Filamentary structures of ionized gas in Cygnus X

Context. Ionized gas probes the influence of massive stars on their environment. The Cygnus X region (d similar to 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 x 10(4) M-circle dot) 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 degrees x 4 degrees (similar to 100 pc x 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 10(3) < EM [pc cm(-6)] < 10(6). As filamentary structure is a prominent feature of the emission, we use DisPerSE and FilChaP 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 (beta = -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 less than or similar to n(e) [cm(-3)] less than or similar to 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 (similar to 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.