Hα Images of Early-Type Galaxies with Hot Gas

We present Hα imaging observations of 13 early type galaxies with known x-ray fluxes from the Einstein Observatory data, and long-slit spectroscopy of nine of these. We have detected Hα emission in the central regions of ten objects. The line emission appears extended over a radius of 5-10 kpc, is generally peaked on the nucleus with regular elliptical isophotes, similar to the red continuum, although with a steeper radial distribution, Low surface brightness filamentary structure is also seen in a few cases (NGC 4406 and NGC 5846). To study the different phases of the interstellar medium in early type galaxies, we have compared the emissions due to hot (x ray) and warm (Hα) gas. On average, galaxies with a larger content of hot gas also show a more powerful line emission. However, the scatter in the relation is significant, and suggests that other parameters must play a role. The depth of the gravitational potential and/or the shape of the objects are probably important in determining the amount of gas present in early type galaxies. Other galaxian properties, such as the optical luminosity, the radio power, or the far-infrared emission do not seem to relate with the Hα emission. We estimate that post-asymptotic giant branch stars provide sufficient ionizing flux to explain the observed line emission.