Ca ii K observations of QSOs in the line - of - sight to the Magellanic Bridge ⋆

We describe medium-resolution spectroscopic observations taken with the ESO Multi-Mode Instrument (emmi) in the Ca ii K line (λair=3933.661Å) towards 7 QSOs located in the line-of-sight to the Magellanic Bridge. At a spectral resolution R= λ/∆λ = 6,000, five of the sightlines have a signal-to-noise (S/N) ratio of∼ 20 or higher. Definite Ca absorption due to Bridge material is detected towards 3 objects, with probable detection towards two other sightlines. Gas-phase Ca ii K Bridge and Milky Way abundances or lower limits for the all sightlines are estimated by the use of Parkes 21-cm H i emission line data. These data only have a spatial resolution of 14 arcminutes compared with the optical observations which have milli-arcsecond resolution. With this caveat, for the three objects with sound Ca ii K detections, we find that the ionic abundance of Ca ii K relative to H i, A=log(N(CaK)/N(H i)) for low-velocity Galactic gas ranges from –8.3 to –8.8 dex, with H i column densities varying from 3−6×10 cm. For Magellanic Bridge gas, the values of A are ∼ 0.5 dex higher, ranging from ∼ –7.8 to –8.2 dex, with N(H i)=1–5×10 cm. Higher values of A correspond to lower values of N(H i), although numbers are small. For the sightline towards B 0251–675, the Bridge gas has two different velocities, and in only one of these is Ca ii tentatively detected, perhaps indicating gas of a different origin or present-day characteristics (such as dust content), although this conclusion is uncertain and there is the possibility that one of the components could be related to the Magellanic Stream. Higher signal-to-noise Ca ii K data and higher resolution H i data are required to determine whether A changes with N(H i) over the Bridge and if the implied difference in the metalicity of the two Bridge components towards B 0251-675 is real.