A new catalogue of HI supershells in the outer part of the Milky Way

L. A. SUAD; CAIAFA, C. F; ARNAL, E. M.; CICHOWOLSKI, S.

Florianópolis

Conferencia; LARIM 2013; 2013

IAU

The interstellar medium, when viewed in the neutral hydrogen (HI) line emission, reveals a complex structure that is evidenced by the presence of inhomogeneities like shells, supershells, arcs, cavities and loops. In particular HI supershells are minima in the galactic HI emission distribution surrounded completely, or partially, by walls of enhanced HI emission. Supershells are believed to be created, most likely, by the action of the powerful stellar winds of massive stars and their subsequent explosion as supernova. Nevertheless, other mechanisms like gamma-ray bursts or high velocity cloud infall have been proposed for those shells wich have larger linear sizes.In this work we present a new catalogue of HI supershell candidates ithe outer part of the Galaxy. Several catalogues of shells and/or supershells have been made using either a visual inspection (Heiles 1979, 1984; McClure-Griffiths et. al 2002) or automatic algorithms (Ehlerov & Palous 2005, 2013; Daigle et al. 2007). In this new catalogue we have used a combination of both visual and automatic identification procedures. One of the main features of our algorithm has to do with the fact that it is able to recognize structures even when they are not completely surrounded by walls of HI emission. In this respect it is quite different from the automatic detection algorithms used up to now by other researchers. We used theLeiden-Argentine-Bonn survey (Kalberla et al. 2005). The catalogue covers most of the second and third galactic quadrants, except for a region 30° wide centered at 180° (90° < l < 165° and 195° < l < 270°; -50° < b < 50°). We have considered as supershell candidates those structures satisfying thefollowing criteria: I) a linear diameter larger than 200 pc, II) an angular diameterlarger than 2°, and III) a velocity extent of at least 6.5 km/s. A totalof 575 supershells have been catalogued. Only 55% of them are completelysurrounded by "walls" of enhanced HI emission.In this work we also analyze the main statistical parameters of the supershellcandidates. From the OB associations and open clusters catalogueswe have looked for correlations between these objects and the supershells.Finally, we have compared our catalogue with the ones made by other authors.