Giant molecular filaments in the Milky Way^⋆

Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: ragan@mpia.de

Received: 10 January 2014
Accepted: 20 June 2014

Abstract

Throughout the Milky Way, molecular clouds typically appear filamentary, and mounting evidence indicates that this morphology plays an important role in star formation. What is not known is to what extent the dense filaments most closely associated with star formation are connected to the surrounding diffuse clouds up to arbitrarily large scales. How are these cradles of star formation linked to the Milky Way’s spiral structure? Using archival Galactic plane survey data, we have used multiple datasets in search of large-scale, velocity-coherent filaments in the Galactic plane. In this paper, we present our methods employed to identify coherent filamentary structures first in extinction and confirmed using Galactic Ring Survey data. We present a sample of seven giant molecular filaments (GMFs) that have lengths on the order of ~100 pc, total masses of 10⁴–10⁵ M_⊙, and exhibit velocity coherence over their full length. The GMFs we study appear to be inter-arm clouds and may be the Milky Way analogs to spurs observed in nearby spiral galaxies. We find that between 2 and 12% of the total mass (above ~10²⁰ cm^-2) is “dense” (above 10²² cm^-2), where filaments near spiral arms in the Galactic midplane tend to have higher dense gas mass fractions than those further from the arms.