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We present millimeter molecular-line complemented by optical observations,along with a reanalysis of archival centimeter HI and continuum data, to inferthe global dynamics and determine where dense molecular gas and massive starspreferentially form in the circumnuclear starburst ring of the barred-spiralgalaxy NGC 7552. We find diffuse molecular gas in a pair of dust lanes eachrunning along the large-scale galactic bar, as well as in the circumnuclearstarburst ring. We do not detect dense molecular gas in the dust lanes, butfind such gas concentrated in two knots where the dust lanes make contact withthe circumnuclear starburst ring. When convolved to the same angular resolutionas the images in dense gas, the radio continuum emission of the circumnuclearstarburst ring also exhibits two knots, each lying downstream of an adjacentknot in dense gas. The results agree qualitatively with the idea that massivestars form from dense gas at the contact points, where diffuse gas is channeledinto the ring along the dust lanes, and later explode as supernovae downstreamof the contact points. Based on the inferred rotation curve, however, thepropagation time between the respective pairs of dense gas and centimetercontinuum knots is about an order of magnitude shorter than the lifetimes of OBstars. We discuss possible reasons of this discrepancy, and conclude thateither the initial mass function is top-heavy or massive stars in the ring donot form exclusively at the contact points where dense molecular gas isconcentrated.

Formation of Dense Molecular Gas and Stars at the Circumnuclear Starburst Ring in the Barred Galaxy NGC 7552