Warm gas and spatial variations of molecular excitation in the nuclear region of IC 342

Abstract

We present maps of the J = 1->0 and 2->1 lines of ^12^CO, ^13^CO, and C^18^O in the nuclear region of the galaxy IC 342. We find that the ^13^CO and C^18^O J = 2->1 to J = 1->0 line ratios provide the most significant constraints on the physical conditions of the molecular interstellar medium. The observations taken with the IRAM 30 m telescope show that molecular gas in the nucleus sampled by the ^12^CO line is warm and that physical conditions vary with position in the galaxy. If all isotopic CO line emission originates from the same material, the molecular gas in the central 1 kpc region of IC 342 has a kinetic temperature of >= 20 K and a molecular hydrogen density of ~ 3 x 10^3^ cm^-3^. At distances ~ 500 pc from the center, the kinetic temperature is significantly lower (between 10 K and 20 K). Along a ridge 500 pc east of the center, there may be CO emission from optically thin gas at a temperature of ~40 K. Alternatively the ^12^CO line emission originates in a small amount of warm gas associated with photodissociation regions and/or a warm interclump medium in star-forming regions. In this case, the ^13^CO and C^18^O emission measures the bulk of cold (~15 K) and dense (~ 10^4^ cm^-3^) gas in molecular clumps. New far-infrared measurements of the [O I] 63 micron fine-structure line indicate that the atomic and molecular gas pressures are both a few times 10^5^ cm^-3^ K, 10 times larger than in our Galaxy.