ALCHEMI, an ALMA Comprehensive High-resolution Extragalactic Molecular Inventory

Survey presentation and first results from the ACA array

¹ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
e-mail: smartin@eso.org
² Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan
⁶ Department of Astronomy, School of Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-1855, Japan
⁷ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ Department of Physics, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama, Kanagawa 223–8522, Japan
¹⁰ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
¹¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans, 08193 Barcelona, Spain
¹² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹³ Astronomy Department, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904–4325, USA
¹⁴ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁵ INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
¹⁶ Jodrell Bank Centre for Astrophysics, Department of Physics & Astronomy, School of Natural Sciences, The University of Manchester, Manchester M13 9PL, UK
¹⁷ Intituto de Astrofísica de Andalucia (CSIC), Glorieta de al Astronomia s/n, 18008 Granada, Spain
¹⁸ Observatorio Astronómico Nacional (OAN-IGN), Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
¹⁹ Cosmic Dawn Center, DTU Space, Technical University of Denmark, Elektrovej 327, Kgs. Lyngby 2800, Denmark
²⁰ Department of Physics and Astronomy, University College London, Gower Street, London WC1E6BT, UK
²¹ Astron. Dept., Faculty of Science, King Abdulaziz University, PO Box 80203 Jeddah 21589, Saudi Arabia
²² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, PR China
²³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
²⁴ Research Center for the Early Universe, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
²⁵ New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA
²⁶ National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
²⁷ Institute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
²⁸ Department of Physics, General Studies, College of Engineering, Nihon University, Tamura-machi, Koriyama, Fukushima 963-8642, Japan

Received: 16 June 2021
Accepted: 14 September 2021

Abstract

Context. The interstellar medium is the locus of physical processes affecting the evolution of galaxies which drive or are the result of star formation activity, supermassive black hole growth, and feedback. The resulting physical conditions determine the observable chemical abundances that can be explored through molecular emission observations at millimeter and submillimeter wavelengths.

Aims. Our goal is to unveiling the molecular richness of the central region of the prototypical nearby starburst galaxy NGC 253 at an unprecedented combination of sensitivity, spatial resolution, and frequency coverage.

Methods. We used the Atacama Large Millimeter/submillimeter Array (ALMA), covering a nearly contiguous 289 GHz frequency range between 84.2 and 373.2 GHz, to image the continuum and spectral line emission at 1.6″(∼28 pc) resolution down to a sensitivity of 30 − 50 mK. This article describes the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) large program. We focus on the analysis of the spectra extracted from the 15″ (∼255 pc) resolution ALMA Compact Array data.

Results. We modeled the molecular emission assuming local thermodynamic equilibrium with 78 species being detected. Additionally, multiple hydrogen and helium recombination lines are identified. Spectral lines contribute 5 to 36% of the total emission in frequency bins of 50 GHz. We report the first extragalactic detections of C₂H₅OH, HOCN, HC₃HO, and several rare isotopologues. Isotopic ratios of carbon, oxygen, sulfur, nitrogen, and silicon were measured with multiple species.

Concluison. Infrared pumped vibrationaly excited HCN, HNC, and HC₃N emission, originating in massive star formation locations, is clearly detected at low resolution, while we do not detect it for HCO⁺. We suggest high temperature conditions in these regions driving a seemingly “carbon-rich” chemistry which may also explain the observed high abundance of organic species close to those in Galactic hot cores. The L_vib/L_IR ratio was used as a proxy to estimate a 3% contribution from the proto super star cluster to the global infrared emission. Measured isotopic ratios with high dipole moment species agree with those within the central kiloparsec of the Galaxy, while those derived from ¹³C/¹⁸O are a factor of five larger, confirming the existence of multiple interstellar medium components within NGC 253 with different degrees of nucleosynthesis enrichment. The ALCHEMI data set provides a unique template for studies of star-forming galaxies in the early Universe.