2006A&A...453..459N

We study the distribution of the molecular gas in the Andromeda galaxy (M31) and compare this with the distributions of the atomic gas and the emission from cold dust at λ175µm. We obtained a new ¹²CO(J=1-0)-line survey of the Andromeda galaxy with the highest resolution to date (23", or 85pc along the major axis), observed On-the-Fly with the IRAM 30-m telescope. We fully sampled an area of 2x0.5deg² with a velocity resolution of 2.6km/s. In several selected regions we also observed the ¹²CO(2-1)-line. Emission from the ¹²CO(1-0) line was detected from galactocentric radius R=3kpc to R=16kpc with a maximum in intensity at R∼10kpc. 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 R=4kpc and R=12kpc 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 15kpc along the western arms reaches about 20 compared to 4 for HI at an angular resolution of 45". For a constant conversion factor X_CO, 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 R≃10kpc. The apparent gas-to-dust ratios N(HI)/I₁₇₅ and (N(HI)+2N(H₂))/I₁₇₅ increase by a factor of ∼20 between the centre and R≃14kpc, whereas the ratio 2N(H₂)/I₁₇₅ only increases by a factor of 4. 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 X_CO with radius seems unlikely. The observed gradients affect the cross-correlations between gas and dust. In the radial range R=8-14kpc 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 M31 within a radius of 18kpc is M (H₂)=3.6x10⁸M_☉ at the adopted distance of 780kpc. This is 7% of the total neutral gas mass in M31.