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

Abstract

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 ( N H 2 ), volume density ( n H 2 ), and temperature (T kin) at 27 pc resolution. Two distinct components are detected: a low-density component with ( n H 2 , T kin ) ∼ ( 10 3.3 cm − 3 , 85 K ) and a high-density component with ( n H 2 , T kin ) ∼ ( 10 4.4 cm − 3 , 110 K ) , separated at n H 2 ∼ 10 3.8 cm − 3 . NGC 253 has ∼10 times the high-density gas mass and ∼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 ∼30 difference in their star formation efficiencies (SFEs), contradicting the dense-gas mass to star formation rate scaling law. The n H 2 histogram toward NGC 253 exhibits a shallow declining slope up to n H 2 ∼ 10 6 cm − 3 , while that of the GC steeply drops in n H 2 ≳ 10 4.5 cm − 3 and vanishes at 105 cm−3. Their dense-gas mass fraction ratio becomes consistent with their SFEs when the threshold n H 2 for the dense gas is taken at ∼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.

The ALCHEMI Atlas: Principal Component Analysis Reveals Starburst Evolution in NGC 253

“…Here, we examine whether each PC has any clear relationship with physical parameters such as column densities, kinetic temperatures, and volume densities. Figure 26 shows the relationship between the PC scores in our results and physical conditions derived by Tanaka et al (2024) from the ALCHEMI data. These physical parameters were obtained from the hierarchical Bayesian analysis method developed by Tanaka et al (2018).…”

“…For example, it takes ∼10 6 yr for a cloud to move 100 pc if it is moving with a velocity of 100 km s −1 , a few times the freefall time of a cloud with n = 10 4 cm −3 . Note, though, that the star formation threshold is likely higher in the CMZ of NGC 253 than that in the Galactic center and the Galactic disk (Tanaka et al 2024).…”

“…These physical parameters were obtained from the hierarchical Bayesian analysis method developed by Tanaka et al (2018). Tanaka et al (2024) present results from different sets of transitions. Among their results, we use their "high-HB" model derived from species with higher critical densities such as HCN, HCO + , and their isotopologues, HC 3 N, and SiO, instead of results derived from CO isotopologues.…”

“…These results suggest that PC1 primarily represents the overall molecular gas content. The volume densities tend to be higher in high-column-density regions in Tanaka et al (2024), and this is likely why PC1 and the volume densities have a positive correlation. The high-density regions are also the regions with high star formation rates, which explains the trend of increasing PC1 scores with increasing temperatures.…”

“…This survey also made the first extragalactic detection of a phosphorus-bearing molecule, phosphorus nitride (Haasler et al 2022). Tanaka et al (2024) make use of the multitransition and multimolecule data to accurately map the physical conditions (gas densities, temperatures, and molecular column densities) using hierarchical Bayesian analysis and found high densities compared to those in the center of our own Galaxy.…”