New insights on the dense molecular gas in NGC 253 as traced by HCN and HCO+

We have imaged the central similar to 1 kpc of the circumnuclear starburst disk in the galaxy NGC 253 in the HCN( 1 -> 0), HCO+( 1 -> 0), and CO( 1 -> 0) transitions at 60 pc resolution using the Owens Valley Radio Observatory Millimeter Wavelength Array (OVRO). We have also obtained Atacama Pathfinder Experiment (APEX) observations of the HCN( 4 -> 3) and the HCO+( 4 -> 3) lines of the starburst disk. We find that the emission from the HCN(1 -> 0) and HCO+( 1 -> 0) transitions, both indicators of dense molecular gas, trace regions which are nondistinguishable within the uncertainties of our observations. Even though the continuum flux varies by more than a factor 10 across the starburst disk, the HCN/HCO+ ratio is constant throughout the disk, and we derive an average ratio of 1.1 +/- 0.2. From an excitation analysis we find that all lines from both molecules are subthermally excited and that they are optically thick. This subthermal excitation implies that the observed HCN/HCO+ line ratio is sensitive to the underlying chemistry. The constant line ratio thus implies that there are no strong abundance gradients across the starburst disk of NGC 253. This finding may also explain the variations in L-HCN'/LHCO+' between different star-forming galaxies both nearby and at high redshifts.