Cyanoacetylene in IC 342: An Evolving Dense Gas Component with Starburst Age

TL;DR

This study maps dense gas tracer HC_3N in galaxy IC 342, revealing its distribution, excitation, and implications for starburst evolution and dense gas dispersal.

Contribution

First imaging of HC_3N lines in an external galaxy, providing insights into dense gas conditions and starburst evolution.

Findings

HC_3N emission is weak at the starburst site but brighter in the northern arm.

Dense gas is dense (~10^4-10^5/cc) and cool (T_K < 40 K).

Starburst may be dispersing dense gas, affecting star formation efficiency.

Abstract

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.