4-8GHz TMRT obs. of WISE point sources

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{deg}).

The Galactic spatial distribution of H2CO absorption line detections yields a scale height of ~71.8pc. Compared to the WISE infrared colors and magnitudes of 6.7GHz 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.