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Abridged: We investigated the detailed radio structure of the nucleus of theSombrero galaxy using high-resolution, quasi-simultaneous, multi-frequency,phase-referencing VLBA observations. We obtained the VLBI images toward thisnucleus, with unprecedented sensitivities and resolutions, at the sevenfrequencies between 1.4 and 43 GHz, where those at 15, 24 and 43 GHz are thefirst clear VLBI detections. At 43 GHz, the nuclear structure was imaged on alinear scale under 100 Schwarzschild radii. For the first time, we havediscovered the presence of the extended structure in this nucleus, which isdirecting from the radio core in two sides toward northwest/southeastdirections. The nuclear structure shows a clear spatial gradient on the radiospectra, which is similar to that commonly seen in more luminous AGN withpowerful relativistic jets. Moreover, the radio core shows afrequency-dependent size with an elongated shape, and the position of the corealso tends to be frequency dependent. A set of these new findings provideevidence that the central engine of the Sombrero is powering radio jets. Basedon the observed brightness ratio of jet-to-counter jet, core position shift andits comparison with a theoretical model, we constrained the followingfundamental physical parameters for the M 104 jets: (1) the northern side isthe approaching jet, whereas the southern side is receding: (2) the inclinationangle of the jet is relatively close to our line-of-sight, probably less than~25degrees: (3) the jet intrinsic velocity is highly sub-relativistic at aspeed less than ~0.2c. The derived pole-on nature of the M 104 jet is inaccordance with the previous argument that M 104 contains a true type II AGN,i.e., the broad line region of this nucleus is actually absent or intrinsicallyweak, if the plane of the presumed circumnuclear torus is perpendicular to theaxis of the radio jets.

Evidence for nuclear radio jet and its structure down to <100 Schwarzschild radii in the center of the Sombrero galaxy (M 104, NGC 4594)