A Search for Narrow Vertical Structures in the Canadian Galactic Plane Survey

Abstract

Worms are defined to be dusty, atomic hydrogen (H I) structures that are observed in low-resolution data to rise perpendicular to the Galactic plane. Data from the 1' resolution Canadian Galactic Plane Survey (CGPS) were systematically searched for narrow vertical H I structures that could be resolved worms. Another motivation for the search was to explore the scenario in which mushroom-shaped worms such as GW 123.4-1.5, studied by English and collaborators, could be generated by a single supernova. However, no other vertical structures of mushroom-shape morphology were found. We also examined objects previously classified as worm candidates by Koo and collaborators; only seven have a significant portion of their structure falling in the CGPS range of l = 74°–147°, -35 < b < +55. Apart from GW 123.4-1.5, we could not confirm that any of these are coherent structures that extend toward the Milky Way's halo. However, a list of 10 narrow vertical structures found in our search is furnished; one structure is 500 pc tall, thus extending from the Galactic plane into the halo. We provide details about these narrow vertical structures, including comparisons between H I, radio continuum, IR, and CO observations. Our search was conducted by visual inspection, and we describe the limitations of this approach since it indicates that only six disk-halo features may exist throughout the Milky Way. We also discuss the possible origins of structures at high latitudes and the relationship between mushroom-shaped clouds and old supernova remnants.