Journal Article

Black hole clustering in cosmological hydrodynamic simulations: evidence for mergers

Colin Degraf, Tiziana Di Matteo and Volker Springel

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 413, issue 2, pages 1383-1394
Published in print May 2011 | ISSN: 0035-8711
Published online April 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18221.x
Black hole clustering in cosmological hydrodynamic simulations: evidence for mergers

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We examine the clustering properties of a population of quasars drawn from fully hydrodynamic cosmological simulations that directly follow black hole (BH) growth. We find that the BH correlation function is best described by two distinct components: contributions from BH pairs occupying the same dark matter halo (‘1-halo term’, ξBH,1h) which dominate at scales below ∼300 kpc h−1, and contributions from BHs occupying separate haloes (‘2-halo term’, ξBH,2h) which dominate at larger scales. From the 2-halo BH term, we find a typical host halo mass for faint-end quasars (those probed in our simulation volumes) ranging from M∼ 1011 to a few 1012 M from z= 5 to 1 respectively. The BH correlation function shows a luminosity dependence as a function of redshift, though weak enough to be consistent with observational constraints. At small scales, the high resolution of our simulations allows us to probe the 1-halo clustering in detail, finding that ξBH,1h follows an approximate power law, lacking the characteristic decrease in slope at small scales found in 1-halo terms for galaxies and dark matter. We show that this difference is a direct result of a boost in the small-scale quasar bias caused by galaxies hosting multiple quasars (1-subhalo term) following a merger event, typically between a large central subgroup and a smaller, satellite subgroup hosting a relatively small BH. We show that our predicted small-scale excess caused by such mergers is in good agreement with both the slope and amplitude indicated by recent small-scale measurements. Finally, we note the excess to be a strong function of halo mass, such that the observed excess is well matched by the multiple BHs of intermediate mass (107–108 M) found in hosts of M∼ 4–8 × 1011 M, a range well probed by our simulations.

Keywords: black hole physics; methods: numerical; galaxies: active; galaxies: evolution; galaxies: nuclei; quasars: general

Journal Article.  8733 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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