The gaseous content of galaxies
The characterizing feature of disc galaxies is the presence of an extended disc of neutral gas, visible via the hyperfine transition at 21-cm (HI). HI discs are often more extended than optical discs, showing the stellar component, see Figure above. The outer parts of these discs, roughly beyond the radius where the stellar component is fading out, are very often warped with respect to the inner disc. In same cases, like the one below, warps are very symmetric but large non-axisymmetries are also often observed. The origin of warps is still not fully understood, current models point at interactions between the galaxy disc and companion object and/or the surrounding environment.


A second feature clear from the above image is the spatial coexistence between HI and UV emission, the latter emitted by massive young stars and thus manifesting recent star formation. Note in particular the emission coming from the region of the warp, where the HI disc seems to show the presence of spiral arms with relatively higher HI density, all these regions are also sites of star formation. The association between star formation and HI is usually disputed because stars form out of molecular gas, in fact whenever HI reaches column densities larger than about 1 Mo/pc^2 there is also star formation but in regions of high star formation rates, the gas component is totally dominated by molecular gas.