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

Abstract

Worms are defined to be dusty, atomic hydrogen (HI) structures which 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 HI structures which could be resolved worms. Another motivation for the search was to explore the scenario that mushroom-shaped worms like GW123.4-1.5, 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 7 have a significant portion of their structure falling in the CGPS range of l=74 deg to 147 deg, -3.5 deg < b < +5.5 deg. Apart from GW123.4-1.5 we could not confirm that any of these are coherent structures that extend towards 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 HI, 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 6 disk-halo features may exist throughout the Milky Way. We also discuss possible origins of structures at high latitudes and the relationship between mushroom-shaped clouds and old supernova remnants.