Fast-growing SMBHs in Fast-growing Galaxies, at High Redshifts: the Role of Major Mergers as Revealed by ALMA

TL;DR

This study investigates the role of major galaxy mergers in the rapid growth of supermassive black holes at high redshifts, using ALMA observations of quasars and their companions to understand different fueling mechanisms.

Contribution

It provides observational evidence linking major mergers to SMBH growth at high redshift, while also highlighting the diversity of host galaxy properties and accretion processes.

Findings

Major mergers are observed in some high-redshift quasars, supporting their role in SMBH growth.

Not all high-SFR quasar hosts show signs of mergers, indicating alternative fueling mechanisms.

ALMA detects companion galaxies at significant distances, implying diverse pathways for SMBH accretion.

Abstract

We present a long-term, multi-wavelength project to understand the epoch of fastest growth of the most massive black holes by using a sample of 40 luminous quasars at z~4.8. These quasars have rather uniform properties, with typical accretion rates and black hole masses of L/L_Edd~0.7 and M_BH~10^9 M_sol. The sample consists of "FIR-bright" sources with a previous Herschel/SPIRE detection, suggesting SFR > 1000 M_sol/yr, as well as of "FIR-faint" sources for which Herschel stacking analysis implies a typical SFR of ~400 M_sol/yr. Six of the quasars have been observed by ALMA in [CII]{\lambda}157.74 micron line emission and adjacent rest-frame 150 micron continuum, to study the dusty cold ISM. ALMA detected companion, spectroscopically confirmed sub-mm galaxies (SMGs) for three sources - one FIR-bright and two FIR-faint. The companions are separated by ~14-45 kpc from the quasar hosts, and we interpret them as major galaxy interactions. Our ALMA data therefore clearly support the idea that major mergers may be important drivers for rapid, early SMBH growth. However, the fact that not all high-SFR quasar hosts are accompanied by interacting SMGs, and their ordered gas kinematics observed by ALMA, suggest that other processes may be fueling these systems. Our analysis thus demonstrates the diversity of host galaxy properties and gas accretion mechanisms associated with early and rapid SMBH growth.