Interstellar absorption lines in the directions of extragalactic objects – I

Observations of Na I and Ca II interstellar lines with resolutions as fine as 24 and 17 km s⁻¹ respectively have been obtained in the directions of the extragalactic sources NGC 985, 1068, 3C 273, 1916–588 and Mkn 509. They all show a component with $$|V_\text {LSR}|\leqslant 10\enspace\text {km s}^{-1}$$ which we attribute primarily to gas in the galactic disc with |z| ≼ 1 kpc based on comparisons with high-latitude stars. Higher velocity components of both NaI and Ca II were discovered in 3C 273 at −22 and +28 km s⁻¹ and in Mkn 509 at +61 km s⁻¹ and an extended negative velocity wing already has been noted in SN 1980n in NGC 1316. The negative velocity in 3C 273 and the wing in SN 1980n are inconsistent with halo gas corotating with the disc or even with the circular velocity decreasing with increasing distance from the plane. Our data also show that the Magellanic Clouds and Stream are not on typical sight lines through our halo. The ratio of Ca II to Na I column densities is high compared with typical stars shining through H I clouds in the galactic plane, indicating that both the low- and high-velocity clouds in our survey are more typical of intercloud gas where calcium is much less depleted.