Extreme star formation in the host galaxies of the fastest growing super-massive black holes at z=4.8

TL;DR

This study presents Herschel observations of 25 high-redshift AGNs, revealing extreme star formation rates in some and suggesting a link between galaxy mergers, black hole growth, and star formation in the early universe.

Contribution

First detailed Herschel-based analysis of star formation in z=4.8 AGNs, connecting extreme starburst activity with galaxy mergers and black hole growth.

Findings

Five sources have SFRs of 2800-5600 M_sol/yr.

Mean SFR of 700 +/- 150 M_sol/yr from stacking.

Evidence of merger-driven starburst activity at high redshift.

Abstract

We report new Herschel observations of 25 z=4.8 extremely luminous optically selected active galactic nuclei (AGNs). Five of the sources have extremely large star forming (SF) luminosities, L_SF, corresponding to SF rates (SFRs) of 2800-5600 M_sol/yr assuming a Salpeter IMF. The remaining sources have only upper limits on their SFRs but stacking their Herschel images results in a mean SFR of 700 +/- 150 M_sol/yr. The higher SFRs in our sample are comparable to the highest observed values so far, at any redshift. Our sample does not contain obscured AGNs, which enables us to investigate several evolutionary scenarios connecting super-massive black holes and SF activity in the early universe. The most probable scenario is that we are witnessing the peak of SF activity in some sources and the beginning of the post-starburst decline in others. We suggest that all 25 sources, which are at their peak AGN activity, are in large mergers. AGN feedback may be responsible for diminishing the SF activity in 20 of them but is not operating efficiently in 5 others.