We sort 4683 molecular clouds between 10°<ℓ<65°from the Bolocam Galactic Plane Survey based on observational diagnostics of star formation activity: compact 70 μm sources, mid-IR color-selected YSOs, H 2 O and CH 3 OH masers, and UCH IIregions. We also present a combined NH 3 -derived gas kinetic temperature and H 2 O maser catalog for 1788 clumps from our own GBT 100 m observations and from the literature. We identify a subsample of 2223 (47.5%) starless clump candidates (SCCs), the largest and most robust sample identified from a blind survey to date. Distributions of flux density, flux concentration, solid angle, kinetic temperature, column density, radius, and mass show strong (>1 dex) progressions when sorted by star formation indicator. The median SCC is marginally subvirial (α∼0.7) with >75% of clumps with known distance being gravitationally bound (α<2). These samples show a statistically significant increase in the median clump mass of ΔM∼170-370 M e from the starless candidates to clumps associated with protostars. This trend could be due to (i) mass growth of the clumps atM 200 440 -M e Myr −1 for an average freefall 0.8 Myr timescale, (ii) a systematic factor of two increase in dust opacity from starless to protostellar phases, and/or (iii)a variation in the ratio of starless to protostellar clump lifetime that scales as ∼M −0.4 . By comparing to the observed number of CH 3 OH maser containing clumps, we estimate the phaselifetime of massive (M>10 3 M e ) starless clumps to be 0.37±0.08 Myr (M/103 M e ) −1; the majority (M<450 M e ) have phaselifetimes longer than their average freefall time.

The Bolocam Galactic Plane Survey (BGPS) is a 1.1 mm continuum survey of dense clumps of dust throughout the Galaxy covering 170 square degrees. We present spectroscopic observations using the Heinrich Hertz Submillimeter Telescope of the dense gas tracers, HCO + and N 2 H + 3 − 2, for all 6194 sources in the Bolocam Galactic Plane Survey v1.0.1 catalog between 7.5 • ≤ l ≤ 194 • . This is the largest targeted spectroscopic survey of dense molecular gas in the Milky Way to date. We find unique velocities for 3126 (50.5%) of the BGPS v1.0.1 sources observed. Strong N 2 H + 3 − 2 emission (T mb > 0.5 K) without HCO + 3 − 2 emission does not occur in this catalog. We characterize the properties of the dense molecular gas emission toward the entire sample. HCO + is very sub-thermally populated and the 3-2 transitions are optically thick toward most BGPS clumps. The median observed line width is 3.3 km/s consistent with supersonic turbulence within BGPS clumps. We find strong correlations between dense molecular gas integrated intensities and 1.1 mm peak flux and the gas -2kinetic temperature derived from previously published NH 3 observations. These intensity correlations are driven by the sensitivity of the 3 − 2 transitions to excitation conditions rather than by variations in molecular column density or abundance. We identify a subset of 113 sources with stronger N 2 H + than HCO + integrated intensity, but we find no correlations between the N 2 H + /HCO + ratio and 1.1 mm continuum flux density, gas kinetic temperature, or line width. Selfabsorbed profiles are rare (1.3%).

With a goal toward deriving the physical conditions in external galaxies, we present a study of the ammonia (NH 3 ) emission and absorption in a sample of star-forming systems. Using the unique sensitivities to kinetic temperature afforded by the excitation characteristics of several inversion transitions of NH 3 , we have continued our characterization of the dense gas in star-forming galaxies by measuring the kinetic temperature in a sample of 23 galaxies and one galaxy offset position selected for their high infrared luminosity. We derive kinetic temperatures toward 13 galaxies, 9 of which possess multiple kinetic temperature and/or velocity components. Eight of these galaxies exhibit kinetic temperatures >100 K, which are in many cases at least a factor of two larger than kinetic temperatures derived previously. Furthermore, the derived kinetic temperatures in our galaxy sample, which are in many cases at least a factor of two larger than derived dust temperatures, point to a problem with the common assumption that dust and gas kinetic temperatures are equivalent. As previously suggested, the use of dust emission at wavelengths greater than 160 μm to derive dust temperatures, or dust heating from older stellar populations, may be skewing derived dust temperatures in these galaxies to lower values. We confirm the detection of high-excitation OH 2 Π 3/2 J = 9/2 absorption toward Arp 220. We also report the first detections of non-metastable NH 3 inversion transitions toward external galaxies in the (2,1) (NGC 253, NGC