THE ANATOMY OF AN ISOLATED SPIRAL GALAXY - NGC-4414

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 optica l edge of the disk. The (CO)-C-13 lines and thermal dust continuum emi ssion 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 o f a variation in the CO excitation temperature. Combining the temperat ure variation with the galactic size-linewidth relation and the virial theorem enables us to estimate how the N(H-2)/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 becaus e there is probably little or no star formation and neutral gas in the nucleus of NGC 4414, we avoid problems associated with conditions pro per to galactic nuclei but thus make no predictions for these regions. In order to check the N(H-2)/I-CO function obtained above, we used ou r (CO)-C-13(1-0) and (CO)-C-13(2-1) data, in conjunction with the (CO) -C-12 and assuming the (CO)-C-13 to be optically thin, to derive absol ute (CO)-C-13 abundances as a function of radius. The resulting abunda nces are in good agreement with galactic observations. The millimeter- wave thermal emission from dust is a second independent test. While gr ain cross-sections and, to a lesser extent, dust temperatures are subj ect to debate, commonly used values yield gas masses quite close to th ose we estimate from our analysis. Unusually low cross-sections or tem peratures and a peculiar variation are required to fit the gas mass re sulting from the use of a constant N(H-2)/I-CO ratio. All of our obser vations support the variation and range of N(H-2)/I-CO(1-0) that we pr opose here for NGC 4414. It should however be borne in mind that a num ber of assumptions about molecular clouds have been made and that we h ave 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 h ave 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 rest case because of its inner and outer cutoffs in the I-III region distribution and because it has probably not suffered tidal in teractions with other galaxies recently. We find that both cutoff radi i are well reproduced using the simple Q criterion for gravitational i nstability.