Radio recombination line observations towards Spitzer infrared bubbles with the TianMa radio telescope
Many of the Spitzer infrared bubbles identified by the Milky Way Project (MWP) are suggested to be H II regions in nature. More than 70 per cent of the ∼5000 known bubbles do not have radio recombination line (RRL) observations, hence have not been confirmed as H II regions. A systematic RRL survey should be helpful to identify the nature of the bubbles. With the Shanghai TianMa 65-m radio telescope, we searched for RRLs towards 216 selected Spitzer bubbles by simultaneously observing 19 RRLs in the C band (4-8 GHz). RRLs are detected in the directions of 75 of the 216 targets. 31 of the 75 RRL sources are classified as new detections, which are possibly from new H II regions or diffuse warm ionized medium; 36 of them are probably from the outskirts of nearby bright H II regions, rather than bubble-encircled ionized gas; and the detected RRLs towards 8 bubbles are identified from known H II regions. For 58 of the 75 RRL sources, we obtained their distances after resolving the kinematic distance ambiguity by combining the results of the H2CO absorption method, the H I emission/absorption method, and the H I self-absorption method. The low detection rate of new H II regions implies that a number of MWP bubbles in the DR1 catalogue are too faint if they are H II regions.