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Astron. Astrophys. 357, 37-50 (2000)

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7. Summary

We have presented HST FOS spectra of the QSO pair HS 1216+5032 AB. Our results are summarized as follows:

  1. Metal systems. Three strong and complex CIV absorption systems are observed in both HST spectra at [FORMULA]. The velocity span along the LOSs, [FORMULA] km s-1, is large and suggests the gas where this CIV occurs might be associated with a cluster of galaxies. Differences in the line strengths suggest that, if these systems arise in coherent structures, they must have characteristic transverse sizes of [FORMULA]kpc. Alternatively, the systems may be composed of a large number of cloudlets correlated in redshift.

    In the spectrum of B, a strong absorption feature at [FORMULA] Å is identified with two MgII doublets at [FORMULA]. If this identification is correct, these lines could arise in a low-redshift damped Ly[FORMULA] system.

  2. The BAL systems in B: The spectrum of QSO image B shows BAL troughs by HI , CII , CIII , NIII , NV , OVI , and possibly SIV and SVI arising in absorption systems at outflow velocities from the QSO of up to 5000 km s-1. The BAL troughs arise probably as a consequence of absorption by a mixture of broad and narrow components.

  3. Ly[FORMULA] absorbers. Due to the redshift path blocked out by the BAL troughs in B the number of detected lines is small. Selecting lines not associated with metal lines, with [FORMULA] km s-1 and [FORMULA] Å yields four lines common to both spectra and five lines without counterpart in the other spectrum. For [FORMULA] Å lines the numbers are two and four, respectively. Using a maximum likelihood technique, most probably diameters for spherical clouds of 192 and 256 [FORMULA] kpc are found for [FORMULA] Å and [FORMULA] Å lines, respectively. The [FORMULA] limits derived using the cumulative distribution of the probability function are [FORMULA] [FORMULA] kpc and [FORMULA] [FORMULA] kpc for the respective samples at [FORMULA]. Assuming that the absorbers are filamentary structures lying perpendicular to the LOSs, transverse dimensions almost [FORMULA] smaller than for spherical clouds are found. In both cases, the results of our analysis do not confirm the claim that the characteristic size of the Ly[FORMULA] absorbers increases with decreasing redshift.

    Independently of the cloud models used, we note that there are significant equivalent width differences between lines in A and B. Also, there appears to be a trend of larger equivalent width differences with increasing line strength, while no velocity differences between common lines is found. This provides evidence that the absorbers are coherent entities. The results for each line sample suggest that the absorbers must have a smooth gas density distribution, with lower density gas being more extended.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000