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We present moderately-high resolution echelle observations of the nucleus ofNGC 5195, the line of sight to which samples intervening interstellar materialassociated with the outer spiral arm of M51. Our spectra reveal absorption frominterstellar Na I, K I, Ca II, and CH+, and from a number of diffuseinterstellar bands (DIBs), at a velocity close to that exhibited by H I 21 cmemission from M51 at the position of NGC 5195. The H I column density impliedby the equivalent width of the 5780.5 DIB, based on the relationship betweenW(5780.5) and N(H I) derived for sight lines in the local Galactic interstellarmedium, is consistent with the column density obtained from the integrated H Iemission. The H2 column density predicted from the observed column density of KI, using the Galactic relationship between N(K I) and N(H2), is comparable toN(H I), suggesting a high molecular fraction (~0.65) for the M51 gas toward NGC5195. The DIBs toward NGC 5195 are, on average, ~40% weaker than would beexpected based on the K I column density, a further indication that the gas inthis direction has a high molecular content. The M51 material is characterizedalso by a high N(Na I)/N(Ca II) ratio (>11), indicative of a high degree of Cadepletion, and a high W(5797.1)/W(5780.5) ratio (~1.6), suggestive of either avery weak ambient radiation field or a significantly shielded environment. Ahigh N(CH+)/N(CH) ratio (>2.3) for the M51 material toward NGC 5195 may be theresult of enhanced turbulence due to interactions between M51 and itscompanion.

The Interstellar Line of Sight to the Interacting Galaxy NGC 5195