Volume Density Structure of the Central Molecular Zone NGC 253 through ALCHEMI Excitation Analysis

We present a spatially resolved excitation analysis for the central molecular zone (CMZ) of the starburst galaxy NGC 253 using the data from the Atacama Large Millimeter/submillimeter Array Comprehensive High-resolution Extragalactic Molecular Inventory, whereby we explore parameters distinguishing NGC 253 from the quiescent Milky Way's Galactic center (GC). Non-LTE analyses employing a hierarchical Bayesian framework are applied to Band 3-7 transitions from nine molecular species to delineate the position-position-velocity distributions of column density ( NH2 ), volume density ( nH2 ), and temperature (T kin) at 27 pc resolution. Two distinct components are detected: a low-density component with (nH2,Tkin)similar to(103.3cm-3,85K) and a high-density component with (nH2,Tkin)similar to(104.4cm-3,110K) , separated at nH2 similar to 103.8cm-3 . NGC 253 has similar to 10 times the high-density gas mass and similar to 3 times the dense-gas mass fraction of the GC. These properties are consistent with their HCN/CO ratio but cannot alone explain the factor of similar to 30 difference in their star formation efficiencies (SFEs), contradicting the dense-gas mass to star formation rate scaling law. The nH2 histogram toward NGC 253 exhibits a shallow declining slope up to nH2 similar to 106cm-3 , while that of the GC steeply drops in nH2 greater than or similar to 104.5cm-3 and vanishes at 105 cm-3. Their dense-gas mass fraction ratio becomes consistent with their SFEs when the threshold nH2 for the dense gas is taken at similar to 104.2-4.6 cm-3. The rich abundance of gas above this density range in the NGC 253 CMZ, or its scarcity in the GC, is likely to be the critical difference characterizing the contrasting star formation in the centers of the two galaxies.