Molecular gas in SAURON early-type galaxies: detection of 13CO and HCN emission

International audienceIn a pilot project to study the relationship between star formation and molecular gas properties in nearby normal early-type galaxies, we have obtained observations of dense molecular gas tracers in the four galaxies of the SAURON sample with the strongest 12CO emission. We used the Institut de Radio Astronomie Millimetrique (IRAM) 30-m telescope 3- and 1-mm heterodyne receivers to observe 13CO(J = 1-0), 13CO(J = 2-1), HCN(J = 1-0) and HCO+(J = 1-0). We report the detection of 13CO emission in all four SAURON sources and HCN emission in three sources, while no HCO+ emission was found to our detection limits in any of the four galaxies. We find that the 13CO/12CO ratios of three SAURON galaxies are somewhat higher than those in galaxies of different Hubble types. The HCN/12CO and HCN/13CO ratios of all four SAURON galaxies resemble those of nearby Seyfert and dwarf galaxies with normal star formation rates, rather than those of starburst galaxies. The HCN/HCO+ ratio is found to be relatively high (i.e. >1) in the three SAURON galaxies with detected HCN emission, mimicking the behaviour in other star-forming galaxies but being higher than in starburst galaxies. When compared to most galaxies, it thus appears that 13CO is enhanced (relative to 12CO) in three out of four SAURON galaxies and HCO+ is weak (relative to HCN) in three out of three galaxies