We present the first VLBI detection of HCN molecular absorption in the nearby active galactic nucleus NGC 1052 .
Utilizing the 1 mas resolution achieved by the Korean VLBI Network , we have spatially resolved the HCN absorption against a double-sided nuclear jet structure .
Two velocity features of HCN absorption are detected significantly at the radial velocity of 1656 and 1719 km s ^ { -1 } , redshifted by 149 and 212 km s ^ { -1 } with respect to the systemic velocity of the galaxy .
The column density of the HCN molecule is estimated to be 10 ^ { 15 } — 10 ^ { 16 } cm ^ { -2 } , assuming an excitation temperature of 100—230 K. The absorption features show high optical depth localized on the receding jet side , where the free-free absorption occurred due to the circumnuclear torus .
The size of the foreground absorbing molecular gas is estimated to be on approximately one-parsec scales , which agrees well with the approximate size of the circumnuclear torus .
HCN absorbing gas is likely to be several clumps smaller than 0.1 pc inside the circumnuclear torus .
The redshifted velocities of the HCN absorption features imply that HCN absorbing gas traces ongoing infall motion inside the circumnuclear torus onto the central engine .