Interstellar CA II in the galactic halo and in QSO absorption systems

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Galactic Structure, Interstellar Matter, Quasars, Absorption Spectra, Calcium, Halos

Scientific paper

The equivalent width distribution of Ca II K absorption lines seen in lines-of-sight through the galactic halo from an unbiased sample of extragalactic sightlines is constructed, and the absorption by the interstellar medium of the Galaxy is compared with that seen in the spectra of QSOs lying behind low-redshift galaxies. The Ca II absorption seen in several QSO spectra apparently arises in material deposited inhomogeneously around the foreground galaxy, perhaps as a result of earlier galaxy interactions or mergers. It is demonstrated that, for other QSO sightlines, such as those toward 0446 - 208 and 2020 - 370, the measured column densities of the absorbing gas can be reproduced by assuming that the quasar line-of-sight passes through the inclined disk of the intervening galaxy. In contrast, it is found that such a model has difficulty in explaining the nondetection of Ca II in the pair IC 1746 - PHL 1226, and requires the diameter of the galaxy in the foreground of the quasar 1543 + 489 to be not less than 105/h kpc if responsible for the absorption. Since W-lambda(K) in the Galaxy is correlated with the number of components comprising the line, the large values of W-lambda(K) seen in most QSO-galaxy pairs can similarly be accounted for by assuming that a line is composed of many individual absorbers spread out over larger velocities than those found locally. This explanation is consistent with absorption attributed to a sightline through an inclined disk.

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