1998ApJ...506..135G

We present deep Hα and [S II] emission-line images of three galaxies, the Sab spiral M81, the Sc spiral M51, and its companion, NGC 5195, in our ongoing study of the diffuse ionized gas (DIG) in spiral galaxies. M81 and M51 show widespread DIG with the characteristic enhanced [S II]/Hα line intensities also seen in other galaxies. A large fraction of the Hα emission in these systems, 0.40-0.50, is contributed by DIG, which is in agreement with results obtained for other disk galaxies. In spite of a drastic difference in the star formation rate per unit disk area and the difference in Hubble types, the overall properties of the DIG appear similar in both galaxies. In detail, however, differences do appear. There is some indication that the [S II]/Hα ratio is systematically lower in M51 than it is in M81. There is also a radial dependence on the [S II]/Hα ratio in M51 that is not seen in M81. The Hα emission from the general bulge area of M81, while diffuse in nature, has a distinctly higher [S II]/Hα intensity ratio than disk DIG elsewhere in M81. This ionized gas in the bulge is most likely not photoionized by massive stars, the dominant source of ionization for DIG in galactic disks. Overall, our results reinforce the important role of DIG in the ISM of spiral galaxies in general. We detect significant emission from the companion to M51, NGC 5195. We find clumps of Hα emission in an 800x800 pc region offset from the nucleus and spectacular kiloparsec-long filaments reminiscent of the outflow cones in starburst galaxies. We attribute these structures to the recent passage of NGC 5195 through the disk plane of M51, which led to a burst of star formation some 70 million years ago.