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Session 40 - The Interstellar Medium.
Display session, Tuesday, June 09
The bright supernova in M81, SN1993J, recently afforded a rare opportunity to probe the ISM in an external nearby galaxy via absorption line spectra (Bowen et al 1994). HST GHRS (G270M) Mg I and Mg II spectra of SN1993J reveal strong, broad absorption lines over a range of 450 km s^-1. KPNO coude feed spectra of the supernova show three corresponding clearly separated absorption complexes. The ISM of M81, the ISM of the Milky Way and an intergalactic cloud of gas are thought to be responsible for these features. The gas cloud is presumably the result of tidal stripping of M81 by M82 and NGC 3077. The Seyfert nucleus of M81 is bright enough to provide a second sightline, separated from the supernova by 2.6' on the sky or 5 kpc in the galactic plane of M81. We obtained GHRS Mg II spectra of the nucleus which show strong broad absorption lines, similar to that of SN1993J, but with some significant velocity differences. However, corresponding KPNO coude feed Na I spectra of the nucleus reveal a single sharp feature at a heliocentric velocity of 0 km s^-1. This is attributable to the Milky Way ISM alone. No Na I absorption arising from M81 is seen either for the disk or the tidally stripped material. One possible explanation for the missing absorption lines is that tidal stripping of gas by the companion galaxies creates a patch of cold, clumpy gas in front of SN1993J on a scale small enough that it does not extend across the nucleus. Our results help constrain the nature of the gas in M81 and gas in the disk and halo of the Milky Way. We use studies of nearby galactic systems to interpret absorption line characteristics of high redshift damped Lyman \alpha absorbers. Related posters with similar analysis of the absorption spectra along the line of sight of QSOs are presented at this meeting (Bauer et al Roth et al) along with complimentary imaging data (Jim amp; Roth).
keywords: M81 - SN1993J - ISM - absorption lines
Program listing for Tuesday