NGC 4526 gas, High Velocity Clouds, and Galactic Halo gas: the Interstellar Medium towards SN 1994D

Abstract

We present spectroscopic observations of supernova 1994D in NGC 4526, an S0$_3$ galaxy in the Virgo cluster 15 Mpc distant. The datasets consist of the interstellar Ca\II\ and Na\I\ lines towards the supernova at high spectral resolution (FWHM 6 \kms), \halpha\ and [N\,II] observations at lower resolution (FWHM 33 \kms) of the nucleus of NGC 4526 and the supernova, obtained with the WHT at La Palma, and 21 cm spectra obtained with the 100 m Effelsberg Radiotelescope in the field of NGC 4526. The velocity of the gas in NGC 4526 determined from our \halpha\ spectra is +625 \kms\ at the centre and +880 \kms\ at the supernova position. Our value of the systemic velocity is higher than the previous value of +450 \kms. In our high resolution spectra we detect Ca\II\ and Na\I\ absorption at +714 \kms\ 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. We detect multi-component Ca\II\ and Na\I\ absorption lines in the range from +204 to +254 \kms\ which originate in a complex of High Velocity Clouds (HVCs) located at a distance $\ll$ 1 Mpc, in the surroundings of the Milky Way. The gas appears to have near solar abundances, low dust content, and a diluted UV radiation field. At $-$29 \kms, we find weak Ca\II\ absorption and weak H\I\ emission. This component may originate in gas infalling onto the Galactic disk. Finally, close 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) $\simeq$ 0.05.