Abstract:
The Very Long Baseline Array has been used to image the milliarcsecond-scale 15-GHz radio emission from the Seyfert galaxies Mrk~231 and Mrk~348 at two epochs separated by about 1.7~yr. Both galaxies contain parsec-scale double radio sources, whose components have brightness temperatures of $10^9$--$10^{11}$~K, indicating that they are generated by synchrotron emission and probably associated with radio jets. The nuclear components of both galaxies are identified by strong variability between epochs, implying that the double sources are apparently one-sided jets. In Mrk~348, the relative component speed is $0.074\pm 0.035c$ at a separation of 0.5~pc, while the apparent speed in Mrk~231 is $0.139\pm 0.052c$ at a separation of 1.1~pc (using $H_0=65$~km~s$^{-1}$~Mpc$^{-1}$). The lack of observed counterjets is probably due to free-free absorption by torus gas having ionized densities $n_e > 2\times 10^5$~cm$^{-3}$ within a parsec of the nuclei. This gas density is consistent with X-ray absorption measurements for both galaxies, and also is similar to that inferred in H$_2$O megamaser galaxies. Except for radio power, the overall properties of the Seyfert radio sources are similar to compact symmetric objects, whose one-sided jets also could be due to free-free absorption by gas having $n_e\sim 5\times 10^4$~cm$^{-3}$ on scales of 5--10~pc.
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Questions: Heino Falcke, hfalcke@mpifr-bonn.mpg.de