2015ApJ...801...63M

We present spatially resolved (∼50 pc) imaging of molecular gas species in the central kiloparsec of the nearby starburst galaxy NGC 253, based on observations taken with the Atacama Large Millimeter/submillimeter Array. A total of 50 molecular lines are detected over a 13 GHz bandwidth imaged in the 3 mm band. Unambiguous identifications are assigned for 27 lines. Based on the measured high CO/C¹⁷O isotopic line ratio ( ≳ 350), we show that ¹²CO(1-0) has moderate optical depths. A comparison of the HCN and HCO⁺ with their ¹³C-substituted isotopologues shows that the HCN(1-0) and HCO⁺(1-0) lines have optical depths at least comparable to CO(1-0). H¹³CN/H¹³CO⁺ (and H¹³CN/HN¹³C) line ratios provide tighter constraints on dense gas properties in this starburst. SiO has elevated abundances across the nucleus. HNCO has the most distinctive morphology of all the bright lines, with its global luminosity dominated by the outer parts of the central region. The dramatic variation seen in the HNCO/SiO line ratio suggests that some of the chemical signatures of shocked gas are being erased in the presence of dominating central radiation fields (traced by C₂ H and CN). High density molecular gas tracers (including HCN, HCO⁺, and CN) are detected at the base of the molecular outflow. We also detect hydrogen β recombination lines that, like their α counterparts, show compact, centrally peaked morphologies, distinct from the molecular gas tracers. A number of sulfur based species are mapped (CS, SO, NS, C₂S, H₂CS, and CH₃SH) and have morphologies similar to SiO.