NGC-4526 GAS, HIGH-VELOCITY CLOUDS, AND GALACTIC HALO GAS - THE INTERSTELLAR-MEDIUM TOWARDS SN 1994D

We present spectroscopic observations of supernova 1994D in NGC 4526, an SO3 galaxy in the Virgo cluster 15 Mpc distant. The datasets consis t of the interstellar Ca II and Na I lines towards the supernova at hi gh spectral resolution (FWHM 6 km s(-1)), H alpha and [N II] observati ons at lower resolution (FWHM 33 km s(-1)) of the nucleus of NGC 4526 and the supernova, obtained with the William Herschel Telescope at La Palma, and 21 cm spectra obtained with the 100 m Effelsberg Radioteles cope in the field of NGC 4526. The velocity of the gas in NGC 4526 det ermined from our H alpha spectra is +625 km s(-1) at the centre (syste mic velocity) and +880 km s(-1) at the supernova position. Our value o f the systemic velocity is higher than the value of +450 km s(-1) freq uently quoted in the literature. In our high resolution spectra we det ect Ca II and Na I absorption at +714 km s(-1) which is produced in in terstellar gas in NGC 4526. To our knowledge this is the first detecti on of interstellar absorption originating in a galaxy of early morphol ogical type. The ratio N(Na-0)/N(Ca+) similar or equal to 4 suggests a n origin in cold gas at rest velocity relative to its galactic environ ment. The lack; of multiple components indicates a relatively simple s tructure of the interstellar medium in the inner region of NGC 4526, a t 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 +254 km s(-1) which origi nate in a complex of High Velocity Clouds (HVCs) located at a distance << 1 Mpc, in the surroundings of the Milky Way. This rare detection o f HVCs in absorption enables the study of the properties of the gas us ing the Ca+ and Na-0 column densities, combined with the H-0 column de nsity taken from the literature at +215 km s(-1) in the same line of s ight. We find N(Na-0)/N(Ca+) similar or equal to 0.1-0.3, in our Galax y the signature of high velocity gas. The Ca+/H-0 and Na-0/H-0 column density ratios are extremely high compared to Milky Way interstellar v alues; the gas appears to have near solar abundances, very low dust co ntent, and a diluted ultraviolet radiation field. This is entirely con sistent with Galactic fountain models, in which hot gas is expelled in to the outer halo, and subsequently cools. At -29 km s(-1), we find we ak Ca II absorption and weak H I emission. This component has properti es similar to those of the warm gas around the Sun and may originate i n gas infalling onto the Galactic disk, perhaps associated with the ex tended complexes of Galactic halo gas at intermediate negative velocit ies which are present in the northern Galactic hemisphere. Finally, cl ose to rest velocity, we find both warm and cold gas located beyond 65 pc, 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) similar or equal to 0.05.