ATLASGAL-selected massive clumps in the inner Galaxy IV. Millimeter
  hydrogen recombination lines from associated HII regions
  Observations of millimeter wavelength radio recombination lines (mm-RRLs) are
used to search for HII regions in an unbiased way that is complementary to many
of the more traditional methods previously used. We carried out targeted mm-RRL
observations (39$\leq$ principal quantum number ($n$) $\leq$65 and $\Delta n$ =
1, 2, 3 and 4, i.e. H$n\alpha$, H$n\beta$, H$n\gamma$ and H$n\delta$) using the
IRAM 30m and Mopra 22m telescopes. We observed 976 compact dust clumps selected
from a catalog of $\sim$10,000 ATLASGAL sources. We detected H$n\alpha$ mm-RRL
emission toward 178 clumps; H$n\beta$, H$n\gamma$ and H$n\delta$ were also
detected toward 65, 23, and 22 clumps, respectively. This is the largest sample
of mm-RRLs detections published to date. Comparing the positions of these
clumps with radio continuum surveys we identified compact radio counterparts
for 134 clumps. The nature of the other 44 detections is unclear, but 8
detections are thought to be potentially new HII regions. Broad linewidths are
seen toward nine clumps (linewidth > 40 km s$^{-1}$) revealing significant
turbulent motions within the ionized gas. We find that the linewidth of the
H$^{13}$CO$^{+}$ (1$-$0) emission is significantly wider than those without
mm-RRL detection. We also find a correlation between the integrated fluxes of
the mm-RRLs and the 6 cm continuum flux densities of their radio counterparts.
We find that the mm-RRL emission is associated with HII regions with
$n_{e}<10^{5}$ cm$^{-3}$ and HII region diameter >0.03 pc. We detected mm-RRLs
toward 178 clumps and identified eight new HII region candidates. The broad
mm-RRL from nine clumps may indicate that they arise in very young
hyper-compact HII regions. The mm-RRLs trace the radio continuum sources
detected by high-resolution observations and their line parameters show
associations with the embedded radio sources and their parental molecular
clumps.
