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

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.

27 pages, 23 figures, accepted for publication in A&A

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