Collimation of the relativistic jet in the quasar 3C 273
  The collimation of relativistic jets launched from the vicinity of
supermassive black holes (SMBHs) at the centers of active galactic nuclei (AGN)
is one of the key questions to understand the nature of AGN jets. However,
little is known about the detailed jet structure for AGN like quasars since
very high angular resolutions are required to resolve these objects. We present
very long baseline interferometry (VLBI) observations of the archetypical
quasar 3C 273 at 86 GHz, performed with the Global Millimeter VLBI Array, for
the first time including the Atacama Large Millimeter/submillimeter Array. Our
observations achieve a beam-formed angular resolution down to $\sim$60 ${\rm
\mu}$as, resolving the innermost part of the jet ever on scales of $\sim 10^5$
Schwarzschild radii. Our observations, including close-in-time High Sensitivity
Array observations of 3C 273 at 15, 22, and 43 GHz, suggest that the inner jet
collimates parabolically, while the outer jet expands conically, similar to
jets from other nearby low luminosity AGN. We discovered the jet collimation
break at $\sim8\times10^{6}$ Schwarzschild radii, providing the first
compelling evidence for structural transition in a quasar jet. The location of
the collimation break for 3C 273 is farther downstream the sphere of
gravitational influence (SGI) from the central SMBH. With the results for other
AGN jets, our results show that the end of the collimation zone in AGN jets is
governed not only by the SGI of the SMBH but also by the more diverse
properties of the central nuclei.
