2016A&A...585A..44M

The central kiloparsec region of the Andromeda galaxy is relatively gas poor, while the interstellar medium appears to be concentrated in a ring-like structure at about 10kpc radius. The central gas depletion has been attributed to a possible head-on collision 200Myr ago, supported by the existence of an offset inner ring of warm dust. We present new IRAM 30m radio telescope observations of the molecular gas in the central region, and the detection of CO and its isotopes ¹³CO(2-1) and C¹⁸O(2-1), together with the dense gas tracers, HCN(1-0) and HCO+(1-0). A systematic study of the observed peak temperatures with non-local thermal equilibrium simulations shows that the detected lines trace dense regions with n_H2 in the range 2.5x10⁴-5.6x10⁵cm^–3, while the gas is very clumpy with a beam filling factor of 0.5-2x10^–2. This is compatible with the dust mass derived from the far-infrared emission, assuming a dust-to-gas mass ratio of 0.01 with a typical clump size of 2pc. We also show that the gas is optically thin in all lines except for ¹²CO(1-0) and ¹²CO(2-1), CO lines are close to their thermal equilibrium condition at 17-20K, the molecular hydrogen density is larger than critical, and HCN and HCO+ lines have a subthermal excitation temperature of 9K with a density smaller than critical. The average ¹²CO/¹³CO line ratio is high (∼21), and close to the ¹²CO/C¹⁸O ratio (∼30) that was measured in the north-western region and estimated in the south-east stacking. The fact that the optically thin ¹³CO and C¹⁸O lines have comparable intensities means that the secondary element ¹³C is depleted with respect to the primary ¹²C, as is expected just after a recent star formation. This suggests that there has been a recent starburst in the central region, supporting the head-on collision scenario.