Extremely High-Velocity Gas in the Galaxy Arp 220, Revealed with Ammonia Absorption Lines

Abstract

Abstract We observed ammonia $(J, K) = (1, 1), (2, 2), (3, 3)$, and $(4, 4)$ transitions at a wavelength of 1.3 cm toward a prototypical ultraluminous infrared galaxy, Arp 220, with the Nobeyama 45-m radio telescope. We detected extremely wide absorption lines at the $(1, 1)$ and $(3, 3)$ transitions. The maximum total velocity width was $\sim 1800 \,\mathrm{km} \,\mathrm{s}^{-1}$. Such wide molecular absorption lines were detected for the first time in galaxies. The absorption lines are formed by ammonia only in front of the central compact continuum emission ($\sim {1{}^{\mathrm {\prime \prime }}} = 370 \,\mathrm{pc}$). The present results clearly indicate the existence of extremely high-velocity motion in the central compact region of Arp 220. A possible origin of such motion is rapidly rotating gas, suggesting the existence of an active galactic nucleus, or outflowing or inflowing gas.