Detection of H II regions and ionized interarm gas in the Sombrero Galaxy (NGC 4594).

Spectroscopic observations of NGC 4594 unambiguously show the presence of H II regions and ionized interarm gas in that galaxy. The brightest H II regions appear to be excited by early O-type stars, giving evidence for the formation of high-mass stars. Ionized gas of low density pervades the visible disk of the galaxy and apparently corotates with the stars. The unusually high rotational velocities reveal a very massive galaxy. When combined with available photometric data, these velocities also indicate a large radial increase of the mass-to-light ratio similar to that observed in M31.