1995A&A...300..881K

We present spectroscopic observations of supernova 1994D in NGC 4526, an S0₃ galaxy in the Virgo cluster 15Mpc distant. The datasets consist of the interstellar Ca II and Na I lines towards the supernova at high spectral resolution (FWHM 6km/s), H α and [N II] observations at lower resolution (FWHM 33km/s) of the nucleus of NGC 4526 and the supernova, obtained with the William Herschel Telescope at La Palma, and 21cm spectra obtained with the 100m Effelsberg Radiotelescope in the field of NGC 4526. The velocity of the gas in NGC 4526 determined from our H α spectra is +625km/s at the centre (systemic velocity) and +880km/s at the supernova position. Our value of the systemic velocity is higher than the value of +450km/s frequently quoted in the literature. In our high resolution spectra we detect Ca II and Na I absorption at +714km/s which is produced in interstellar gas in NGC 4526. To our knowledge this is the first detection of interstellar absorption originating in a galaxy of early morphological type. The ratio N(Na⁰)/N(Ca⁺)≃4 suggests an origin in cold gas at rest velocity relative to its galactic environment. The lack of multiple components indicates a relatively simple structure of the interstellar medium in the inner region of NGC 4526, at least in the particular line of sight to the supernova at the border of the nuclear ring of dust. We detect multi-component Ca II and Na I absorption lines in the range from +204 to +254km/s which originate in a complex of High Velocity Clouds (HVCs) located at a distance ≪1Mpc, in the surroundings of the Milky Way. This rare detection of HVCs in absorption enables the study of the properties of the gas using the Ca⁺ and Na⁰ column densities, combined with the H⁰ column density taken from the literature at +215km/s in the same line of sight. We find N(Na⁰)/N(Ca⁺)=0.1-0.3, in our Galaxy the signature of high velocity gas. The Ca⁺/H⁰ and Na⁰/H⁰ column density ratios are extremely high compared to Milky Way interstellar values; the gas appears to have near solar abundances, very low dust content, and a diluted ultraviolet radiation field. This is entirely consistent with Galactic fountain models, in which hot gas is expelled into the outer halo, and subsequently cools. At -29km/s, we find weak Ca II absorption and weak H I emission. This component has properties similar to those of the warm gas around the Sun and may originate in gas infalling onto the Galactic disk, perhaps associated with the extended complexes of Galactic halo gas at intermediate negative velocities which are present in the northern Galactic hemisphere. Finally, close to rest velocity, we find both warm and cold gas located beyond 65pc, probably associated with high latitude gas at the border of Loop I. The total reddening of the supernova, estimated using the standard Milky Way gas-to-dust ratio, is E(B-V)≃0.05.