A H2CO and H110 α survey of H ii regions with the 25-m radio telescope of Nanshan Station

Abstract

Context. Accurate distances are necessary for determining the physical properties of massive star-formation regions and the structure of our Galaxy. One way to approach these studies is by determining the kinematic distances of H ii regions. Aims. This study determines the kinematic distances of H ii regions located in the inner Galaxy and studies their spatial distribution. Methods. We observed 251 H ii regions with the 25-m radio telescope of Nanshan Station, looking for H2CO absorption lines and H110α radio recombination lines (RRLs). The kinematic distances were calculated using the velocity of H110α lines and a model of the Galactic rotation. The kinematic distance ambiguity was resolved using the H2CO velocities. Results. We detected the H110α RRLs in 28 sources and H2CO absorption lines in 59 sources. In the latter case, 43 features had not previously been observed. H2CO and H110α lines were simultaneously detected toward 23 H ii regions. We resolved the kinematic distance ambiguities for 14 H ii regions and 20 intervening molecular clouds. Conclusions. Our statistical analysis shows that UCH ii regions are tightly confined to the Galactic plane, and their vertical height distribution is about 25.4 pc. The corresponding value of the H ii regions is 28.6 pc. There is a good statistical relationship between the fluxes of H2CO and infrared 100 μm, for those H ii regions with both H2CO absorption lines and H110α RRLs detected. This suggests that there is a weak correlation between the continuum fluxes at 6 cm and infrared 100 μm. © 2011 ESO.