Detection of CA II absorption by a high-velocity cloud in the direction of the quasar PKS 0837 - 120

Abstract

We present optical absorption spectroscopy of the Ca II K and H lines along the sight line to the quasar PKS 0837-120, which lies in the direction of a high-velocity cloud (HVC) detected in H I 21-cm emission at V(LSR) = + 105 km s-1. Our data show Ca II absorption due to the HVC as well as a lower velocity interstellar component. This is the fifth case in which optical absorption lines due to heavy elements in an HVC have been reported. We also give results of additional radio observations of the HVC. The column density ratio N(Ca II)/N(H I) for the HVC is approximately 1.6 x 10(-7), with an uncertainty of approximately a factor of 2. This calcium abundance is approximately one tenth the solar ratio of Ca/H, and higher than the level of Ca II found in the other detections of this species in an HVC. Thus the gas in the HVC towards PKS 0837-120 has undergone considerable heavy element enrichment and little depletion of Ca on to grains. These facts might suggest an origin for the HVC such as the sweeping up of interstellar material by a supernova explosion; however, they would also be consistent with other origins such as a galactic fountain, infall of intergalactic gas, shreds of a warped disc, or tidal debris from the Magellanic Clouds.