paper

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

arXiv:2308.00993 · doi:10.1051/0004-6361/202346102

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 $I^» \sim$ 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 $≳$ 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 $≳$ 1.8 $\times 1 0^{22}$ ~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 $≳ 5 \times 1 0^{21}$ cm $^{- 2}$ . The ortho-H $_{2}$ CO fractional abundance with respect to H $_{2}$ has a mean value of 7.0 $\times 1 0^{- 10}$ . 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