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Date and Time of the Query: 2018-12-16 T09:26:25 PST
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For refcode 2006A&A...453..459N:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
2006A&A...453..459N Molecular gas in the Andromeda galaxy Ch. Nieten; N. Neininger; M. Guelin; H. Ungerechts; R. Lucas; E. M. Berkhuijsen; R. Beck; R. Wielebinski Received 13 November 2003 / Accepted 5 December 2005 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 microns. Methods. We obtained a new CO(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 2deg x 0.5deg with a velocity resolution of 2.6 km s^-1^. In several selected regions we also observed the CO(2-1)-line. Results. Emission from the CO(1-0) line was detected from galactocentric radius R=3 kpc to R=16 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 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 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 ~= 10 kpc. The apparent gas-to-dust ratios N(H I)/I_175_ and (N(H I)+2N(H_2_))/I_175 increase by a factor of 20 between the centre and R ~= 14 ~ kpc , whereas the ratio 2N(H_2_)/I_175 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 (H_2_) = 3.6 x 10^8^ ~ M_sun_ 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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