Molecular gas in the Andromeda galaxy

A&A 453, 459-475 (2006)
DOI: 10.1051/0004-6361:20035672

Molecular gas in the Andromeda galaxy

Ch. Nieten¹, N. Neininger^{1, 2, 3}, M. Guélin³, H. Ungerechts⁴, R. Lucas³, E. M. Berkhuijsen¹, R. Beck¹ and R. Wielebinski¹

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 $\lambda 175\, \mu$ m.
Methods.We obtained a new $\element[][12]{CO}$ (J = 1-0)-line survey of the Andromeda galaxy with the highest resolution to date ( $23\arcsec$ , or 85 pc along the major axis), observed On-the-Fly with the IRAM 30-m telescope. We fully sampled an area of $2\degr\times 0\fdg 5$ with a velocity resolution of $2.6\rm {\, km\, s^{-1}}$ . In several selected regions we also observed the $\element[][12]{CO}$ (2-1)-line.
Results.Emission from the $\element[][12]{CO}$ (1-0) line was detected from galactocentric radius R=3 kpc to R=16 kpc with a maximum in intensity at $R\sim 10$ 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 $\ion{H}{i}$ at an angular resolution of $45\arcsec$ . 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 $R\simeq 10$ kpc. The apparent gas-to-dust ratios $N(\ion{H}{i})/I_{175}$ and $(N(\ion{H}{i})+2N(\mathrm{H}_2))/I_{175}$ increase by a factor of ~20 between the centre and $R\simeq 14\rm {\, kpc}$ , 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 X_CO 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 $M (\mbox{H}_2) = 3.6\times 10^8\, {M}_{\odot}$ at the adopted distance of 780 kpc. This is 7% of the total neutral gas mass in M 31.

Key words: ISM: molecules -- galaxies: individual: M 31 -- galaxies: ISM -- galaxies: spiral -- radio lines: galaxies

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