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

TL;DR

This study uses the GLOSTAR survey to analyze formaldehyde absorption in Cygnus X, revealing widespread absorption, magnetic field alignment, and insights into molecular cloud dynamics and structure at multiple scales.

Contribution

First large-scale detection of H2CO absorption across Cygnus X, linking molecular gas properties with magnetic fields and turbulence at various spatial scales.

Findings

Widespread H2CO absorption over >50 pc in Cygnus X.

Magnetic field alignment with velocity gradients at high column densities.

Molecular clouds exhibit supersonic motions and complex turbulence behavior.

Abstract

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. As part of the GLOSTAR Galactic plane survey, we performed large scale simultaneous H$_{2}$CO (1$_{1,0}$-1$_{1,1}$) spectral line and radio continuum imaging observations toward Cygnus X at $\lambda\sim$6 cm with the Karl G. Jansky Very Large Array and the Effelsberg-100 m radio telescope. Our Effelsberg observations reveal widespread H$_{2}$CO (1$_{1,0}$-1$_{1,1}$) absorption with a spatial extent of $\gtrsim$50 pc in Cygnus~X for the first time. On large scales of 4.4 pc, the relative orientation between local velocity gradient and magnetic field tends to be more parallel at H$_{2}$ column densities of $\gtrsim$1.8$\times 10^{22}$~cm$^{-2}$. On the smaller scale of 0.17 pc, our VLA+Effelsberg combined data reveal H$_{2}$CO absorption only toward three bright H{\scriptsize II} regions. Our observations demonstrate that H$_{2}$CO (1$_{1,0}$-1$_{1,1}$) is commonly optically thin. Kinematic analysis supports the assertion that molecular clouds generally exhibit supersonic motions on scales of 0.17-4.4 pc. We show a non-negligible contribution of the cosmic microwave background radiation in producing extended absorption features in Cygnus X. Our observations suggest that H$_{2}$CO ($1_{1,0}-1_{1,1}$) can trace molecular gas with H$_{2}$ column densities of $\gtrsim 5 \times 10^{21}$ cm$^{-2}$. The ortho-H$_{2}$CO fractional abundance with respect to H$_{2}$ has a mean value of 7.0$\times 10^{-10}$. 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.