A complete survey of filaments in Cygnus X

¹ School of Astronomy and Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, PR China
² Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, PR China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 31 October 2025
Accepted: 8 January 2026

Abstract

Context. Filamentary gas structures are widely observed in molecular clouds and are suggested to play a key role in the star formation processes. However, existing observations of molecular filaments are still biased toward case studies and small samples, and a large sample study from a single giant molecular cloud is lacking.

Aims. We aim to carry out a complete census of filaments in Cygnus X and obtain their physical properties, relations with dense cores, magnetic field (B field), and HII regions.

Methods. We extracted 2633 filaments and 6551 cores from the column-density map of Cygnus X, using the most updated getsf algorithm. We then investigated the mass functions for the cores on and separate from the filaments, compared the orientations of the filaments with that of the B field obtained with the Planck data, derived the radial column-density profiles of the filaments close to HII regions, and calculated the distances between the identified young stellar objects and filament spines. We also re-extracted filaments at the resolution of the Planck 353 GHz dust-emission map to study their relationship with the B field.

Results. The filaments in Cygnus X have a typical width of 0.5 pc. More than 93% of high-mass cores (≥20 M_⊙) are located on filaments. The core mass function constructed from the cores on the filaments (onCMF) shows a power law in the high-mass-end (> 10 M_⊙) with a slope of −2.30, whereas the high-mass end of the core mass function (CMF) derived from the cores outside the filaments (outCMF) has a much steeper power-law distribution with a slope of −2.83. The core mass corresponding to the peak of onCMF is much less than the Bonnor-Ebert mass, but that corresponding to the peak of outCMF is well comparable to the Bonnor-Ebert mass. Filaments re-extracted from the column-density map smoothed to an angular resolution identical to the Planck 353 GHz map are mostly perpendicular to the B field, except that those of the lowest column densities are parallel to the B field. The transition from parallel to perpendicular occurs at a column-density equivalent to A_V = 10 mag. Most prominent filamentary gas structures and high-mass cores appear to be preferentially located along the boundaries of HII regions or at the intersections of multiple HII regions. The filaments close to the HII region boundaries show a steeper column-density profile on the side toward the HII region compared to that on the opposite side.

Conclusions. The formation of more massive cores has a stronger dependence on filaments, and the latter may provide a mass reservoir for the former to grow in mass via accretion. The B field plays a crucial role in filament formation, and the type-O mode where filaments form at the tip of converging flows along an oblique MHD shock front may be prevalent in Cygnus X. In this context, expanding HII regions in the complex induces shocks that compress the surrounding gas, creating inhomogeneity and dense clumps, and forming filaments. The global picture of filaments, cores, and star formation in Cygnus X is apparently consistent with the bubble-filament paradigm proposed in the literature.