Faint, Large-Scale Hα Filaments in the Milky Way

During the initial data reduction of the Wisconsin Hα Mapper (WHAM) Hα Sky Survey, we have discovered several very long ( ~30°-80°) filaments superposed on the diffuse Hα 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 ≈ 0.5-1.5 R) structures are presented. The first is an 80° long, 2° wide arch that extends nearly perpendicular to the Galactic plane at ℓ=225° and attains a maximum latitude of +51° near ℓ=240° before reaching the southern boundary of our survey map at ℓ=270°, b =+42°. The vertical portion of this feature between b =+10° and +25° is associated with a single radial velocity component centered at v LSR =+16 km s -1 with a FWHM of 27 km s -1 . A decrease in the velocity is observed from b =+33° through +48° as the feature arches toward higher Galactic longitudes. At this end, the emission component is centered near v LSR =-20 km s -1 . Where this feature appears to meet the Galactic plane near ℓ=225°, it is directly above the H II region surrounding CMa R1/OB1. A second filament consists of a ~25°-30° long arc spanning ℓ=210°-240° at b =+30° to +40°. The radial velocity of this feature increases systematically from 0 km s -1 at ℓ=215°, b =+38° to +18 km s -1 at ℓ=236°, b =+28°. Both features have rather constant intensities along their entire lengths, ranging from 0.5 to 1.5 R (EM = 1-3 cm -6 pc) with no obvious trends.