Filamentary structures of ionized gas in Cygnus X^★

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903 USA
e-mail: kemig@nrao.edu
² Leiden Observatory, Leiden University, PO Box 9513 Leiden, 2300 RA The Netherlands
³ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
⁴ RAL Space, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX UK
⁵ Green Bank Observatory, PO Box 2 Green Bank, WV 24944 USA
⁶ Department of Astronomy, University of Maryland, College Park, MD 20742 USA
⁷ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁸ Institut Universitaire de France (IUF), Paris, France
⁹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL The Netherlands
¹⁰ Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, Dwingeloo, 7990 AA The Netherlands
¹¹ SURF/SURFsara, Science Park 140, Amsterdam, 1098 XG The Netherlands
¹² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn, 53121 Germany

Received: 5 November 2021
Accepted: 12 May 2022

Abstract

Context. Ionized gas probes the influence of massive stars on their environment. The Cygnus X region (d ~ 1.5 kpc) is one of the most massive star-forming complexes in our Galaxy, within which the Cyg OB2 association (age of 3–5 Myr and stellar mass 2 × 10⁴ M_⊙) has a dominant influence.

Aims. We observe the Cygnus X region at 148 MHz using the Low Frequency Array (LOFAR) and take short-spacing information into account during image deconvolution into account. Together with data from the Canadian Galactic Plane Survey, we investigate the morphology, distribution, and physical conditions of low-density ionized gas in a 4° × 4° (~100 pc × 100 pc) region at a resolution of 2′ (0.9 pc).

Methods. The Galactic radio emission in the region analyzed is almost entirely thermal (free-free) at 148 MHz, with emission measures (EM) of 10³ < EM [pc cm⁻⁶] < 10⁶. As filamentary structure is a prominent feature of the emission, we use DisPerSE and Fil ChaP to identify filamentary ridges and characterize their radial (EM) profiles.

Results. The distribution of radial profiles has a characteristic width of 4.3 pc and a power-law distribution (β = −1.8 ± 0.1) in peak EM down to our completeness limit of 4200 pc cm⁻⁶. The electron densities of the filamentary structure range between 10 ≲ n_e [cm⁻³] ≲ 400 with a median value of 35 cm⁻³, remarkably similar to [N II] surveys of ionized gas.

Conclusions. Cyg OB2 may ionize at most two-thirds of the total ionized gas and the ionized gas in filaments. More than half of the filamentary structures are likely photoevaporating surfaces flowing into a surrounding diffuse (~5 cm⁻³) medium. However, this is likely not the case for all ionized gas ridges. A characteristic width in the distribution of ionized gas indicates that the stellar winds of Cyg OB2 create a fraction of the ionized filaments through swept-up ionized gas or dissipated turbulence.