The Neutral Medium

We consider the physical conditions of the neutral medium within, and in the environments of, galaxies. The basic physical and morphological properties of the neutral medium within galaxy disks are now quite well-constrained. Systematic variations in temperature and phasebalance (of cool versus warm neutral gas) are indicated as a function of both radius and zheight. Interestingly, the cool medium line-widths are observed to be dominated by turbulent energy injection within cells of 10 pc to 1 kpc size. Deep new observations reveal that 5–10% of the neutral medium is associated within an extended halo which rotates more slowly and experiences radial inflow. Much of this component is likely to be associated with a “galactic fountain” type of phenomenon. However, compelling evidence is also accumulating for the importance of tidal disruption of satellites as well as continuous accretion (of both diffuse and discrete components) in fueling galaxy halos and disks. Continued fueling is even observed on scales of 100’s of kpc in galaxy environments, where the neutral component is likely to be merely a trace constituent of a highly ionized plasma.