First Detection of Millimeter/Submillimeter Extragalactic H2O Maser Emission

We report the first detection of an extragalactic millimeter wavelength H2O maser at 183 GHz toward NGC 3079 using the Submillimeter Array (SMA) and a tentative submillimeter wave detection of the 439 GHz maser toward the same source using the James Clerk Maxwell Telescope (JCMT). These H2O transitions are known to exhibit maser emission in star-forming regions and evolved stars. NGC 3079 is a well-studied nuclear H2O maser source at 22 GHz with a time-variable peak flux density in the range 312 Jy. The 183 GHz H2O maser emission, with a peak flux density of 0.5 Jy (7 detection), also originates from the nuclear region of NGC 3079 and is spatially coincident with the dust continuum peak at 193 GHz (53 mJy integrated). Peak emission at both 183 and 439 GHz occurs in the same range of velocity as that covered by the 22 GHz spectrum. We estimate the gas-to-dust ratio of the nucleus of NGC 3079 to be 150, comparable to the Galactic value of 160. Discovery of maser emission in an active galactic nucleus beyond the long-known 22 GHz transition opens the possibility of future position-resolved radiative transfer modeling of accretion disks and outflows <1 pc from massive black holes.