Evidence for nuclear radio jet and its structure down to <100
  Schwarzschild radii in the center of the Sombrero galaxy (M 104, NGC 4594)
  Abridged: We investigated the detailed radio structure of the nucleus of the
Sombrero galaxy using high-resolution, quasi-simultaneous, multi-frequency,
phase-referencing VLBA observations. We obtained the VLBI images toward this
nucleus, with unprecedented sensitivities and resolutions, at the seven
frequencies between 1.4 and 43 GHz, where those at 15, 24 and 43 GHz are the
first clear VLBI detections. At 43 GHz, the nuclear structure was imaged on a
linear scale under 100 Schwarzschild radii. For the first time, we have
discovered the presence of the extended structure in this nucleus, which is
directing from the radio core in two sides toward northwest/southeast
directions. The nuclear structure shows a clear spatial gradient on the radio
spectra, which is similar to that commonly seen in more luminous AGN with
powerful relativistic jets. Moreover, the radio core shows a
frequency-dependent size with an elongated shape, and the position of the core
also tends to be frequency dependent. A set of these new findings provide
evidence that the central engine of the Sombrero is powering radio jets. Based
on the observed brightness ratio of jet-to-counter jet, core position shift and
its comparison with a theoretical model, we constrained the following
fundamental physical parameters for the M 104 jets: (1) the northern side is
the approaching jet, whereas the southern side is receding: (2) the inclination
angle 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 a
speed less than ~0.2c. The derived pole-on nature of the M 104 jet is in
accordance 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 intrinsically
weak, if the plane of the presumed circumnuclear torus is perpendicular to the
axis of the radio jets.
