In order to check the function obtained above, we used our (1-0) and (2-1) data, in conjunction with the and assuming the to be optically thin, to derive absolute abundances as a function of radius. The resulting abundances are in good agreement with galactic observations. The millimeter-wave thermal emission from dust is a second independent test. While grain cross-sections and, to a lesser extent, dust temperatures are subject to debate, commonly used values yield gas masses quite close to those we estimate from our analysis. Unusually low cross-sections or temperatures and a peculiar variation are required to fit the gas mass resulting from the use of a constant ratio. All of our observations support the variation and range of that we propose here for NGC 4414. It should however be borne in mind that a number of assumptions about molecular clouds have been made and that we have no means of verifying them for the clouds in NGC 4414.
As such an analysis is not yet available for other external spiral galaxies, we have applied our knowledge of the gas and stellar distributions to the question of what controls star formation on large scales. NGC 4414 is an ideal test case because of its inner and outer cutoffs in the HII region distribution and because it has probably not suffered tidal interactions with other galaxies recently. We find that both cutoff radii are well reproduced using the simple Q criterion for gravitational instability.