Search

The Milky Way is a barred spiral galaxy with bar lanes that bring gas toward the Galactic center. Gas flowing along these bar lanes often overshoots, and instead of accreting onto the Central Molecular Zone (CMZ), it collides with the bar lane on the opposite side of the Galaxy. We observed G5, a cloud that we believe is the site of one such collision, near the Galactic center at ( ℓ , b ) = ( +5.4, −0.4) with the Atacama Large Millimeter/submillimeter Array/Atacama Compact Array. We took measurements of the spectral lines ^12 CO J = 2 → 1, ^13 CO J = 2 → 1, C ^18 O J = 2 → 1, H _2 CO J = 3 _03 → 2 _02 , H _2 CO J = 3 _22 → 2 _21 , CH _3 OH J = 4 _22 → 3 _12 , OCS J = 18 → 17, and SiO J = 5 → 4. We observed a velocity bridge between two clouds at ∼50 and ∼150 km s ^−1 in our position–velocity diagram, which is direct evidence of a cloud–cloud collision. We measured an average gas temperature of ∼60 K in G5 using H _2 CO integrated-intensity line ratios. We observed that the ^12 C/ ^13 C ratio in G5 is consistent with optically thin, or at most marginally optically thick ^12 CO. We measured $1.5\times {10}^{19}\,{\mathrm{cm}}^{-2}\,{({\rm{K}}\,\mathrm{km}\,{{\rm{s}}}^{-1})}^{-1}$ for the local X _CO , 10–20× less than the average Galactic value. G5 is strong direct observational evidence of gas overshooting the CMZ and colliding with a bar lane on the opposite side of the Galactic center.

Recent Galactic plane surveys of dust continuum emission at long wavelengths have identified a population of dense, massive clumps with no evidence for ongoing star formation. These massive starless clump candidates are excellent sites to search for the initial phases of massive star formation before the feedback from massive star formation affects the clump. In this study, we search for the spectroscopic signature of inflowing gas toward starless clumps, some of which are massive enough to form a massive star. We observed 101 starless clump candidates identified in the Bolocam Galactic Plane Survey (BGPS) in J = 1−0 using the 12 m Arizona Radio Observatory telescope. We find a small blue excess of for the complete survey. We identified six clumps that are good candidates for inflow motion and used a radiative transfer model to calculate mass inflow rates that range from 500 to 2000 Myr−1. If the observed line profiles are indeed due to large-scale inflow motions, then these clumps will typically double their mass on a freefall time. Our survey finds that massive BGPS starless clump candidates with inflow signatures in J = 1−0 are rare throughout our Galaxy. [ABSTRACT FROM AUTHOR]

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, H2O and CH3OH masers, and UCH ii regions. We also present a combined NH3-derived gas kinetic temperature and H2O 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⊙ from the starless candidates to clumps associated with protostars. This trend could be due to (i) mass growth of the clumps at M⊙ 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 CH3OH maser containing clumps, we estimate the phase lifetime of massive (M > 103M⊙) starless clumps to be 0.37 ± 0.08 Myr (M/103M⊙)−1; the majority (M < 450 M⊙) have phase lifetimes longer than their average freefall time. [ABSTRACT FROM AUTHOR]