Cyanoacetylene in IC 342: An Evolving Dense Gas Component with Starburst
  Age
  We present the first images of the J=5-4 and J=16-15 lines of the dense gas
tracer, cyanoacetylene, HC_3N, in an external galaxy. The central 200 pc of the
nearby star-forming spiral galaxy, IC 342, was mapped using the VLA and the
Plateau de Bure Interferometer. HC_3N(5-4) line emission is found across the
nuclear mini-spiral, but is very weak towards the starburst site, the location
of the strongest mid-IR and radio emission. The J=16-15 and 10-9 lines are also
faint near the large HII region complex, but are brighter relative to the 5-4
line, consistent with higher excitation. The brightest HC_3N emission is
located in the northern arm of the nuclear minispiral, 100 pc away from the
radio/IR source to the southwest of the nucleus. This location appears less
affected by ultraviolet radiation, and may represent a more embedded, earlier
stage of star formation. HC_3N excitation temperatures are consistent with
those determined from C^{18}O; the gas is dense, 10^{4-5}/cc, and cool, T_K ~<
40 K. So as to not violate limits on the total H_2 mass determined from
C^{18}O, at least two dense components are required to model IC 342's giant
molecular clouds. These observations suggest that HC_3N(5-4) is an excellent
probe of the dense, quiescent gas in galaxies. The high excitation combined
with faint emission towards the dense molecular gas at the starburst indicates
that it currently lacks large masses of very dense gas. We propose a scenario
where the starburst is being caught in the act of dispersing or destroying its
dense gas in the presence of the large HII region. This explains the high star
formation efficiency seen in the dense component. The little remaining dense
gas appears to be in pressure equilibrium with the starburst HII region.
