A 2–3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946^⋆

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: eibensteiner@astro.uni-bonn.de
² Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
⁴ New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA
⁵ National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
⁶ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII 3, 28014 Madrid, Spain
⁷ Department of Astronomy, The Ohio State University, 4055 McPherson Laboratory, 140 West 18th Avenue, Columbus, OH 43210, USA
⁸ 4-183 CCIS, University of Alberta, Edmonton, Alberta T6G 2E1, Canada
⁹ Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹⁰ Institut de Radioastronomie Millimétrique (IRAM), 300 Rue de la Piscine, 38406 Saint Martin d’Hères, France
¹¹ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
¹² European Southern Observatory, Karl-Schwarzschild Straße 2, 85748 Garching bei München, Germany
¹³ Univ. Lyon, Univ. Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
¹⁴ National Radio Astronomy Observatory (NRAO), 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁵ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
¹⁶ Universität Heidelberg, Zentrum für Astronomie, Institüt für Theoretische Astrophysik, Albert-Ueberle-Strasse 2, 69120 Heidelberg, Germany
¹⁷ Department of Astronomy, Xiamen University, Xiamen, Fujian 361005, PR China
¹⁸ Purple Mountain Observatory, Chinese Academy of Sciences (CAS), Nanjing 210023, PR China
¹⁹ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
²⁰ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, INF 205, 69120 Heidelberg, Germany
²¹ Department of Physics, Tamkang University, No. 151, Yingzhuan Rd., Tamsui Dist., New Taipei City 251301, Taiwan
²² Center for Astrophysics and Space Sciences, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA

Received: 9 November 2021
Accepted: 20 December 2021

Abstract

The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new 2 − 4″ (∼75 − 150 pc at 7.7 Mpc) observations at 2 and 3 mm covering the central 50″ (∼1.9 kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO⁺, HNC, CS, HC₃N, N₂H⁺, C₂H, CH₃OH, and H₂CO. We complemented these with published 1 mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density Σ_SFR on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 – the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO⁺, and HNC). We find that HCO⁺ correlates best with Σ_SFR, but the dense gas fraction (f_dense) and star formation efficiency of the dense gas (SFE_dense) fits show different behaviours than expected from large-scale disc observations. The SBE has a higher Σ_SFR, f_dense, and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2−1)/CO(1−0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO⁺/HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies.