Molecular gas in the Andromeda galaxy

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: c.nieten@zeiss.de;NNeininger@kpmg.com
² Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ Institut de Radioastronomie Millimétrique, 300 rue de la piscine, 38406 St. Martin d' Hères, France e-mail: guelin@iram.fr
⁴ Instituto de Radioastronomía Milimétrica, Avenida Divina Pastora 7, 18012 Granada, Spain

Received: 13 November 2003
Accepted: 5 December 2005

Abstract

Aims.We study the distribution of the molecular gas in the Andromeda galaxy (M 31) and compare this with the distributions of the atomic gas and the emission from cold dust at m.

Methods.We obtained a new (J = 1-0)-line survey of the Andromeda galaxy with the highest resolution to date (, or 85 pc along the major axis), observed On-the-Fly with the IRAM 30-m telescope. We fully sampled an area of with a velocity resolution of . In several selected regions we also observed the (2-1)-line.

Results.Emission from the (1-0) line was detected from galactocentric radius  kpc to  kpc with a maximum in intensity at  kpc. The molecular gas traced by the (velocity-integrated) (1-0)-line intensity is concentrated in narrow arm-like filaments, which often coincide with the dark dust lanes visible at optical wavelengths. Between  kpc and  kpc the brightest CO filaments define a two-armed spiral pattern that is described well by two logarithmic spirals with a pitch angle of 7°–8°. The arm-interarm brightness ratio averaged over a length of 15 kpc along the western arms reaches about 20 compared to 4 for at an angular resolution of . For a constant conversion factor , the molecular fraction of the neutral gas is enhanced in the spiral arms and decreases radially from 0.6 on the inner arms to 0.3 on the arms at  kpc. The apparent gas-to-dust ratios and increase by a factor of ~20 between the centre and , whereas the ratio only increases by a factor of 4.

Conclusions.Either the atomic and total gas-to-dust ratios increase by a factor of ~20 or the dust becomes colder towards larger radii. A strong variation of with radius seems unlikely. The observed gradients affect the cross-correlations between gas and dust. In the radial range –14 kpc total gas and cold dust are well correlated; molecular gas correlates better with cold dust than atomic gas. The mass of the molecular gas in M 31 within a radius of 18 kpc is at the adopted distance of 780 kpc. This is 7% of the total neutral gas mass in M 31.