ALMA detection of 321 GHz water maser emission in the radio galaxy NGC 1052

The Atacama Large Millimeter/submillimeter Array (ALMA) serendipitously detected H₂O $J_{K_{\rm a}, K_{\rm c}} = 10_{2,9}$-9_{3, 6} emission at 321 GHz in NGC 1052. This is the first submillimeter maser detection in a radio galaxy and the most luminous 321 GHz H₂O maser known to-date with the isotropic luminosity of $1090\, L_{\odot }$. The line profile consists of a broad velocity component with FWHM = 208 ± 12 km s^-1 straddling the systemic velocity and a narrow component with FWHM = 44 ± 3 km s^-1 blueshifted by 160 km s^-1. The profile is significantly different from the known 22 GHz 6_{1, 6}-5_{2, 3} maser which shows a broad profile redshifted by 193 km s^-1. The submillimeter maser is spatially unresolved with a synthesized beam of ${0{^{\prime \prime}_{.}}68} \times {0{^{\prime \prime}_{.}}56}$ and coincides with the continuum core position within 12 pc. These results indicate amplification of the continuum emission through high-temperature (>1000 K) and dense [n(H₂O) > 10⁴ cm^-3] molecular gas in front of the core.