Molecular Gas in SAURON Early-Type Galaxies: Detection of 13CO and HCN
  Emission
  In a pilot project to study the relationship between star formation and
molecular gas properties in nearby normal early-type galaxies, we used the IRAM
30m telescope to observe the 13CO(J=1-0), 13CO(J=2-1), HCN(J=1-0) and
HCO+(J=1-0) line emission in the four galaxies of the SAURON sample with the
strongest 12CO emission. 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.
  All three galaxies detected in HCN follow the standard HCN-infrared
luminosity and dense gas fraction-star formation efficiency correlations. As
already suggested by 12CO observations, when traced by infrared radiation, star
formation in the three SAURON galaxies thus appears to follow the same physical
laws as in galaxies of different Hubble types. The star formation rate and
fraction of dense molecular gas however do not reach the high values found in
nearby starburst galaxies, but rather resemble those of nearby normal
star-forming galaxies.
