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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

[EQUATION]

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

[EQUATION]

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

[EQUATION]

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
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