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

The Atacama Large Millimeter/submillimeter Array (ALMA) serendipitously detected H$_2$O $J_{Ka, Kc} = 10_{2,9} - 9_{3,6}$ emission at 321 GHz in NGC 1052. This is the first submillimeter maser detection in a radio galaxy and the most luminous 321-GHz H$_2$O maser known to date with the isotropic luminosity of 1090 $L_{\odot}$. The line profile consists of a broad velocity component with FWHM $= 208 \pm 12$ km s$^{-1}$ straddling the systemic velocity and a narrow component with FWHM $= 44 \pm 3$ 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 ($n({\rm H}_2{\rm O}) > 10^4$ cm$^{-3}$) molecular gas in front of the core