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 a radio galaxy and most luminous 321-GHz known to date with isotropic luminosity of 1090 $L_{\odot}$. line profile consists broad velocity component FWHM $= 208 \pm 12$ km s$^{-1}$ straddling systemic narrow 44 3$ blueshifted by 160 s$^{-1}$. significantly different from 22-GHz $6_{1,6} 5_{2,3}$ which shows redshifted 193 spatially unresolved synthesized beam $0^{\prime \prime}.68 \times 0^{\prime \prime}.56$ coincides continuum core position within 12 pc. These results indicate amplification through high-temperature ($>1000$ K) dense ($n({\rm H}_2{\rm O}) > 10^4$ cm$^{-3}$) molecular gas front