Molecular gas in the Andromeda galaxy

We present a new 12CO(J=1–0)–line survey of the Andromeda galaxy, M 31, 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 mapped an area of about 2 ×0. 5 which was tightly sampled on a grid of 9 with a velocity resolution of 2.6 km s−1. The r.m.s. noise in the velocity-integrated map is around 0.35 K km s−1 on the Tmb-scale. Emission from the 12CO(1–0) line is detected from galactocentric radius R = 3 kpc to R = 16 kpc, but peaks in intensity at R ∼ 10 kpc. Some clouds are visible beyond R = 16 kpc, the farthest of them at R = 19.4 kpc. The molecular gas traced by the (1–0) line is concentrated in narrow arm-like filaments, which often coincide with the dark dust lanes visible at optical wavelengths. The H  arms are broader and smoother than the molecular arms. Between R = 4 kpc and R = 12 kpc the brightest CO filaments and the darkest dust lanes define a two-armed spiral pattern that is well described by two logarithmic spirals with a constant pitch angle of 7–8. Except for some bridge-like structures between the arms, the interarm regions and the central bulge are free of emission at our sensitivity. The arm–interarm brightness ratio averaged over a length of 15 kpc along the western arms reaches about 20 compared to 4 for H  at an angular resolution of 45 . In several selected regions we also observed the CO(2–1)–line on a finer grid. Towards the bright CO emission in our survey we find normal ratios of the (2–1)–to–(1–0) line intensities which are consistent with optically thick lines and thermal excitation