The growth of typical star-forming galaxies and their super massive black holes across cosmic time since z~2

TL;DR

This study investigates the co-evolution of star-forming galaxies and their supermassive black holes from redshift 2 to 0, revealing a roughly constant ratio of black hole accretion to star formation over 11 billion years.

Contribution

It provides the first comprehensive analysis of the relative growth rates of galaxies and black holes across cosmic time using multi-wavelength data from the COSMOS field.

Findings

Black hole accretion rates decline from z=2.23 to 0, mirroring star formation rate density.

Typical galaxies grow their stellar mass much faster than their black holes.

A small fraction (~3%) of galaxies show rapid black hole growth, exceeding typical rates.

Abstract

Understanding galaxy formation and evolution requires studying the interplay between the growth of galaxies and the growth of their black holes across cosmic time. Here we explore a sample of Ha-selected star-forming galaxies from the HiZELS survey and use the wealth of multi-wavelength data in the COSMOS field (X-rays, far-infrared and radio) to study the relative growth rates between typical galaxies and their central supermassive black holes, from z=2.23 to z=0. Typical star-forming galaxies at z~1-2 have black hole accretion rates (BHARs) of 0.001-0.01 Msun/yr and star formation rates (SFRs) of ~10-40 Msun/yr, and thus grow their stellar mass much quicker than their black hole mass (~3.3 orders of magnitude faster). However, ~3% of the sample (the sources detected directly in the X-rays) show a significantly quicker growth of the black hole mass (up to 1.5 orders of magnitude quicker growth than the typical sources). BHARs fall from z=2.23 to z=0, with the decline resembling that of star formation rate density or the typical SFR. We find that the average black hole to galaxy growth (BHAR/SFR) is approximately constant for star-forming galaxies in the last 11 Gyrs. The relatively constant BHAR/SFR suggests that these two quantities evolve equivalently through cosmic time and with practically no delay between the two.