Molecular Abundances in Galaxies

Abstract

Two topics are discussed here. The first is an observational study on molecular abundance in nearby (< 10 Mpc) gas‐rich galaxies based on our ammonia survey and on data in literature. As a result, a systematically peculiar molecular abundance was found in a famous starburst galaxy M 82 regarding the formation mechanisms of molecules. We discussed possible reasons for this peculiarity. We think that formation of molecules on dust grain is not effective, and/or that evaporation of molecules from dust to gas‐phase is not effective in M 82. The second topic is our recent trial to detect ammonia toward a more distant galaxy. We observed ammonia toward a prototypical ultraluminous infrared galaxy, Arp 220 (∼ 77 Mpc), with the Nobeyama 45‐m radio telescope. Extremely wide absorption lines were detected at the (1,1) and (3,3) transitions. The maximum total velocity width was ∼ 1800 km s−1. Such wide molecular absorption lines were detected for the first time in galaxies. The absorption lines are formed only by foreground ammonia of the central compact continuum emission (∼ 1″=370 pc). The present results clearly indicate the existence of extremely high‐velocity gas in the central compact region. A possible origin of such motion is rapidly rotating gas, suggesting the existence of an active galactic nucleus, or outflowing or inflowing gas. The obtained column density 1.8 × 1017 cm−2 is the largest among galaxies where ammonia is already detected.