A WATER-VAPOR GIGA-MASER IN THE ACTIVE GALAXY TXFS2226-184

ACTIVE galactic nuclei are thought to be powered by gas falling into a massive black hole; the different types of active galaxy mag arise be cause we view them through a thick torus of molecular gas at varying a ngles of inclination(1). One way to determine whether the black hole i s surrounded by a torus, which would obscure the accretion disk around the black hole along certain lines of sight, is to search for water m asers, as these exist only in regions with plentiful molecular gas. Si nce the first detection(2) of an extragalactic water maser in 1979, th ey have come to be associated primarily with active galaxies, and have even been used to probe the mass of the central engine(3). Here we re port the detection of a water giga-maser in the radio galaxy TXFS2226- 184. The strength of the emission supports a recently proposed theory of maser pumping(4) that allows for even more powerful masers, which m ight be detectable at cosmological distances. Water masers may accordi ngly provide a may to determine distances to galaxies outside the usua l distance ladder, providing an independent calibration of the Hubble constant(3,5).