Structure and fragmentation of a high line-mass filament: Nessie

Abstract

Context.An increasing number of hundred-parsec-scale, high line-mass filaments are being detected in the Galaxy. Their evolutionary path, including fragmentation towards star formation, is virtually unknown. Aims. We characterize the fragmentation within the hundred-parsec-scale, high line-mass Nessie filament, covering size-scales in the range ∼0.1-100 pc. We also connect the small-scale fragments to the star-forming potential of the cloud. Methods. We combine near-infrared data from the VISTA Variables in the Via Lactea (VVV) survey with mid-infrared Spitzer/GLIMPSE data to derive a high-resolution dust extinction map for Nessie. We then apply a wavelet decomposition technique on the map to analyze the fragmentation characteristics of the cloud. The characteristics are then compared with predictions from gravitational fragmentation models. We compare the detected objects to those identified at a resolution approximately ten times lower from ATLASGAL 870 µm dust emission data. Results. We present a high-resolution extinction map of Nessie (200 full-width-half-max, FWHM, corresponding to 0.03 pc). We estimate the mean line mass of Nessie to be ∼627 M pc−1 and the distance to be ∼3.5 kpc. We find that Nessie shows fragmentation at multiple size scales. The median nearest-neighbor separations of the fragments at all scales are within a factor of two of the Jeans' length at that scale. However, the relationship between the mean densities of the fragments and their separations is significantly shallower than expected for Jeans' fragmentation. The relationship is similar to the one predicted for a filament that exhibits a Larson-like scaling between size-scale and velocity dispersion; such a scaling may result from turbulent support. Based on the number of young stellar objects (YSOs) in the cloud, we estimate that the star formation rate (SFR) of Nessie is ∼371 M Myr−1; similar values result if using the number of dense cores, or the amount of dense gas, as the proxy of star formation. The star formation efficiency is 0.017. These numbers indicate that by its star-forming content, Nessie is comparable to the Solar neighborhood giant molecular clouds like Orion A.