Seyfert galaxies come in two main types ( types 1 and 2 ) and the difference is probably due to obscuration of the nucleus by a torus of dense molecular material .
The inner edge of the torus is expected to be ionized by optical and ultraviolet emission from the active nucleus , and will radiate direct thermal emission ( e.g .
NGC 1068 ) and will cause free-free absorption of nuclear radio components viewed through the torus ( e.g .
Mrk 231 , Mrk 348 , NGC 2639 ) .
However , the nuclear radio sources in Seyfert galaxies are weak compared to radio galaxies and quasars , demanding high sensitivity to study these effects .
We have been making sensitive phase referenced VLBI observations at wavelengths between 21 and 2 cm where the free-free turnover is expected , looking for parsec-scale absorption and emission .
We find that free-free absorption is common ( e.g .
in Mrk 348 , Mrk 231 , NGC 2639 , NGC 1068 ) although compact jets are still visible , and the inferred density of the absorber agrees with the absorption columns inferred from X-ray spectra ( Mrk 231 , Mrk 348 , NGC 2639 ) .
We find one-sided parsec-scale jets in Mrk 348 and Mrk 231 , and we measure low jet speeds ( typically \leq 0.1 c ) .
The one-sidedness probably is not due to Doppler boosting , but rather is probably free-free absorption .
Plasma density required to produce the absorption is N _ { \mathrm { e } } \geq 2 \times 10 ^ { 5 } cm ^ { -3 } assuming a path length of 0.1 pc , typical of that expected at the inner edge of the obscuring torus .