Probing the Growth of Massive Black Holes with Black Hole–Host Galaxy Spin Correlations

Abstract

Supermassive black holes are commonly found at the centers of their host galaxies, but their formation still remains an open question. In light of the tight correlation between the black hole (BH) mass and the velocity dispersions of the bulge component of the host galaxy, a BH–host galaxy coevolution scenario has been established. Such a description, however, still contains many theoretical uncertainties, including puzzles about the formation of BH seeds at high redshifts and the growth channel fueling these seeds. In this work, we systematically analyze the signatures of different growth channels on massive BH (MBH) spins. We show that different growth channels can be partially distinguished with the magnitudes of MBH spins inferred from extreme-mass-ratio inspirals detected by the Laser Interferometer Space Antenna. In addition, we propose measuring the correlation between the directions of MBH spins and their host galaxy spins, which is possible for extreme-mass-ratio inspirals happening in low-redshift galaxies ( ). With the inclusion of spin direction correlation different formation channels shall be significantly better constrained.