inclusion of these data would not change our scaleheight value in a significant way.

We should note here that [FORMULA] relies almost exclusively on the LMC measurements. To set an approximate lower limit for the scaleheight we applied the fit procedure to a dataset where the extragalactic stars are excluded. The result is [FORMULA] kpc, and points to a substantially larger scaleheight of O VI in the halo than previous measurements do. The upper [FORMULA] limit is [FORMULA]20 kpc.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998