2019ApJ...871..170M

The nuclear starburst within the central ∼15'' (∼250 pc; 1'' ≃ 17 pc) of NGC 253 has been extensively studied as a prototype for the starburst phase in galactic evolution. Atacama Large Millimeter/submillimeter Array (ALMA) imaging within receiver Bands 6 and 7 has been used to investigate the dense gas structure, kinetic temperature, and heating processes that drive the NGC 253 starburst. A total of 29 transitions from 15 molecular species/isotopologues have been identified and imaged at 1.''5-0.''4 resolution, allowing for the identification of five of the previously studied giant molecular clouds within the central molecular zone (CMZ) of NGC 253. Ten transitions from the formaldehyde (H₂CO) molecule have been used to derive the kinetic temperature within the ∼0.''5-5'' dense gas structures imaged. On ∼5'' scales we measure T_K >= 50 K, while on size scales <=1'' we measure T_K >= 300 K. These kinetic temperature measurements further delineate the association between potential sources of dense gas heating. We have investigated potential heating sources by comparing our measurements to models that predict the physical conditions associated with dense molecular clouds that possess a variety of heating mechanisms. This comparison has been supplemented with tracers of recently formed massive stars (Brγ) and shocks ([Fe II]). Derived molecular column densities point to a radially decreasing abundance of molecules with sensitivity to cosmic-ray and mechanical heating within the NGC 253 CMZ. These measurements are consistent with radio spectral index calculations that suggest a higher concentration of cosmic-ray-producing supernova remnants within the central 10 pc of NGC 253.