Faint, large-scale Hα filaments in the Milky Way

During the initial data reduction of the Wisconsin Her Mapper (WHAM) H(alpha)Sky Survey, we have discovered several very long (similar to 30 degrees-80 degrees) filaments superposed 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 approximate to 0.5-1.5 R) structures are presented. The first is an 80 degrees long, 2 degrees wide arch that extends nearly perpendicular to the Galactic plane at l = 225 degrees and attains a maximum latitude of +51 degrees near l = 240 degrees before reaching the southern boundary of our survey map at l = 270 degrees, b = +42 degrees. The vertical portion of this feature between b = +10 degrees and +25 degrees 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 degrees through +48 degrees 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 degrees, it is directly above the H II region surrounding CMa R1/OB1. A second filament consists of a similar to 25 degrees-30 degrees long are spanning l = 210 degrees-240 degrees at b = +30 degrees to +40 degrees. The radial velocity of this feature increases systematically from 0 km s(-1) at l = 215 degrees, b = +38 degrees to +18 km s(-1) at l = 236 degrees, b = +28 degrees. 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.