[AG Abs.15 P81]

Total line of sight from SN 1987A to SN 1993J in UV absorption


We reexamine the NEWSIPS reduced IUE spectra of SN 1993J in M81. Column 
densities are determined for Fe II, Mg I, Mg II, O I, S II, Si II, and Zn II, 
as well as for C IV, Si IV and Al III. In neutral gas eight velocity components
can be identified between -180 and +200 km/s. Presumably, only three of these 
are of Galactic origin, the others are gas in the disk and either in the halo or
in the vicinity of M81. Also the clouds seen in the highly ionised atoms 
probably are located near M81, except a weak local component.

Indicators for an enlarged radiation density in the three components between 
70 and 170 km/s are revealed by an analysis of the ionisation ratios of Mg and 
Na. If these components are indeed located in front of M81, the source of
ionising photons should rather be the intergalactic medium of the M81 group than
the galactic radiation field of M81.

A unique possibility arises from the locations of SN 1993J and SN 1987A at
opposite positions in the sky. We combine both IUE spectra to a single line of
sight from the backside of the LMC through all of the Milky Way, the
intergalactic medium towards the M81 group, and the halo and the disk of M81
itself. We therefore cross three galaxies along a line of sight about
perpendicular to the plane of the local group, at a length of 3.8 Mpc.

The absorption along this long line of sight is compared to typical absorption
along quasar sight lines. From the supernova sight line we get column densities
in low ionised atoms about one order of magnitude larger than in highly ionised
atoms, while along typical quasar sight lines lower ionisation stages are
equally or less prominent than higher ions. From this we derive that the
contribution of galaxy disk gas plays only a minor role in quasar spectra, the
absorption mainly occurs in the halos of the crossed galaxies.