2026ApJS..282...15L

We conducted a 6 cm formaldehyde (H₂CO) line survey across the northern sky, targeting 3149 high-mass star-forming region candidates selected from the all-sky Wide-field Infrared Survey Explorer (WISE) point-source catalog using the Shanghai TianMa 65 m Radio Telescope. This survey detected H₂CO absorption lines at 1156 positions and four known maser sources, achieving a detection rate of 36.7%. Among these, 14 candidates exhibited hyperfine structure splitting, all located at high Galactic latitudes (b > 3°). The Galactic spatial distribution of H₂CO absorption line detections yields a scale height of ∼71.8 pc. Compared to the WISE infrared colors and magnitudes of 6.7 GHz CH₃OH masers and hydrogen radio-recombination lines (HRRLs), the distribution of H₂CO absorption lines closely resembles that of HRRLs, albeit with higher [W4] magnitudes. The detection of these three lines in the same observational sample enables the classification of H₂CO absorption lines into four groups representing distinct evolutionary stages. Variations in H₂CO detection proportion, line width, and intensity across these groups probably reflect the influence of star formation feedback on the circumstellar medium during protostellar evolution. Observational updates over six years (2015-2021) of the 6 cm H₂CO maser in NGC 7538 IRS1 demonstrate that the flux density diverged from the prediction of the rotating ellipsoidal model, highlighting the need for further data to elucidate the maser variability mechanism.