Missing link found? The "runaway" path to supermassive black holes

Observations of stellar kinematics, gasdynamics, and masers around galactic nuclei have now firmly established that many galaxies host central superma ssive black holes (SMBHs) with masses in the range of similar to 10(6)-10(9 ) M-.. However, how these SMBHs formed is not well understood. One reason f or this situation is the lack of observations of intermediate-mass BHs (IMB Hs), which could bridge the gap between stellar mass BHs and SMBHs. Recentl y, this missing link (i.e., an IMBH) has been found in observations made by ASCA and Chandra of the central region of the starburst galaxy M82. Subseq uent observations by Subaru have revealed that this IMBH apparently coincid es with a young compact star cluster. Based on these findings, we suggest a new formation scenario for SMBHs. In this scenario, IMBHs first form in yo ung compact star clusters through runaway merging of massive stars. While t hese IMBHs are forming, the host star clusters sink toward the galactic nuc leus through dynamical friction and upon evaporation deposit their IMBHs ne ar the galactic center. The IMBHs then form binaries and eventually merge v ia gravitational radiation, forming an SMBH.