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

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, HC 3 N, N 2 H + , C 2 H, CH 3 OH, and H 2 CO. We complemented these with published 1mm 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.

Star Formation in the Central Molecular Zone of the Milky Way

“…Similar results have been found in nearby galaxies also (e.g. Usero et al 2015;Bigiel et al 2016;Querejeta et al 2019;Jiménez-Donaire et al 2019;Bešlić et al 2021;Eibensteiner et al 2022). However, the exact nature and magnitude of this dependence remains an unresolved question.…”

“…The gas continues its journey from the CND towards the centre through the so-called Galactic centre minispiral (also known as the Sgr A West HII region), a system of orbiting filamentary streamers that are photoionised by the high-mass stars located within the central parsec (Lo and Claussen 1983;Ekers et al 1983;Nitschai et al 2020;Heywood et al 2022). The minispiral streamers have an associated mass inflow rate of ∼ 10 −3 M yr −1 (or ∼ 0.1% of the inflow rate from large-scales; §4.1.1) into the central few arcseconds (Jackson et al 1993;Genzel et al 1994).…”

“…The dashed horizontal line shows the scaling relation of Lada et al (2012), and the dotted line shows a factor of ten below this relation. Right (inset panel): The M dense and SFR/M dense , normalised to the Lada et al (2012) relation.external galaxies have found higher dense gas fractions and lower dense gas star formation efficiencies (SFR/M dense ), similar to the CMZ (see e.g Usero et al 2015;Bigiel et al 2016;Jiménez-Donaire et al 2019;Querejeta et al 2019;Bešlić et al 2021;Eibensteiner et al 2022)…”