The role of stellar relaxation in the formation and evolution of the first massive black holes

Hidenobu Yajima, Sadegh Khochfar

Research output: Contribution to journalArticlepeer-review

28 Citations (Scopus)


We present calculations on the formation of massive black holes of 105 M at z > 6, which can be the seeds of supermassive black holes at z ≳ 6. Under the assumption of compact star cluster formation in merging galaxies, star clusters in haloes of ~ 108-109M can undergo rapid core collapse, leading to the formation of very massive stars (VMSs) of ~1000M that collapse directly into black holes with similar masses. Star clusters in haloes of ≳109M experience Type II supernovae before the formation of VMSs, due to long core-collapse timescales. We also model the subsequent growth of black holes via accretion of residual stars in clusters. Two-body relaxation refills the loss cones of stellar orbits efficiently at larger radii and resonant relaxation at small radii is the main driver for accretion of stars on to black holes. As a result, more than 90 percent of stars in the initial cluster are swallowed by the central black holes before z = 6. Using dark matter merger trees, we derive black hole mass functions at z =6-20. Themass function ranges from 103-105M at z ≲15. Majormerging of galaxies of ≳4 × 108M at z ~ 20 leads successfully to the formation of ≳ 105M black holes by z ≳ 10, which could be the potential seeds of supermassive black holes seen today.

Original languageEnglish
Article numberstw124
Pages (from-to)2423-2432
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 2016 Apr 11


  • Galaxies: formation
  • Galaxies: high-redshift
  • Galaxies: star clusters: general
  • Quasars: supermassive black holes
  • Stars: black holes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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