page_title: Sagittarius A
section_title: Sagittarius A West
hierarchical_section_title: Sagittarius A / Sagittarius A West
caption_attribution_description: The Sagittarius A West complex of ionized gas here observed in the Bracket gamma line of ionized Hydrogen) has the apparent shape of a three-arm spiral. The image above was produced using data obtained with the BEAR spectro-imager on the Canada-France-Hawaii Telescope. The color shades represent different radial velocities, explicited on the spectrum in the upper-left inset, which corresponds to the selected pixel.
context_page_description: Sagittarius A or Sgr A is a complex radio source at the center of the Milky Way which contains a supermassive black hole. It is located in the constellation Sagittarius, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the spiral arms of the Milky Way.
  It consists of three components, the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A*. These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West.
context_section_description: Sgr A West has the appearance of a three-arm spiral, from the point of view of the Earth.  For this reason, it is also known as the "Minispiral". This appearance and nickname are misleading, though: the three-dimensional structure of the Minispiral is not that of a spiral. It is made of several dust and gas clouds, which orbit and fall onto Sagittarius A* at velocities as high as 1,000 kilometers per second. The surface layer of these clouds is ionized. The source of ionisation is the population of massive stars (more than one hundred OB stars have been identified so far) that also occupy the central parsec.
  Sgr A West is surrounded by a massive, clumpy torus of cooler molecular gas, the Circumnuclear Disk (CND). The nature and kinematics of the Northern Arm cloud of Sgr A West suggest that it once was a clump in the CND, which fell due to some perturbation, perhaps the supernova explosion responsible for Sgr A East. The Northern Arm appears as a very bright North—South ridge of emission, but it extends far to the East and can be detected as a dim extended source.
  The Western Arc (outside the field of view of the image shown in the right) is interpreted as the ionized inner surface of the CND. The Eastern Arm and the Bar seem to be two additional large clouds similar to the Northern Arm, although they do not share the same orbital plane. They have been estimated to amount for about 20 solar masses each.
  On top of these large scale structures (of the order of a few light-years in size), many smaller cloudlets and holes inside the large clouds can be seen. The most prominent of these perturbations is the Minicavity which is interpreted as a bubble blown inside the Northern Arm by the stellar wind of a massive star, which is not clearly identified.
caption: Surface brightness and velocity field of the inner part of Sagittarius A West
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