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

¹ APS division, Dept. of Pure and Applied Physics, Queen's University, University Road, Belfast, BT7 1NN, UK e-mail: j.smoker@qub.ac.uk
² European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
³ Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Australia Telescope National Facility, CSIRO, PO Box 76, Epping N.S.W, 1710, Australia
⁵ Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706, USA


Received: 15 May 2005
Accepted: 28 July 2005

Abstract

We describe medium-resolution spectroscopic observations taken with the ESO Multi-Mode Instrument (emmi) in the Ca ii K line ( Å) towards 7 QSOs located in the line-of-sight to the Magellanic Bridge. At a spectral resolution , 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 arcmin 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, (Ca K)/N(H i)) for low-velocity Galactic gas ranges from –8.3 to –8.8 dex, with H i column densities varying from 3- cm^-2. 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) =  cm^-2. 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.