Tsukuba Uchu Forum

145th Uchu Forum

Volume density structure of the NGC 253 CMZ through ALCHEMI excitation analysis: what is the critical difference between NGC253 and Milky Way?

Kunihiko Tanaka

Keio University

NGC 253 is an archetypal starburst galaxy in the nearby universe, whose relative proximity (D ~ 3.5 Mpc) makes it an ideal target for studying the physical and chemical environments conducive to highly active star formation (SF). In particular, its similarity to the Milky Way (MW), traditionally understood as a quiescent galaxy, provides clues to understanding the critical conditions required to sustain high star formation rates (SFR) in galactic central regions; both NGC 253 and the MW possess a central molecular zone (CMZ) with similar average densities and dense-gas fractions, yet they differ by nearly two orders of magnitude in their star formation rates. We aim to elucidate the origin of the differences between NGC 253 and the MW by leveraging the unprecedentedly rich molecular line survey data of NGC 253 obtained through the ALMA large program ALCHEMI (ALMA Comprehensive High-resolution Extragalactic Molecular Inventory). This dataset provides a complete spectral survey in the 84-373 GHz frequency range with at 27 pc spatial resolution. With this talk, we present position-position-velocity 3-dimensional maps of the hydrogen column density, volume density, gas kinetic temperature, and the abundances of major molecular species within the NGC 253 CMZ. These maps are constructed through a physical condition analysis using 32 transitions from 13 species derived from the ALCHEMI data, employing hierarchical Bayesian probability modeling which we newly developed for analysing large-scale multi-transition molecular data. These maps are compared with those of the MW obtained from previous studies, with a particular focus on discerning differences in the volume density distribution function. Through this study, we seek to address the following key questions: (1) What does the traditional "dense gas fraction" defined by the HCN/CO ratio represent? (2) Is the SF critical volume density of $10^{4.6}\ \rm{cm}^{-3}$ valid for explaing the SF activities in galactic centers?, and (3) What critical difference is responsible for the contrasting SF activities observed in the two galaxies? Image