Formation of Dense Molecular Gas and Stars at the Circumnuclear
  Starburst Ring in the Barred Galaxy NGC 7552
  We present millimeter molecular-line complemented by optical observations,
along with a reanalysis of archival centimeter HI and continuum data, to infer
the global dynamics and determine where dense molecular gas and massive stars
preferentially form in the circumnuclear starburst ring of the barred-spiral
galaxy NGC 7552. We find diffuse molecular gas in a pair of dust lanes each
running along the large-scale galactic bar, as well as in the circumnuclear
starburst ring. We do not detect dense molecular gas in the dust lanes, but
find such gas concentrated in two knots where the dust lanes make contact with
the circumnuclear starburst ring. When convolved to the same angular resolution
as the images in dense gas, the radio continuum emission of the circumnuclear
starburst ring also exhibits two knots, each lying downstream of an adjacent
knot in dense gas. The results agree qualitatively with the idea that massive
stars form from dense gas at the contact points, where diffuse gas is channeled
into the ring along the dust lanes, and later explode as supernovae downstream
of the contact points. Based on the inferred rotation curve, however, the
propagation time between the respective pairs of dense gas and centimeter
continuum knots is about an order of magnitude shorter than the lifetimes of OB
stars. We discuss possible reasons of this discrepancy, and conclude that
either the initial mass function is top-heavy or massive stars in the ring do
not form exclusively at the contact points where dense molecular gas is
concentrated.
