  Absorption in the J{K-K+} = 2{11}-2{12} transition of formaldehyde at 2 cm
towards the ultracompact HII regions C1 and C2 of W 58 has been observed with
the VLA with an angular resolution of ~0.2'' and a velocity resolution of ~1
km/s. The high resolution continuum image of C1 (ON 3) shows a partial shell
which opens to the NE. Strong H2CO absorption is observed against W 58 C1. The
highest optical depth (tau > 2) occurs in the SW portion of C1 near the edge of
the shell, close to the continuum peak. The absorption is weaker towards the
nearby, more diffuse compact HII region C2, tau<~0.3. The H2CO velocity (-21.2
km/s) towards C1 is constant and agrees with the velocity of CO emission,
mainline OH masers, and the H76 alpha recombination line, but differs from the
velocity of the 1720 MHz OH maser emission (~-13 km/s). Observations of the
absorption in the J{K-K+} = 1{10}-1{11} transition of formaldehyde at 6 cm
towards W 58 C1 and C2 carried out earlier with the WSRT at lower resolution
(~4''x7'') show comparable optical depths and velocities to those observed at 2
cm. Based on the mean optical depth profiles at 6 cm and 2 cm, the volume
density of molecular hydrogen n(H2) and the formaldehyde column density N(H2CO)
were determined. The n(H2) is ~6E4 /cm**3 towards C1. N(H2CO) for C1 is ~8E14
/cm**2 while that towards C2 is ~8E13 /cm**2.
