Letter to the Editor

Discovery of a 30-degree-long ultraviolet arc in Ursa Major

¹ Rudjer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia
e-mail: abracco@irb.hr
² Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, Paris, France
³ University of Wisconsin-Whitewater, Department of Physics, 800 West Main St, Whitewater, WI 53190, USA
⁴ Radboud University, Department of Astrophysics/IMAPP, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁵ IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France
⁶ MDW Sky Survey, New Mexico Skies Observatory, Mayhill, NM 88339, USA

Received: 17 March 2020
Accepted: 3 April 2020

Abstract

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. We present the discovery of a 30-degree-long arc of ultraviolet emission with a thickness of only a few arcminutes: the Ursa Major arc. This 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 Hα 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 extragalactic 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.