2008A&A...479...75H

Studying molecular gas in the central regions of the star burst galaxies NGC4945 and Circinus enables us to characterize the physical conditions and compare them to previous local and high-z studies. We estimate temperature, molecular density and column densities of CO and atomic carbon. Using model predictions we give a range of estimated CO/C abundance ratios. Using the new NANTEN2 4 m sub-millimeter telescope in Pampa La Bola, Chile, we observed for the first time CO 4-3 and [CI] ³P₁-³P₀ at the centers of both galaxies at linear scale of 682 pc and 732 pc respectively. We compute the cooling curves of ¹²CO and ¹³CO using radiative transfer models and estimate the physical conditions of CO and [CI]. The centers of NGC4945 and Circinus are very [CI] bright objects, exhibiting [CI] ³P₁-³P₀ luminosities of 91 and 67K.km/s.kpc², respectively. The [CI] ³P₁-³P₀/CO 4-3 ratio of integrated intensities are large at 1.2 in NGC4945 and 2.8 in Circinus. Combining previous CO J= 1-0, 2-1 and 3-2 and ¹³CO J= 1-0, 2-1 studies with our new observations, the radiative transfer calculations give a range of densities, n(H₂)=10³-3x10⁴cm^–3, and a wide range of kinetic temperatures, T_kin=20-100K, depending on the density. To discuss the degeneracy in density and temperature, we study two representative solutions. In both galaxies the estimated total [CI] cooling intensity is stronger by factors of ∼1-3 compared to the total CO cooling intensity. The CO/C abundance ratios are 0.2-2, similar to values found in Galactic translucent clouds. Our new observations enable us to further constrain the excitation conditions and estimate the line emission of higher-J CO- and the upper [CI]-lines. For the first time we give estimates for the CO/C abundance ratio in the center regions of these galaxies. Future CO J=7-6 and [CI] 2-1 observations will be important to resolve the ambiguity in the physical conditions and confirm the model predictions.