Journal Article

The merger rates and mass assembly histories of dark matter haloes in the two Millennium simulations

Onsi Fakhouri, Chung-Pei Ma and Michael Boylan-Kolchin

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 406, issue 4, pages 2267-2278
Published in print August 2010 | ISSN: 0035-8711
Published online August 2010 | e-ISSN: 1365-2966 | DOI: https://dx.doi.org/10.1111/j.1365-2966.2010.16859.x
The merger rates and mass assembly histories of dark matter haloes in the two Millennium simulations

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We construct merger trees of dark matter haloes and quantify their merger rates and mass growth rates using the joint data set from the Millennium and Millennium-II simulations. The finer resolution of the Millennium-II simulation has allowed us to extend our earlier analysis of halo merger statistics to an unprecedentedly wide range of descendant halo mass (1010M0≲ 1015 M), progenitor mass ratio (10−5≲ξ≤ 1) and redshift (0 ≤z≲ 15). We update our earlier fitting form for the mean merger rate per halo as a function of M0, ξ and z. The overall behaviour of this quantity is unchanged: the rate per unit redshift is nearly independent of z out to z∼ 15; the dependence on halo mass is weak (∝M0.130); and it is nearly a power law in the progenitor mass ratio (∝ξ−2). We also present a simple and accurate fitting formula for the mean mass growth rate of haloes as a function of mass and redshift. This mean rate is 46 M yr−1 for 1012 M haloes at z= 0, and it increases with mass as ∝M1.1 and with redshift as (1 +z)2.5 (for z≳ 1). When the fit for the mean mass growth rate is integrated over a halo's history, we find excellent match to the mean mass assembly histories of the simulated haloes. By combining merger rates and mass assembly histories, we present results for the number of mergers over a halo's history and the statistics of the redshift of the last major merger.

Keywords: galaxies: formation; cosmology: theory; dark matter; large-scale structure of Universe

Journal Article.  9363 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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