We present the first images of the J=5–4 and J=16–15 lines of the dense gas tracer , cyanoacetylene , HC _ { 3 } N , 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 _ { 3 } N ( 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 _ { 3 } N 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 _ { 3 } N excitation temperatures are consistent with those determined from C ^ { 18 } O ; the gas is dense , 10 ^ { 4 - 5 } ~ { } cm ^ { -3 } , 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 _ { 3 } N ( 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 .