VLBI study of water maser emission in the Seyfert 2 galaxy NGC 5793. I. Imaging blueshifted emission and the parsec-scale jet

We present the first result of VLBI observations of the blueshifted water m aser emission from the type 2 Seyfert galaxy NGC 5793, which we combine wit h new and previous VLBI observations of continuum emission at 1.7, 5.0, 8.4 , 15, and 22 GHz. Maser emission was detected earlier in single-dish observ ations and found to have both red- and blueshifted features relative to the systemic velocity. We could image only the blueshifted emission, which is located 3.6 pc southwest of the 22 GHz continuum peak. The blueshifted emis sion was found to originate in two clusters that are separated by 0.7 mas ( 0.16 pc). No compact continuum emission was found within 3.6 pc of the mase r spot. A compact continuum source showing a marginally inverted spectrum b etween 1.7 and 5.0 GHz was found 4.2 pc southwest of the maser position. Th e spectral turnover might be due to synchrotron self-absorption caused by a shock in the jet owing to collision with dense gas, or it might be due to free-free absorption in an ionized screen, possibly the inner part of a dis k, foreground to the jet. The water maser may be part of a maser disk. If s o, it would be rotating in the opposite sense to the highly inclined galact ic disk observed in CO emission. We estimate a binding mass within 1 pc of the presumed nucleus to be on the order of 10(7) M-.. Alternatively, the ma ser emission could result from the amplification of a radio jet by foregrou nd circumnuclear molecular gas. In this case, the high blueshift of the mas er emission might mean that the masing region is moving outward away from t he molecular gas surrounding an active nucleus.