ATLASGAL-selected massive clumps in the inner Galaxy IV. Millimeter hydrogen recombination lines from associated HII regions

Aims. Observations of millimeter wavelength radio recombination lines (mm-RRLs) are used to search for H i i regions in an unbiased way that is complementary to many of the more traditional methods previously used (e. g., radio continuum, far-infrared colors, maser emission). The mm-RRLs can be used to derive physical properties of H i i regions and to provide velocity information of ionized gas. Methods. We carried out targeted mm-RRL observations (39 <= principal quantum number (n) <= 65 and Delta n = 1, 2, 3, and 4, named Hn alpha, Hn beta, Hn gamma, and Hn delta) using the IRAM 30 m and Mopra 22 m telescopes. In total, we observed 976 compact dust clumps selected from a catalog of similar to 10 000 sources identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The sample was selected to ensure a representative mix of star-forming and quiescent clumps such that a variety of di ff erent evolutionary stages is represented. Approximately half of the clumps are mid-infrared quiet while the other half are mid-infrared bright. Results. We detected Hn ff mm-RRL emission toward 178 clumps; Hn alpha, Hn gamma, and Hn 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, confirming their association with known H i i regions. The nature of the other 44 detections is unclear, but 8 detections are thought to be potentially new H i i regions while the mm-RRL emission from the others may be due to contamination from nearby evolved H i i regions. Broad linewidths are seen toward nine clumps (linewidth > 40 km s(-1)) revealing significant turbulent motions within the ionized gas; in the past, such wide linewidths were found toward very compact and dense H i i regions. We find that the systemic velocity of the associated dense molecular gas, traced by (HCO+)-C-13 (1-0), is consistent with the mm-RRL velocities and confirms them as embedded H i i regions. We also find that the linewidth of the (HCO)-C-13 + (1-0) emission is significantly wider than those without mm-RRL detection, indicating a physical connection between the embedded H II region and their natal environments. We also find a correlation between the integrated fluxes of the mm-RRLs and the 6 cm continuum flux densities of their radio counterparts (the correlation coefficient, rho, is 0.70). By calculating the electron densities we find that the mm-RRL emission is associated with H II regions with ne < 105 cm 3 and H i i region diameter > 0.03 pc. Conclusions. We detected mm-RRLs toward 178 clumps and identified eight new H II region candidates. The broad mm-RRL from nine clumps may indicate that they arise in very young hyper-compact H II 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.