Discovery of a Thirty-Degree Long Ultraviolet Arc in Ursa Major
  Our view of the interstellar medium of the Milky Way and the universe beyond
is affected by the structure of the local environment in the Solar
neighborhood. Here, we present the discovery of a thirty-degree long arc of
ultraviolet emission with a thickness of only a few arcminutes: the Ursa Major
Arc. It consists of several arclets seen in the near- and far-ultraviolet bands
of the GALEX satellite. A two-degree section of the arc was first detected in
the H{\alpha} optical spectral line in 1997; additional sections were seen in
the optical by the team of amateur astronomers included in this work. This
direction of the sky is known for very low hydrogen column density and dust
extinction; many deep fields for extra-galactic and cosmological investigations
lie in this direction. Diffuse ultraviolet and optical interstellar emission
are often attributed to scattering of light by interstellar dust. The lack of
correlation between the Ursa Major Arc and thermal dust emission observed with
the Planck satellite, however, suggests that other emission mechanisms must be
at play. We discuss the origin of the Ursa Major Arc as the result of an
interstellar shock in the Solar neighborhood.
