A global view on star formation: The GLOSTAR Galactic plane survey

VIII. Formaldehyde absorption in Cygnus X^★

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: ygong@mpifr-bonn.mpg.de
² Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51 y 216, 72000 Puebla, Mexico
³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁴ National Radio Astronomy Observatory, 1003 Lopezville RD, Socorro, NM 87801, USA
⁵ Department of Astronomy, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
⁶ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁸ IRAM, 300 rue de la piscine, 38406 Saint-Martin-d’Hères, France
⁹ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK
¹⁰ National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100101, PR China
¹¹ Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100101, PR China
¹² Department of Earth and Space Science, Indian Institute for Space Science and Technology, Trivandrum 695547, India
¹³ German Aerospace Center, Scientific Information, 51147 Cologne, Germany
¹⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁵ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France

Received: 7 February 2023
Accepted: 31 July 2023

Abstract

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 the 4.8 GHz formaldehyde (H₂CO) absorption line in Cygnus X.

Methods. As part of the GLOSTAR Galactic plane survey, we performed large-scale (7º×3º) simultaneous H₂CO (1_1,0–1_1,1) 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 (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 the local velocity gradient and the 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 (1_1,0–1_1,1) absorption only towards three bright HII regions. Our observations demonstrate that H₂CO (1_1,0–1_1,1) is optically thin in general. The 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 to the extended absorption features in Cygnus X. Our observations suggest that H₂CO (1_1,0–1_1,1) can trace molecular gas with H₂ column densities of ≳5 × 10²¹ cm⁻² (i.e., A_V ≳ 5). The ortho-H₂CO fractional abundance with respect to H₂ has a mean value of 7.0 × 10⁻¹⁰. A comparison of the velocity dispersions on different linear scales suggests that the velocity dispersions of the dominant −3 km s⁻¹ velocity component in the prominent DR21 region are nearly identical on scales of 0.17−4.4 pc, which deviates from the expected behavior of classic turbulence.