Supermassive black holes (SMBHs)

The formation and growth of SMBHs and their impact on galaxies is an important, yet poorly understood piece in galaxy formation.

SMBH growth in the early Universe

It is a long-standing puzzle that SMBH ≳ 10⁹M_⊙ have been discovered at z ≳ 5 when the Universe is less than 1 Gyr old. So far, no theory beyong hand-waving provides a reasonble picture how these SMBHs form and grow in such a short time. Several groups (including FIRE) find that BH growth is heavily suppressed by stellar feedback at early times when the host galaxy is clumpy and exhibits bursty star formation, such that SMBHs remain undermassive compared to local BH scaling relations during this phase (e.g. Dubois et al. 2015; Anglés-Alcázar et al. 2017).

Using cosmological zoom-in simulations of massive galaxies (M_halo ≳ 10¹²M_⊙) at z ≳ 5 from the FIRE in the high-redshift Universe suite, we explain why this happens (Ma et al., to be submitted to MNRAS; see figure below): during a starburst, the galactic nucleus (e.g. the central ≲ 100 pc) builds up stellar mass primarily by merging massive star clusters instead of in situ SF, meaning that there is not a long-term gas reservoir in the nuclear region for the BH to grow proportionally.

Figure: Left: Stellar mass growth in the galactic nucleus during a starburst of a M_halo ~ 10¹²M_⊙ halo. The entire stellar mass build-up process takes ≲ 10 Myr. Right: Mass enclosed in the nucleus (top), BH accretion rates (middle), and total mass accreted (bottom). ΔM_BH is calculated in post-processing, assuming an initial BH mass 2×10⁷M_⊙ based on the local M_BH-σ relation. The BH cannot grow proportionally to stay on local BH scaling relations (from Ma et al., to be submitted).