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Galaxy mergers in eagle do not induce a significant amount of black hole growth yet do increase the rate of luminous AGN

Patton, David R.; Johansson, Peter H.; Ellison, Sara L.; Alexander, David M.; Rosario, David J.; Harrison, Chris M.; McAlpine, Stuart

Galaxy mergers in eagle do not induce a significant amount of black hole growth yet do increase the rate of luminous AGN Thumbnail


David R. Patton

Peter H. Johansson

Sara L. Ellison

David J. Rosario

Chris M. Harrison

Stuart McAlpine


We investigate the connection between galaxy–galaxy mergers and enhanced black hole (BH) growth using the cosmological hydrodynamical eagle simulation. We do this via three methods of analysis, investigating: the merger fraction of AGN, the AGN fraction of merging systems and the AGN fraction of galaxies with close companions. In each case, we find an increased abundance of AGN within merging systems relative to control samples of inactive or isolated galaxies (by up to a factor of ≈ 3 depending on the analysis method used), confirming that mergers are enhancing BH accretion rates for at least a subset of the galaxy population. The greatest excess of AGN triggered via a merger are found in lower mass (푀∗ ∼ 1010 M) gas rich ( 푓gas > 0.2) central galaxies with lower mass BHs (푀BH ∼ 107 M) at lower redshifts (푧 < 1). We find no enhancement of AGN triggered via mergers in more massive galaxies (푀∗ & 1011 M). The enhancement of AGN is not uniform throughout the phases of a merger, and instead peaks within the early remnants of merging systems (typically lagging ≈ 300 Myr post-coalescence of the two galaxies at 푧 = 0.5). We argue that neither major (푀∗,1/푀∗,2 ≥ 1 4 ) nor minor mergers ( 1 10 < 푀∗,1/푀∗,2 < 1 4 ) are statistically relevant for enhancing BH masses globally. Whilst at all redshifts the galaxies experiencing a merger have accretion rates that are on average 2–3 times that of isolated galaxies, the majority of mass that is accreted onto BHs occurs outside the periods of a merger. We compute that on average no more than 15% of a BHs final day mass comes from the enhanced accretion rates triggered via a merger.

Journal Article Type Article
Publication Date Jan 1, 2020
Deposit Date Apr 28, 2020
Publicly Available Date Apr 28, 2020
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Public URL


Accepted Journal Article (4.7 Mb)

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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