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The cold neutral medium (CNM) in galaxies

The cold neutral medium is the densest part of the ISM in galaxies. Most of the cold gas exists in the form of molecular gas clouds. The structure of the cold gas is highly clumped and also normally tightly bound to the disk planes of spiral galaxies.

Just like the warm neutral medium, cold neutral gas is not abundant in the halos of spiral galaxies, most likely because it is heated up and therefore, if visible, shows up as part of a different gaseous component. However, small amounts of cold neutral gas can survive being torn out of galaxy disks with the rest of the outflowing, mostly ionized, gas. An example for such molecular gas in a galaxy's "superwind" outflow is the CO gas in the starburst galaxy M 82. This work is so recent, however, that no pictures are available yet in the literature.

Massive star formation has a direct influence on the surrounding gas. One can observe this in different wavebands (gas phases), including the cold neutral medium. An interferometric observation of CO emission from the starburst galaxy NGC 4666 led to the detection of a "hole" in the gas distribution (see image below). The hole is marked by an arrow.

A comparison with an Hα image of NGC 4666 suggests that the hole is located exactly at the footpoint of one of the prominent gas filaments seen in Hα emission. An overlay of the total CO emission (as contours of equal brightness) is shown ontop a grey-scale representation of the Hα image below. Not only is it seen in this total intensity image, but one can also gain kinematic information from CO line observations. The velocity information can be used to deduce the expansion velocity of the shell; its radius can be measured from the total intensity image. This two quantities are needed to estimate the amount of kinetic energy that this expanding structure has. Based on a model of the expansion of such gaseous shells one can then estimate the total amount of energy originally injected into the gas in order to make it expand at the observed rate.