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Colloquium
The molecular ISM and astrochemistry in the starburst galaxy NGC 253: The ALCHEMI survey

Abstract

Properties of the molecular interstellar medium (ISM) can alter future star formation activities. The ISM in starburst galaxies is expected to have different physical properties from the ISM in the Milky Way due to the high star formation activity. This difference should also appear in chemical composition. We conducted the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) survey to analyze this ISM property in one of the nearest starburst galaxies. It is a spectra scan towards the nearby starburst galaxy NGC 253. It covers a wide frequency range of ALMA Bands 3-7, and was conducted as an ALMA Large Program in Cycle 5. The survey detected about 1500 transitions and more than 100 species. They include complex organic molecules that originated from the hot and dense starburst region. Comparing chemical models and observed chemical abundances, our results show 1-2 orders of magnitude higher cosmic-ray ionization rates than the Galactic Center. They also demonstrate multiple signs of shocks. We also conduct a statistical study called principal component analysis (PCA) to extract physical features from the survey images.