Spatially Resolved HCN Absorption Features in the Circumnuclear Region
  of NGC 1052
  We present the first VLBI detection of HCN molecular absorption in the nearby
active galactic nucleus NGC 1052. Utilizing the 1 milliarcsecond 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, redshifted by 149 and 212 km/s 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 the 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 parsec 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.
