1997A&A...318...19B

Our on-going observations of the interstellar medium of NGC 4414 have resulted in CO(1-0) and CO(2-1) detections to very close to the optical edge of the disk. The ¹³CO lines and thermal dust continuum emission have been detected almost as far out. The variation in the beam-independent CO(2-1/ 1-0) line intensity ratio is interpreted in terms of a variation in the CO excitation temperature. Combining the temperature variation with the galactic size-linewidth relation and the virial theorem enables us to estimate how the N(H₂)/ I_CO factor varies as a function of galactocentric radius. One then straightforwardly obtains the distribution of the molecular gas. It should be noted that because there is probably little or no star formation and neutral gas in the nucleus of NGC 4414, we avoid problems associated with conditions proper to galactic nuclei but thus make no predictions for these regions. In order to check the N(H₂)/ I_CO function obtained above, we used our ¹³CO(1-0) and ¹³CO(2-1) data, in conjunction with the ¹²CO and assuming the ¹³CO to be optically thin, to derive absolute ¹³CO 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 N(H₂)/I_CO ratio. All of our observations support the variation and range of N(H₂)/I_CO(1–0) 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.