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The hot ionized medium (HIM) in galaxies

When large numbers of massive stars are created in a galaxy (where "massive" means 8 Solar masses or more), these perish at the end of their relatively short lifetimes in spectacular explosions named "supernovae". Such supernovae do not only emit large amounts of light (which we can pick up with optical telescopes and, if the supernovae are close enough, also with the unaided eye). They also produce light in other wavebands and, ontop of that, expel large amounts of hot plasma (ionized gas). Basically, they shed what used to be the outer parts of the stars themselves. This extremely hot gas (millions of degrees) has a high pressure and expands rapidly.

In the most extreme case of high energy input from supernovae, such hot gas is found also away from the galaxy disks where the stars exploded, out in the halos. Since the hot gas, with temperatures of millions of degrees, emits very energetic radiation, in the form of X-rays, one can use X-ray telescopes to image it. As an example, a set of X-ray images of the nearby starburst galaxy M82 is very suggestive.

Together with the energetic radiation, as part of the hot plasma, heavy elements produced in the atomic fusion processes going in the interior of the supernova progenitor stars are expelled into the ambient medium. Such heavy elements are of great significance, because they are the basic building blocks of life in the Universe. Elements like carbon, oxygen, nitrogen, all the most important constituents of substances known to us on Earth (including ourselves), come from the interior of stars, from where they are released when the star first implodes and then the outer shell is expelled again. These elements, when being heated up to million degrees during such an explosion, emit characteristic line emission. An X-ray spectrum of the starburst galaxy M82, with such emission lines in the regime from about 0.5 to 6.5 keV photon energy, is displayed in the figure below.

Combined X-ray spectrum of M82 obtained with the ROSAT and ASCA satellites. The upper panel displays the source spectrum and the residuals left after subtraction of the multi-component model displayed in the lower panel. The fact that the residuals are small indicates that the adaptation of the model to the observed data is good.

Thus, X-ray imaging is a powerful tool to detect hot gas in galaxies. In particular, when viewing them edge-on, one can detect gas beyond their disks, i.e. hot gaseous halos. Spectroscopy of this emission is one of the most important diagnostic tools for assessing the chemical composition of such gas and thus the distribution of heavy elements created by massive stars, after releasing them back into their environment again. As part of hot gas, these heavy element can leave not only the galaxy disks, but - if the level of energy production in the disk is high enough to allow the expelled gas to reach escape velocity - even leave the galaxy itself into intergalactic space.

Another way of looking at X-ray data, which are obtained with cameras that can register not only the position of incoming photons (like ordinary cameras creating images), but also their energy and time of arrival. One can, for example, create diferrent images, covering different energy ranges, say "soft", "medium" and "hard" energies. From the three subimages, one can then create a colour-coded image (in almost the same way a TV set creates a colour image from a red, green and blue image that are displayed simultaneously), providing "colour" or energy information on the target. Such a colour-composite image is shown below. The sources with the hardest emission show up whitish-yellow, sources with intermediate energies green/turquiose, while the softest emission appears orange/red. The cross marks the position of the galaxy centre. Note that the disk of the galaxy extends from upper left to lower right! Thus, the reddish emission cone in the image is oriented perpendicular to the disk, forming part of an energetic outflow, similar to what is displayed in the overlay of X-ray emission ontop an optical image of M82 (see above). These data of NGC 253 were obtained with the Chandra X-ray satellite.