The Assembly of the First Massive Black Holes

The existence of similar to 10(9) M-circle dot supermassive black holes (SMBHs) within the first billion years of the Universe has stimulated numerous ideas for the prompt formation and rapid growth of black holes (BHs) in the early Universe. Here, we review ways in which the seeds ofmassive BHsmay have first assembled, how they may have subsequently grown as massive as similar to 10(9) M-circle dot, and how multimessenger observations could distinguish between different SMBH assembly scenarios. We conclude the following: The ultrarare similar to 10(9) M-circle dot SMBHs represent only the tip of the iceberg. Early BHs likely fill a continuum from the stellar-mass ( similar to 10(9) M-circle dot) to the supermassive (similar to 10(9)) regimes, reflecting a range of initial masses and growth histories. Stellar-mass BHs were likely left behind by the first generation of stars at redshifts as high as similar to 30, but their initial growth typically was stunted due to the shallow potential wells of their host galaxies. Conditions in some larger, metal-poor galaxies soon became conducive to the rapid formation and growth of massive seed holes, via gas accretion and by mergers in dense stellar clusters. BH masses depend on the environment (such as the number and properties of nearby radiation sources and the local baryonic streaming velocity) and on the metal enrichment and assembly history of the host galaxy. Distinguishing between assembly mechanisms will be difficult, but a combination of observations by the Laser Interferometer Space Antenna (probing massive BH growth via mergers) and by deep multiwavelength electromagnetic observations (probing growth via gas accretion) is particularly promising.