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

Journal Article · Sun Apr 26 00:00:00 EDT 2020 · Monthly Notices of the Royal Astronomical Society

DOI:https://doi.org/10.1093/mnras/staa1123· OSTI ID:1638902

McAlpine, Stuart ^[1]; Harrison, Chris M. ^[2]; Rosario, David J. ^[3]; Alexander, David M. ^[3]; Ellison, Sara L. ^[4]; Johansson, Peter H. ^[1]; Patton, David R. ^[5]

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1. Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2 P.O. Box 64, FI-00014 Helsinki, Finland
2. School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
3. Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
4. Department of Physics and Astronomy, University of Victoria, Finnerty Road, Victoria, British Columbia V8P 1A1, Canada
5. Department of Physics and Astronomy, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9L 0G2, Canada

ABSTRACT

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 (M* ∼ 1010 M⊙) gas rich (fgas > 0.2) central galaxies with lower mass BHs (MBH ∼ 107 M⊙) at lower redshifts (z < 1). We find no enhancement of AGN triggered via mergers in more massive galaxies (M* ≳ 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 z = 0.5). We argue that neither major ($$M_{\mathrm{*,1}} / M_{\mathrm{*,2}} \ge \frac{1}{4}$$) nor minor mergers ($$\frac{1}{10} \lt M_{\mathrm{*,1}} / M_{\mathrm{*,2}} \lt \frac{1}{4}$$) 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 on to BHs occurs outside the periods of a merger. We compute that on average no more than 15 per cent of a BHs final day mass comes from the enhanced accretion rates triggered via a merger.

• Sponsoring Organization: USDOE
• OSTI ID: 1638902
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 4 Vol. 494; ISSN 0035-8711
• Publisher: Oxford University Press
• Country of Publication: United Kingdom
• Language: English