A New Water Vapor Megamaser

We report on the detection of a new megamaser, the 3 sub(1, 3)-2 sub(2, 0) H sub(2)O line ( sub(0) = 183.310 GHz) in Arp 220, using the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescope. The line is about 350 km s super(-1) wide with a total luminosity of 62.5 x 10 super(8) K km s super(-1) pc super(2). Although OH megamasers were first discovered in this source, no emission is seen in the 6 sub(1, 6)-5 sub(2, 3) H sub(2)O transition ( sub(0) = 22.235 GHz), a line otherwise detected as a megamaser in about 50 sources to date. This fact puts interesting constraints on the physical conditions of the central region of Arp 220 that are further strengthened by the HCN and HNC J= 3-2 and J = 1-0 luminosities [in the range (1.5-10) x 10 super(8) K km s super(-1) pc super(2)]. A scenario with 610 super(6) star-forming cores similar to those found in Sgr B2 in the central kiloparsec of Arp 220 would be compatible with these data and would explain the lack of 22 GHz H sub(2)O emission. This result opens up the possibility of using the 183 GHz H sub(2)O line as an additional tool to explore the physical conditions in luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs, respectively) and their starburst or active galactic nucleus (AGN) nature, with a potential interest for high angular resolution observations with the Atacama Large Millimeter Array (ALMA).