Stars are the fundamental building blocks of the Universe and most stars in our Galaxy form in clusters, some of which also produce high mass stars. Although relatively rare, from their birth to their ultimate death as supernovae, these high mass stars dominate the chemical and mechanical evolution of the interstellar medium of galaxies. Despite their importance for understanding phenomena ranging from the dispersal of molecular clouds to the origin of gamma ray bursts and blackholes, the formation and early evolution of these massive stars are poorly understood. Key issues include understanding how gas accumulates into highly condensed clumps, the precursors to clusters, and how these then fragment in to cores, the precursors of individual stars, as well as how massive protostars evolve. Studies of regions which are not yet dominated by star formation and effects of stellar feedback are essential for understanding these processes. In this seminar I will discuss our recent work on such regions selected from the Spitzer Dark Cloud catalogue and the insights they provide into how clusters and massive stars form.

The nucleus of our Galaxy, Sgr A*, is surrounded by a rapidly rotating ring of dense molecular gas, which is referred to as the circumnuclear disk (CND). The CND is considered to be a mass reservoir for feeding the nucleus. Within 300 pc from the nucleus, a number of compact clouds with broad velocity widths, namely high-velocity compact clouds (HVCCs), have been discovered. In my Ph.D course, we conducted observational studies on the high-velocity molecular clouds in the Galactic center such as the CND and HVCCs. First, we observed the CND and its periphery in several molecular lines using the NRO 45 m telescope, and found an emission “bridge” which connects the CND to an adjacent giant molecular cloud, M–0.13–0.08. This emission bridge indicates the physical contact between the CND and M–0.13–0.08, suggesting that M–0.13–0.08 has just fallen into the CND. The physical contact between them may cancel out their angular momentum, dissipate their kinetic energies, and thereby increase the mass accretion rate to the nucleus. This is a significant result that caught a part of the feeding process to the Galactic nucleus. Moreover, we extended the mapping area to 20 pc radius from the nucleus with the JCMT. Two small HVCCs were detected at 10 pc from Sgr A*. Their sizes, kinematics, kinetic energies, and the absences of counterparts in other wavelengths are consistent with the formation scenario assuming the high-velocity plunge of a stellar-mass black hole into a molecular cloud. This is the first observational case that suggests a number of black holes are flying about in the vicinity of the nucleus. Theoretical calculations predicted that more than 10^8 black holes are floating in our Galaxy, and most of them are isolated and dim. Such isolated black holes could be detected as high-velocity features, such as HVCCs.