Molecular gas in NGC 6946

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany
² Harvard Smithsonian Center for Astrophysics 60 Garden St, MS12, Cambridge, MA 02138, USA
³ Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121, Bonn, Germany
⁴ SMTO, Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, Arizona 85721, USA

Corresponding author: W. Walsh, wwalsh@mpifr-bonn.mpg.de

Received: 2 November 2001
Accepted: 25 March 2002

Abstract

We present imaging of molecular gas emission in the star-forming spiral galaxy NGC 6946. Our ¹²CO(1–0) and ¹²CO(3–2) images, made at 22´´  resolution with the IRAM 30-m and the Heinrich Hertz radio telescopes, are the most extensive observations of molecular gas in this galaxy and are among the most extensive observations of molecular gas in any spiral galaxy. The ¹²CO(1–0) emission shows a central concentration aligned in a north-south ellipse, an extended diffuse component plus concentrations in the outer spiral arms. ¹²CO(1–0) is detected across most of the optical disk out to a projected radius of nearly 300´´. The molecular component in NGC 6946 is unusually massive, with . The scale length of the disk in ¹²CO(1–0) is the same, to within 5%, as the VRI, Hα, 15 μm, and FIR disks. The ¹²CO(3–2) data shows broadly similar structure to the ¹²CO(1–0) image out to the observed limit of 200´´, although the arm-interarm contrast of ¹²CO(3–2) is significantly larger than that of ¹²CO(1–0) which suggests that molecules in the spiral arms are warmer or reside longer there. The rotation curve derived from the ¹²CO(1–0) velocity field agrees, within the uncertainties, with the H i rotation curve. The velocity dispersion is  km s^-1 in the inner 2 kpc and  km s^-1 in the disk, without tendency to increase in the spiral arms. Spectra of the ¹³CO(1–0), ¹³CO(2–1), ¹²C¹⁸O(1–0) isotopomers for several positions are used for line ratio studies of the state of the molecular ISM in NGC 6946. In the centre of NGC 6946 our LVG analysis suggests that the beam-averaged gas kinetic temperature is  K, the molecular gas density is  cm^-3 and that line opacities are high, with . A star formation efficiency image for NGC 6946, made from the Hα image divided by the molecular gas image, ranges by over two orders of magnitude with highest values found in the northeastern spiral arm. The λ6 cm polarized emission image, which traces the regular part of the magnetic field, appears anti-correlated with the star formation efficiency. We present an analysis of the ISM in NGC 6946's disk by making 1-D and 2-D comparisons of images made in several wavebands. Using a point-by-point correlation technique, we investigate the distribution and kinematics of the molecular gas and its relation to the neutral and ionized gas, the mid-infrared-emitting dust, the radio continuum and the magnetic field, and find that the molecular gas is closely associated with the 7 μm-emitting dust. The highest correlation between any pair of tracers is found between the mid-infrared emission and the total radio continuum emission at λ6 cm. This cannot be due to dust heating and gas ionization in star-forming regions because the thermal radio emission is less correlated with the mid-infrared emission than the nonthermal emission. A coupling of magnetic fields to gas clouds is proposed as a possible scenario.