TOWARD EXTRAGALACTIC CHEMISTRY: DETECTIONS OF N_2H~+ AND SiO IN NEARBY GALAXIES

摘要

Observations of N_2H~+ and SiO in a sample of nine nearby galaxies have been carried out. N_2H~+ was newly detected in two and confirmed in five. Four galaxies were newly detected in SiO, and emission from one other was confirmed. In addition, two galaxies showed evidence of HCO emission, a possible new extra-galactic molecule. With the exception of M82, the abundance of N_2H~+ is fairly uniform in these sources, with [N_2H~+]/[H_2] ～ 6 x 10~(-11), close to concentrations typically found in Galactic clouds. The SiO abundances of 1-4 x 10~(-11), on the other hand, seem more sensitive to the star formation rate, possibly reflecting the differing fractions of molecular gas that are currently forming massive stars. However, the abundance of this species seems low in M82, a starburst galaxy. In addition, the behavior of the molecules, when compared to CS— which traces dense gas without regard to temperature—is not what was naively expected. These observations suggest that while extragalactic chemistry may be similar to that in the Milky Way, there are substantial differences which have yet to be analyzed.