A global view on star formation: The GLOSTAR Galactic plane survey VIII. Formaldehyde absorption in Cygnus X

Context. Cygnus X is one of the closest and most active high-mass star-forming regions in our Galaxy, making it one of the best laboratories for studying massive star formation.

Aims. We aim to investigate the properties of molecular gas structures on different linear scales with 4.8 GHz formaldehyde (H2CO) absorption line in Cygnus X.

Methods. As part of the GLOSTAR Galactic plane survey, we performed large scale (7◦×3◦) simultaneous H2CO (11,0–11,1) spectral line and radio continuum imaging observations toward Cygnus X at λ ∼6 cm with the Karl G. Jansky Very Large Array and the Effelsberg-100 m radio telescope. We used auxiliary HI, 13CO (1–0), dust continuum, and dust polarization data for our analysis.

Results. Our Effelsberg observations reveal widespread H2CO (11,0–11,1) absorption with a spatial extent of ≳50 pc in Cygnus X for the first time. On large scales of 4.4 pc, the relative orientation between local velocity gradient and magnetic field tends to be more parallel at H2 column densities of ≳1.8×1022 cm−2. On the smaller scale of 0.17 pc, our VLA+Effelsberg combined data reveal H2CO (11,0–11,1) absorption only toward three bright HII regions. Our observations demonstrate that H2CO (11,0–11,1) is commonly optically thin. Kinematic analysis supports the assertion that molecular clouds generally exhibit supersonic motions on scales of 0.17–4.4 pc. We show a non-negligible contribution of the cosmic microwave background radiation in producing extended absorption features in Cygnus X. Our observations suggest that H2CO (11,0 − 11,1) can trace molecular gas with H2 column densities of ≳ 5 × 1021 cm−2 (i.e., AV ≳ 5). The ortho-H2CO fractional abundance with respect to H2 has a mean value of 7.0×10−10. A comparison of velocity dispersions on different linear scales suggests that the dominant −3 km s−1 velocity component in the prominent DR21 region has nearly identical velocity dispersions on scales of 0.17–4.4 pc, which deviates from the expected behavior of classic turbulence.