  Aims. We seek to understand how the 4.8 GHz formaldehyde absorption line is
distributed in the MON R2, S156, DR17/L906, and M17/M18 regions. More
specifically, we look for the relationship among the H2CO, 12CO, and 13CO
spectral lines. Methods. The four regions of MON R2 (60'x90'), S156 (5'0x70'),
DR17/L906 (40'x60'), and M17 /M18 (70'x80')were observed for H2CO (beam 10'),
H110a recombination (beam 10'), 6 cm continuum (beam 10'), 12CO (beam 1'), and
13CO (beam 1'). We compared the H2CO,12CO,13CO, and continuum distributions,
and also the spectra line parameters of H2CO,12CO, and 13CO. Column densities
of H2CO,13CO, and H2 were also estimated. Results. We found out that the H2CO
distribution is similar to the 12CO and the 13CO distributions on a large
scale. The correlation between the 13 CO and the H2CO distributions is better
than between the 12CO and H2CO distributions. The H2CO and the 13CO tracers
systematically provide consistent views of the dense regions. T heir maps have
similar shapes, sizes, peak positions, and molecular spectra and present
similar centr al velocities and line widths. Such good agreement indicates that
the H2CO and the 13CO arise from similar regions.
