Bimodal orientation distribution and head-tail asymmetry of a sample of filamentary molecular clouds

Abstract

ABSTRACT The morphology of molecular clouds is crucial for understanding their origin and evolution. In this work, we investigate the morphology of the filamentary molecular clouds (filaments for short) using a portion of the 12CO(J = 1 − 0) data from the Milky Way Imaging Scroll Painting project. The data cover an area spanning 104.75° < l < 150.25°, |b| < 5.25° in Galactic coordinates, with VLSR ranging from −95 to 25 kms−1. Our primary focus is on the orientation and morphological asymmetry of the filaments. To achieve this, we apply several criteria on the data to create a sample of filaments with well-defined straight shape, and we use elliptical fitting to obtain the orientation of each filament, with an estimated error of ∼1.6° for the orientation. We find that the filament orientation with respect to the Galactic plane exhibits a bimodal distribution, a double-Gaussian fitting of which has two centres located at −38.1° and 42.0°, with 1σ of the two Gaussian functions being 35.4° and 27.4°. We do not find significant correlation between the orientation and other parameters, including the Galactic coordinates, radial velocity, velocity width, and physical scale. A considerable fraction of filaments (≳40 per cent) display head-tail asymmetry, which suggests that mass concentration tends to occur at one end of the filaments.