Large-scale molecular gas distribution in the M17 cloud complex: dense
  gas conditions of massive star formation?
  The non-uniform distribution of gas and protostars in molecular clouds is
caused by combinations of various physical processes that are difficult to
separate. We explore this non-uniform distribution in the M17 molecular cloud
complex that hosts massive star formation activity using the $^{12}$CO
($J=1-0$) and $^{13}$CO ($J=1-0$) emission lines obtained with the Nobeyama 45m
telescope. Differences in clump properties such as mass, size, and
gravitational boundedness reflect the different evolutionary stages of the
M17-H{\scriptsize II} and M17-IRDC clouds. Clumps in the M17-H{\scriptsize II}
cloud are denser, more compact, and more gravitationally bound than those in
M17-IRDC. While M17-H{\scriptsize II} hosts a large fraction of very dense gas
(27\%) that has column density larger than the threshold of $\sim$ 1 g
cm$^{-2}$ theoretically predicted for massive star formation, this very dense
gas is deficient in M17-IRDC (0.46\%). Our HCO$^+$ ($J=1-0$) and HCN ($J=1-0$)
observations with the TRAO 14m telescope, {\nlqb trace all gas with column
density higher than $3\times 10^{22}$ cm$^{-2}$}, confirm the deficiency of
high density ($\gtrsim 10^5$ cm$^{-3}$) gas in M17-IRDC. Although M17-IRDC is
massive enough to potentially form massive stars, its deficiency of very dense
gas and gravitationally bound clumps can explain the current lack of massive
star formation.
