The hierarchical assembly of galaxies and black holes in the first billion years: predictions for the era of gravitational wave astronomy

Abstract

In this work we include black hole (BH) seeding, growth and feedback into our\nsemi-analytic galaxy formation model, Delphi. Our model now fully tracks the,\naccretion- and merger-driven, hierarchical assembly of the dark matter halo,\nbaryonic and BH masses of high-redshift ($z>5$) galaxies. We use a minimal set\nof mass- and $z$-independent free parameters associated with star formation and\nBH growth (and feedback) and include suppressed BH growth in low-mass galaxies\nto explore a number of physical scenarios including: (i) two types of BH seeds\n(stellar and those from Direct Collapse BH; DCBH); (ii) the impact of\nreionization feedback; and (iii) the impact of instantaneous versus delayed\ngalaxy mergers on the baryonic growth. While both reionization feedback and\ndelayed galaxy mergers have no sensible impact on the evolving ultra-violet\nluminosity function, the latter limits the maximum BH masses achieved at these\nhigh-$z$. We then use this model, baselined against all available high-$z$\ngalaxy and BH data-sets, to predict the LISA detectability of merger events at\n$z > 5$. As expected, the merger rate is dominated by stellar BH mergers for\nall scenarios and our model predicts an expected upper limit of about 20\nmergers in the case of instantaneous merging and no reionization feedback over\nthe 4-year mission duration. Including the impact of delayed mergers and\nreionization feedback reduces this to about 12 events over the same\nobservational time-scale.\n