  Observations of H$_{2}$CO lines and continuum at 1.3 mm towards Sgr B2(N) and
Sgr B2(M) cores were carried out with the SMA. We imaged H$_{2}$CO line
absorption against the continuum cores and the surrounding line emission
clumps. The results show that the majority of the dense gas is falling into the
major cores where massive stars have been formed. The filaments and clumps of
the continuum and gas are detected outside of Sgr B2(N) and Sgr B2(M) cores.
Both the spectra and moment analysis show the presence of outflows from Sgr
B2(M) cores. The H$_{2}$CO gas in the red-shifted outflow of Sgr B2(M) appears
to be excited by a non-LTE process which might be related to the shocks in the
outflow.
