Massive filamentary clouds and their role in star formation

Abstract

Filamentary structures are ubiquitous in the interstellar medium of the Milky Way. They are observed at a large range of size-scales from ~ 1 pc within star-forming and quiescent molecular clouds to ~ 100 pc representing molecular clouds themselves. Therefore, filamentary structures play an important role in the early phases of star formation. Filaments in nearby (< 500 pc) molecular clouds were found to be thermally stable against gravitational collapse, and therefore, the fragmentation time-scales are much shorter than the collapse-, which allows the filament to fragment into star-forming clumps. However, the evolutionary scenario of filaments with masses significantly above the thermally critical value is virtually unknown. These massive filaments are found throughout the entire Galaxy, where they channel the gas from large, Galactic to small, star-forming scales.<br /> In this thesis we use various survey observations to investigate the evolution of massive filamentary molecular clouds. Here we first concentrate on the hundred-parsec-scale, massive Nessie filament. We aim to characterize the fragmentation of the cloud covering size-scales in the range ~ 0.1 - 100 pc and to connect the smallest scales to its star-forming potential. Further, we will analyze the kinematics of Nessie to reveal its continuity in position-position-velocity (ppv) space, which is essential to confirm Nessie as a single physical object, and to study the gas motions along and across the filament. For the fragmentation characterization we combine near- and mid-infrared data to derive a high-resolution dust extinction map, from which we extract the cloud fragments at different size-scales. The characteristics are then compared with predictions from gravitational fragmentation models. We find that the median nearest-neighbor separations of the fragments at all scales are similar to the ones predicted for a filament that exhibits a Larson-like scaling between size-scale and velocity dispersion. The kinematic information of Nessie is provided by the 13CO(2-1) and C18O(2-1) molecular lines of the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic Inter Stellar Medium) survey. Although Nessie shows several morphological differences along the filament it is observed as a continuous structure in ppv space.<br /> In the second part of the thesis we analyze the kinematics of 283 filamentary molecular cloud candidates in the Galactic Plane, that were previously identified in the ATLASGAL (APEX Telescope Large Area Survey of the Galaxy) dust continuum data. The ccol and cool data of the SEDIGISM survey allows us to analyze the kinematics of these targets and to determine their physical properties. To do so, we developed an automated algorithm to identify all velocity components along the line- of-sight correlated with the ATLASGAL dust emission, and derive size, mass, and kinematic properties for all velocity components. We find two-third of the filament candidates are coherent structures in ppv space. Also, we find a correlation between the observed mass per unit length and the velocity dispersion of the filament of m ~ sigma^2_v. We show that this relation can be explained by a virial balance between self-gravity and pressure. Another possible explanation could be radial collapse of the filament, but the observation can exclude infall motions close to the free-fall velocity.