The interstellar spectrum towards the supernova 1987A in the Large Magellanic Cloud

Abstract

In recent years, dramatic improvements in spectroscopic capabilities have opened up the interstellar line research to the brightest extragalactic targets, and in particular to stars in the Magellanic Clouds (in the visible, see for example refs 1–3). This allows us to tackle two questions of crucial importance: (1) the existence, structure and composition of a gaseous halo surrounding our Galaxy and/or the Clouds; and (2) the fragmentation of the Clouds, their depth along the line of sight, and to probe the interstellar matter in galaxies less evolved than ours. The supernova 1987A offers a unique opportunity to probe the visible interstellar absorption spectrum towards the Large Magellanic Cloud at high resolution and high signal-to-noise ratio. Here we report the first results of these observations. A Ca II 'forest' is detected through all the velocity range between the Galaxy and the Large Magellanic Cloud and in particular, a component is seen at a velocity of ∼215 km s⁻¹ which may correspond to cooler gas in a halo. A preliminary abundance study is also presented.