CO(4-3) and CO(7-6) maps of the nucleus of NGC253

Context. Molecular line excitation studies of the nuclei of nearby starburst galaxies yield important information on the starburst phenomena, in particular on the temperature and density of the star-forming gas. Such studies also provide templates for high redshift galaxies with even more extreme star formation. Aims. Fundamental constraints on the physical properties in the nuclear regions of external galaxies can be derived from the spectral energy distribution (i.e., integrated flux density vs. rotational quantum number) of CO rotational emission arising from warm gas. Methods. The resolution and sensitivity of the APEX telescope makes it feasible to perform spatially resolved studies of submillimeter (submm) CO emission from the warm, dense gas in nearby starburst nuclei. Using the FLASH dual-channel heterodyne receiver we mapped emission in the CO J = 4-3 and 7-6 lines toward the archetypical nuclear starburst galaxy NGC253. Results. Combining our new observations with data from the literature, we derive the CO line SED in the central 250 pc of NGC253, which peaks near the 6-5 transition and has a shape very similar to that of M82. All CO transitions in the central region can well be fitted with a single temperature/density Large Velocity Gradient (LVG) model. A good match to the observations is found by assuming kinetic gas temperatures that are comparable to the dust temperature (T-kin approximate to 60 K) and a H-2 density of order 10(4) cm(-3). Conclusions. Our very first APEX submm study of a nearby starburst nucleus ( NGC253) meaningfully constrains the physical properties of the star-forming molecular gas it contains. With broader band spectrometers and a chopping secondary coming soon, the impact of APEX on extragalactic astrophysics will be foreseeably significant.