2021MNRAS.504.1733S

Spectral lines from formaldehyde (H₂CO) molecules at cm wavelengths are typically detected in absorption and trace a broad range of environments, from diffuse gas to giant molecular clouds. In contrast, thermal emission of formaldehyde lines at cm wavelengths is rare. In previous observations with the 100 m Robert C. Byrd Green Bank Telescope (GBT), we detected 2 cm formaldehyde emission towards NGC 7538 IRS1 - a high-mass protostellar object in a prominent star-forming region of our Galaxy. We present further GBT observations of the 2 and 1 cm H₂CO lines to investigate the nature of the 2 cm H₂CO emission. We conducted observations to constrain the angular size of the 2 cm emission region based on a East-West and North-South cross-scan map. Gaussian fits of the spatial distribution in the East-West direction show a deconvolved size (at half maximum) of the 2 cm emission of 50 arcsec ± 8 arcsec. The 1 cm H₂CO observations revealed emission superimposed on a weak absorption feature. A non-LTE radiative transfer analysis shows that the H₂CO emission is consistent with quasi-thermal radiation from dense gas (∼10^5–106 cm^–3). We also report detection of four transitions of CH₃OH (12.2, 26.8, 28.3, 28.9 GHz), the (8,8) transition of NH₃ (26.5 GHz), and a cross-scan map of the 13 GHz SO line that shows extended emission (>50 arcsec).