Faint, Large-scale H-Alpha Filaments in the Milky Way

Abstract

During the initial data reduction of the Wisconsin H-Alpha Mapper (WHAM) H-Alpha Sky Survey, we have discovered several very long (~30--80 deg) filaments superimposed on the diffuse H-Alpha background. These features have no clear correspondence to the other phases of the interstellar medium revealed by 21 cm, X-ray, IR, or radio continuum surveys, and they have no readily identifiable origin or source of ionization. In this letter, the data for two of these faint (I_{H-Alpha} = 0.5--1.5 R) structures are presented. The first is an 80 deg-long, 2 deg-wide arch that extends nearly perpendicular to the Galactic plane at l = 225 deg and attains a maximum latitude of +51 deg near l = 240 deg. Where this feature appears to meet the Galactic plane near l = 225 deg, it is directly above the H II region surrounding CMa R1/OB1. A second filament consists of a ~25--30 deg-long arc spanning l = 210--240 deg at b = +30 deg to +40 deg. Both features have measurable velocity trends with position. However, they have rather constant intensities along their entire lengths, ranging from 0.5--1.5 R (EM = 1--3 cm^{-6} pc) with no obvious trends.