2006A&A...448..457U

We present the first high-resolution (5.6"x5.1") images of the emission of silicon monoxide (SiO) in the nucleus of the nearby spiral IC 342, obtained with the IRAM Plateau de Bure Interferometer (PdBI). Using a two-field mosaic, we have simultaneously mapped the emission of the SiO(v=0, J=2-1) and H¹³CO⁺(J=1-0) lines in a region of ∼0.9x1.3kpc² (RAxDec) centered around the nucleus of IC 342. The bulk of the emission in the two lines comes from a ∼290pc spiral arm located to the North and a central component that forms the southern ridge of a r∼80pc nuclear ring that was identified in other interferometer maps of the galaxy. We detect continuum emission at 86.8 GHz in a ∼80-180pc central source. The continuum emission, dominated by thermal free-free bremsstrahlung, is mostly anticorrelated with the observed distribution of SiO clouds. The SiO-to-H¹³CO⁺ intensity ratio is seen to increase by an order of magnitude from the nuclear ring (∼0.3) to the spiral arm (∼3.3). Furthermore the gas kinematics show significant differences between SiO and H¹³CO⁺ over the spiral arm, where the linewidths of SiO are a factor of 2 larger than those of H¹³CO⁺. The average abundance of SiO in the inner r∼320pc of IC 342 is X(SiO)>2x10^–10. This shows that shock chemistry is at work in the inner molecular gas reservoir of IC 342. To shed light on the nature of shocks in IC 342, we have compared the emission of SiO with another tracer of molecular shocks: the emission of methanol (CH₃OH). We find that the significant difference of the abundance of SiO measured between the spiral arm (X(SiO) ∼ a few 10^–9) and the nuclear ring (X(SiO)∼10^–10) is not echoed by a comparable variation in the SiO-to-CH₃OH intensity ratio. This implies that the typical shock velocities should be similar in the two regions. In contrast, the fraction of shocked molecular gas should be ∼5-7 times larger in the spiral arm (up to ∼10% of the available molecular gas mass over the arm region) compared to the nuclear ring. In the light of these results, we revise the validity of the various scenarios that have been proposed to explain the onset of shock chemistry in galaxies and study their applicability to the nucleus of IC 342. We conclude that the large-scale shocks revealed by the SiO map of IC 342 are mostly unrelated to star formation and arise instead in a pre-starburst phase. Shocks are driven by cloud-cloud collisions along the potential well of the IC 342 bar. The general implications for the current understanding of galaxy evolution are discussed.