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 $0^{\mathrm{\prime }\mathrm{\prime }}.68\times 0^{\mathrm{\prime }\mathrm{\prime }}.56$ 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 ($n({\mathrm{H}}_2\mathrm{O})>{10}^4$ cm${}^{-3}$) molecular gas in front of the core