ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253
  We present spatially resolved ($\sim$50 pc) imaging of molecular gas species
in the central kiloparsec of the nearby starburst galaxy NGC 253, based on
observations taken with the Atacama Large Millimeter/submillimeter Array
(ALMA). A total of 50 molecular lines are detected over a 13 GHz bandwidth
imaged in the 3 mm band. Unambiguous identifications are assigned for 27 lines.
Based on the measured high CO/C$^{17}$O isotopic line ratio ($\gtrsim$350), we
show that $^{12}$CO(1-0) has moderate optical depths. A comparison of the HCN
and HCO$^{+}$ with their $^{13}$C-substituted isotopologues shows that the
HCN(1-0) and HCO$^{+}$(1-0) lines have optical depths at least comparable to
CO(1-0). H$^{13}$CN/H$^{13}$CO$^{+}$ (and H$^{13}$CN/HN$^{13}$C) 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
C$_{2}$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 $\beta$ recombination lines that, like their $\alpha$ counterparts,
show compact, centrally peaked morphologies, distinct from the molecular gas
tracers. A number of sulfur based species are mapped (CS, SO, NS, C$_{2}$S,
H$_{2}$CS and CH$_{3}$SH) and have morphologies similar to SiO.
