The warm ionized medium (WIM) in galaxies
Stars in galaxies, especially the most massive ones, emit copious amounts of light, of which the energetic part is capable of ionizing part of their ambient medium. This ionization works in principle like in a fluorescence tube. The gas is subjected to so much energy that it is ionized and then emits characteristic line emission. This is also true for fluorescence lamps. Most of their light is emitted in the form of emission lines (i.e. light at special frequencies, with frequency ranges in between where no or little light is emitted), while old-fashioned wire bulbs emit a continuous light distribution in frequency space, without gaps in between.
While X-ray emission of hot plasmas (see section on the hot ionized medium) consists of both continuum and line emission, there is little optical continuum emission from warm ionized gas. Therefore, the easiest way to detect such gas is by using narrow filters centered on characteristic emission lines to image its distribution. The most prominent emission line of the warm ionized medium is the so-called Hα line of hydrogen. A number of Hα images are displayed in the picture gallery. Also the background images of some pages on this web site are Hα images of edge-on starburst galaxies (in the background of the welcome page one can see the galaxy NGC 4666). As another example, there is a colour image of the Hα emission of NGC 891 (image courtesy of J. Rossa, 2001, Ph.D. thesis, Bochum University). Such line images are created by subtracting, based on a second broad-band image, the underlying (stellar) continuum light. Thus, one obtains images of the distribution of warm ionized gas. Besides Hα many more emission lines exist (most importantly those of Nitrogen, Oxygen and Sulfur) and their absolute and relative strengths compared to each other are our most important diagnostic tools to assess the excitation condition of the gas. Different sources of light illuminating gas lead to different line strengths and ratios. An optical emission line spectrum is displayed in the figure below .
An example of an optical spectrum with such emission lines is displayed in the figure above. The underlying continuum emission is hardly visible. This image is reproduced from an article by Kewley et al. (2001, Astrophyiscal Journal Supplements, 132, 37).
For one of the most impressive cases of an outflow from a galaxy disk, have a look at the background image of this page. This is the nearby starburst galaxy M82, as seen in Hα emission (tracing warm ionized gas). This does not really look like a typical galaxy image at all. And if you want to see where the disk of M82 and how it is oriented (because, YES, there IS one), be surprised and take a peek here! This image shows the same galaxy, at the same orientation, and roughly the same field of view... The Hα image of M82 is also available in colours.