2024MNRAS.528.3668H

The results of high-resolution spectral-line observations of dense molecular gas are presented towards the nuclear region of the type 2 Seyfert galaxy NGC 1068. MERLIN observations of the 22 GHz H₂O maser were made for imaging the known off-nuclear maser emission at radio jet component located about 0.3 arcsec north-east of the radio nucleus in the galaxy. High angular resolution ALMA observations have spatially resolved the molecular gas emissions of HCN and HCO⁺ in this region. The off-nuclear maser spots are found to nearly overlap with a ring-like molecular gas structure and are tracing an evolving shock-like structure, which appears to be energized by interaction between the radio jet and circumnuclear medium. The scenario of the dynamic jet-ISM interaction is further supported by a systematic shift of the centroid velocities of the off-nuclear maser features over a period of 35 yr. The enhanced integrated flux ratios of the HCN to HCO⁺ line emission features at component C suggest a kinetic temperature T_k ≳ 300 K and an H₂ density of ≳ 10⁶ cm^–3, which are conditions where water masers may be formed. The diagnostics of the masering action in this jet-ISM interaction region is exemplary for galaxies hosting off-nuclear H₂O maser emission.