CI and CO in the nearby spiral galaxies IC 342 and Maffei 2

Abstract

We present J = 2-1, J = 3-2, J = 4-3 12CO and 492 GHz [CI] maps as well as J = 2-1 and J = 3-2 13CO measurements of the central regions in the nearby Sc galaxies IC 342 and Maffei 2. In both galaxies, the distribution of CO and [CI] is strongly concentrated towards the center. These centers harbour modest starbursts. Both galaxies have nearly identical 12CO transitional ratios but the relative intensities of their 13CO and [CI] emission lines differ significantly. The observed sets of line intensities require modelling with a multi-component molecular gas. In both galaxies, a dense component must be present (n(H2) ≈ 104-105 cm-3) with kinetic temperatures Tkin = 10-20 K (IC 342) or 20-60 K (Maffei 2), as well as a less dense (IC 342: a few hundred cm-3 at most; Maffei 2: ≈ 3 × 103 cm-3) and hotter (Tkin = 100-150 K) component. In both galaxies, neutral and ionized atomic carbon amounts are between 1.5 and 2.5 times those of CO. In both starburst centers about half to two thirds of the molecular gas mass is associated with the hot PDR phase. The center of IC 342 contains within R = 0.25 kpc an (atomic and molecular) gas mass of 1 × 107 M⊙ and a peak face-on gas mass density of about 70 M⊙ pc-2. For Maffei 2 these numbers are less clearly defined, mainly because of uncertainties in its distance and carbon abundance. We find a gass mass Mgas ≤ 0.5 × 107 M⊙, and a peak face-on gas mass density of about 35 M⊙ pc-2.