Large-scale Map of Millimeter-wavelength Hydrogen Radio Recombination
  Lines around a Young Massive Star Cluster
  We report the first map of large-scale (10 pc in length) emission of
millimeter-wavelength hydrogen recombination lines (mm-RRLs) toward the giant H
II region around the W43-Main young massive star cluster (YMC). Our mm-RRL data
come from the IRAM 30 m telescope and are analyzed together with radio
continuum and cm-RRL data from the Karl G. Jansky Very Large Array and
HCO$^{+}$ 1-0 line emission data from the IRAM 30 m. The mm-RRLs reveal an
expanding wind-blown ionized gas shell with an electron density ~70-1500
cm$^{-3}$ driven by the WR/OB cluster, which produces a total Ly$\alpha$ photon
flux of 1.5 x 10$^{50}$ s$^{-1}$. This shell is interacting with the dense
neutral molecular gas in the W43-Main dense cloud. Combining the high spectral
and angular resolution mm-RRL and cm-RRL cubes, we derive the two-dimensional
relative distributions of dynamical and pressure broadening of the ionized gas
emission and find that the RRL line shapes are dominated by pressure broadening
(4-55 km s$^{-1}$) near the YMC and by dynamical broadening (8-36 km s$^{-1}$)
near the shell's edge. Ionized gas clumps hosting ultra-compact H II regions
found at the edge of the shell suggest that large-scale ionized gas motion
triggers the formation of new star generation near the periphery of the shell.
