HCN and HCO+emission in the disk of M 31

We report observations made with the IRAM 30 m radiotelescope in the HCN(1–0) and HCO+(1–0) lines towards a sample of molecular complexes (GMCs) in the disk of the Andromeda galaxy (M 31). The targets were identified bright CO GMCs selected from the IRAM 30 m CO survey with various morphologies and environments. The clouds vary in galactocentric distances from 2.4 to 15.5 kpc. The HCN and HCO+ emission is easily detected in almost all observed positions, with line widths generally similar to the CO ones and there is a good correlation between the two dense gas tracers. The HCO+ emission is slightly stronger than the HCN, in particular towards GMCs with a strong star formation activity. However the HCO+ emission is weaker than the HCN towards a quiescent cloud in the inner part of M 31, which could be due to a lower abundance of HCO+. We derive ratios between 0.008 and 0.03 and ratios between less than 0.003 and 0.04. We study the radial distribution of the dense gas in the disk of M 31. Unlike our Galaxy the HCO+/CO ratio is lower in the center of M 31 than in the arms, which can be explained by both a lower abundance of HCO+ and different conditions of excitation. Furthermore the HCN/CO and HCO+/CO ratios appear to be higher in the inner spiral arm and weaker in the outer arm.