ALCHEMI: First Results from the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory of NGC253

The dramatic increase in sensitivity and frequency coverage of radio, millimeter, and submillimeter facilities in the last decade has made it possible for the first time to study the chemical complexity of the molecular interstellar medium (ISM) in galaxies beyond the Milky Way. Both spatially resolved observations in nearby galaxies (e.g., Meier & Turner, 2005; Meier+2015; Martin+2015, Izumi+2015) and unresolved surveys (e.g., Krips+2008; Costagliola+2011) have gathered substantial evidence of a great variety in molecular gas composition and excitation across galaxies. Spectral line surveys (e.g., Martin+2011, Aladro+2015, Costagliola+2015) reveal significant differences in the chemical properties of the ISM in starburst galaxies, and suggest that chemistry may be an important diagnostic of evolutionary stage. ISM chemical cycling and enrichment by star formation can be assessed through observations of molecular isotopologues and the derivation of isotopic ratios (e.g., the 12C/13C and 16O/18O ratios: Rangwala+2011, Henkel+2014, Falstad+2015). ISM energy exchange and mechanical feedback from the starburst process can be studied through other molecular species that trace shocked gas and dust (e.g., SiO, HNCO, CH3OH: e.g., Garcia-Burillo+2001, Usero+2006, Meier & Turner 2012). Ultimately, molecular tracers of ISM chemistry probe the fundamental regulation of the star formation process in galaxies. To this end, we are conducting a multi-band spectral line survey within the nearby starburst galaxy NGC253. The ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI), an ALMA Cycle 5 Large Programme, encompasses the entirety of ALMA Bands 3, 4, 6, and 7 (85 to 366 GHz). A complimentary ALMA Cycle 6 program adds imaging of the ALCHEMI field at ALMA Band 5. ALCHEMI images the central several hundred parsec area of NGC253 using both the 12m Array and the Atacama Compact Array (ACA) to cover most of the central molecular zone (50x20 arcsec field) at 1.5 arcsec (25 pc) spatial and 10 km/s spectral resolution, with sensitivity to spatial structure <~ 15 arcsec. These measurements will allow us to resolve the molecular cloud structure on giant molecular cloud scales. As of March 2019 all observations are complete. Analysis to-date has focused on spectral line identification in order to assess the average chemical composition in NGC253 and compare this chemical composition to that within other galaxies and Galactic sources. A preliminary analysis using the MADCUBA (Martin et al, in prep) line identification tool of the ACA observations has uncovered the surprisingly rich molecular complexity even at the relatively low spatial resolution (~10 arcsec) of our ACA measurements of NGC253. This MADCUBA analysis has revealed (1) Spectral line transitions with intensity greater than 1 mJy = 3978, (2) Identification of 50 molecular species and 33 isotopologues, and (3) Seven of these species are new extragalactic identifications. The ALCHEMI collaboration is currently completing its molecular inventory of the NGC253 CMZ and has started the process of studying the detailed physical and chemical environment within the GMCs of NGC253. In this presentation I will present results from a wide range of research projects currently being conducted with the ALCHEMI survey.