Molecular gas in the Andromeda galaxy

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 λ175μm. Methods. We obtained a new 12CO(J = 1-0)-line survey of the Andromeda galaxy with the highest resolution to date (23″, or 85 pc along the major axis), observed On-the-Fly with the IRAM 30-m telescope. We fully sampled an area of 2° × 0°.5 with a velocity resolution of 2.6 km s-1. In several selected regions we also observed the 12CO(2-1)-line. Results. Emission from the 12CO(1-0) line was detected from galactocentric radius R = 3 kpc to R = 16 kpc with a maximum in intensity at R ∼ 10 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 R = 4 kpc and R = 12 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 H I at an angular resolution of 45″. For a constant conversion factor XCO, 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 ≃ 10 kpc. The apparent gas-to-dust ratios N(H I)/I175 and (N(H I) + 2N(H2))/I175 increase by a factor of ∼20 between the centre and R ≃ 14 kpc, whereas the ratio 2N(H2)/I175 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 XCO with radius seems unlikely. The observed gradients affect the cross-correlations between gas and dust. In the radial range R = 8-14 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 M(H2) = 3.6 × 108 M⊙ at the adopted distance of 780 kpc. This is 7% of the total neutral gas mass in M 31. © ESO 2006.