The Bolocam Galactic Plane Survey. V. HCO⁺ and N ₂H⁺ spectroscopy of 1.1mm dust continuum sources

We present the results of observations of 1882 sources in the Bolocam Galactic Plane Survey (BGPS) at 1.1mm with the 10 m Henrich Hertz Telescope simultaneously in HCO⁺ J = 3-2 and N₂H⁺ J = 3-2. We detect 77% of these sources in HCO⁺ and 51% in N ₂H⁺ at greater than 3σ. We find a strong correlation between the integrated intensity of both dense gas tracers and the 1.1mm dust emission of BGPS sources. We determine kinematic distances for 529 sources (440 in the first quadrant breaking the distance ambiguity and 89 in the second quadrant). We derive the size, mass, and average density for this subset of clumps. The median size of BGPS clumps is 0.75pc with a median mass of 330 M_⊙ (assuming T _Dust = 20K). The median HCO ⁺ linewidth is 2.9kms^-1 indicating that BGPS clumps are dominated by supersonic turbulence or unresolved kinematic motions. We find no evidence for a size-linewidth relationship for BGPS clumps. We analyze the effects of the assumed dust temperature on the derived clump properties with a Monte Carlo simulation and find that changing the temperature distribution will change the median source properties (mass, volume-averaged number density, surface density) by factors of a few. The observed differential mass distribution has a power-law slope that is intermediate between that observed for diffuse CO clouds and the stellar initial mass function. BGPS clumps represent a wide range of objects (from dense cores to more diffuse clumps) and are typically characterized by larger sizes and lower densities than previously published surveys of high-mass star-forming regions. This collection of objects is a less-biased sample of star-forming regions in the Milky Way that likely span a wide range of evolutionary states.