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

C. Eibensteiner , A. T. Barnes , F. Bigiel , E. Schinnerer , D. Liu , D. S. Meier , A. Usero , A. K. Leroy , E. Rosolowsky , J. Puschnig , I. Lazar , J. Pety , L. A. Lopez , E. Emsellem (1) , I. Bešlić , M. Querejeta , E. J. Murphy , J. den Brok , A. Schruba , M. Chevance , S. C. O. Glover , Y. Gao , K. Grasha , H. Hassani , J. D. Henshaw , M. J. Jimenez-Donaire , R. S. Klessen , J. M. D. Kruijssen , H. -A. Pan , T. Saito , M. C. Sormani , Y. -H. Teng , T. G. Williams

1 CRAL - Centre de Recherche Astrophysique de Lyon

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.

Based on observations carried out with the IRAM Plateau de Bure Interferometer (PdBI). IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

Mots clés: galaxies: ISM; ISM: molecules; galaxies: individual: NGC 6946; Astrophysics - Astrophysics of Galaxies

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

C. Eibensteiner, A. T. Barnes, F. Bigiel, E. Schinnerer, D. Liu, et al.. A 2-3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946. Astronomy and Astrophysics - A&A, 2022, 659, ⟨10.1051/0004-6361/202142624⟩. ⟨insu-03711531⟩