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Ca ii and Na i absorbers in the Galactic halo


Ca II and Na I absorption towards PKS 1448−232, observed with UVES During the last decades, absorption and emission line measurements have demonstrated that the Milky Way is surrounded by a complex, multi-phase gaseous halo. Embedded in a corona of million-degree gas, neutral and ionised gas clouds move with high radial velocities $(|v| > 40~\mathrm{km \, s}^{-1})$ through the Milky Way halo, giving rise to the population of intermediate- and high-velocity clouds (Wakker 2004). While most of the recent HVC measurements have focussed on the distribution and properties of the large IVC and HVC complexes, CHVCs, and highly-ionised HVCs, relatively little attention has been paid so far to the abundance and distribution of neutral gaseous structures in the halo that have H i column densities below the detection limit of the large 21-cm HVC surveys $($approximately $10^{19}~\mathrm{cm}^{-2})$.

High-velocity Ca ii absorbers

Using archival data taken with the UVES spectrograph at the ESO Very Large Telescope (VLT), we detected high-velocity Ca ii and Na i absorption lines towards 8 out of 13 quasars, suggesting that the Milky Way halo contains a population of low column density neutral gas clumps. Many of the high-velocity Ca ii absorbers probably are associated with known HVCs. For the sightline towards the quasar PKS 1448−232 we carried out additional H i observations with the NRAO Very Large Array (VLA). The observations demonstrate that the Ca ii absorption is connected with low column density H i clumps with angular sizes of a few arcminutes and peak column densities of $\le 7 \times 10^{18}~\mathrm{cm}^{-2}$. Because of the unknown ionisation conditions and the fairly large beam size of the H i data of 2 arcmin, however, we are not able to determine the metallicity of the gas.

HI column density map of gas clumps in the direction of PKS 1448−232, observed with the VLA With typical H i column densities of $\lt 10^{19}~\mathrm{cm}^{-2}$ and a complex velocity component structure, the properties of the Ca ii structures in the Milky Way halo resemble those of strong Mg ii absorbers that are nearly always found within an impact parameter of about $35 \, h^{-1}~\mathrm{kpc}$ of a luminous galaxy (Ding et al. 2005; Bergeron & Boissé 1991). These strong Mg ii systems are believed to sample both disk and halo gas in galaxies. The Ca ii systems we found possibly represent the Galactic counterparts of halo Mg ii systems. The neutral gas fraction probably is only a few percent or less, so that one would expect that the low column density H i clumps in the halo have substantial ionised gaseous envelopes. Therefore it seems likely that the detected Ca ii features also are related to the population of highly ionised HVCs in the halo seen in C iv and O vi absorption (Sembach et al. 2003).

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