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Thin, thick and dark discs in ΛcDM

Read, J. I. ; Lake, G. ; Agertz, O. LU and Debattista, Victor P. (2008) In Monthly Notices of the Royal Astronomical Society 389(3). p.1041-1057
Abstract

In a Λ cold dark matter (ΛCDM) cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has three subhaloes with vmax > 80 km s -1; seven with vmax > 60 km s-1 and 15 with vmax > 40 km s-1 merge at redshift z ≳ 1. Assuming isotropic accretion, a third of these merge at an impact angle θ < 20°and are dragged into the disc plane by dynamical friction. Their accreted... (More)

In a Λ cold dark matter (ΛCDM) cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has three subhaloes with vmax > 80 km s -1; seven with vmax > 60 km s-1 and 15 with vmax > 40 km s-1 merge at redshift z ≳ 1. Assuming isotropic accretion, a third of these merge at an impact angle θ < 20°and are dragged into the disc plane by dynamical friction. Their accreted stars and dark matter settle into a thick disc. The stellar thick disc qualitatively reproduces the observed thick disc at the solar neighbourhood, but is less massive by a factor ∼2 - 10. The dark matter disc contributes ρDDISC = 0.25 - 1ρHALO at the solar position. Although not likely to be dynamically interesting, the dark disc has important implications for the direct detection of dark matter because of its low velocity with respect to the Earth. Higher inclination encounters θ > 20°are twice as likely as low-inclination ones. These lead to structures that closely resemble the recently discovered inner and outer stellar haloes. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (vmax ≳ 80 km s -1) heat the thin disc enough to produce a thick disc. These heated thin-disc stars are essential for obtaining a thick disc as massive as that seen in the Milky Way; they likely comprise some ∼50-90 per cent of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z = 1. Only one in four of our sample Milky Way haloes experiences mergers massive and late enough to fully destroy the thin disc. We conclude that thick, thin and dark discs occur naturally within a ΛCDM cosmology.

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author
; ; and
publishing date
type
Contribution to journal
publication status
published
keywords
Galaxies: formation, Galaxies: kinematics and dynamics, Galaxy: disc, Galaxy: formation, Galaxy: kinematics and dynamics, Galaxy: structure
in
Monthly Notices of the Royal Astronomical Society
volume
389
issue
3
pages
17 pages
publisher
Oxford University Press
external identifiers
  • scopus:51349109740
ISSN
0035-8711
DOI
10.1111/j.1365-2966.2008.13643.x
language
English
LU publication?
no
id
c5a2e6ed-f159-489c-b658-2fa3e25fd817
date added to LUP
2019-02-07 11:24:36
date last changed
2022-04-25 21:26:26
@article{c5a2e6ed-f159-489c-b658-2fa3e25fd817,
  abstract     = {{<p>In a Λ cold dark matter (ΛCDM) cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has three subhaloes with v<sub>max</sub> &gt; 80 km s <sup>-1</sup>; seven with v<sub>max</sub> &gt; 60 km s<sup>-1</sup> and 15 with v<sub>max</sub> &gt; 40 km s<sup>-1</sup> merge at redshift z ≳ 1. Assuming isotropic accretion, a third of these merge at an impact angle θ &lt; 20°and are dragged into the disc plane by dynamical friction. Their accreted stars and dark matter settle into a thick disc. The stellar thick disc qualitatively reproduces the observed thick disc at the solar neighbourhood, but is less massive by a factor ∼2 - 10. The dark matter disc contributes ρ<sub>DDISC</sub> = 0.25 - 1ρ<sub>HALO</sub> at the solar position. Although not likely to be dynamically interesting, the dark disc has important implications for the direct detection of dark matter because of its low velocity with respect to the Earth. Higher inclination encounters θ &gt; 20°are twice as likely as low-inclination ones. These lead to structures that closely resemble the recently discovered inner and outer stellar haloes. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (v<sub>max</sub> ≳ 80 km s <sup>-1</sup>) heat the thin disc enough to produce a thick disc. These heated thin-disc stars are essential for obtaining a thick disc as massive as that seen in the Milky Way; they likely comprise some ∼50-90 per cent of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z = 1. Only one in four of our sample Milky Way haloes experiences mergers massive and late enough to fully destroy the thin disc. We conclude that thick, thin and dark discs occur naturally within a ΛCDM cosmology.</p>}},
  author       = {{Read, J. I. and Lake, G. and Agertz, O. and Debattista, Victor P.}},
  issn         = {{0035-8711}},
  keywords     = {{Galaxies: formation; Galaxies: kinematics and dynamics; Galaxy: disc; Galaxy: formation; Galaxy: kinematics and dynamics; Galaxy: structure}},
  language     = {{eng}},
  month        = {{09}},
  number       = {{3}},
  pages        = {{1041--1057}},
  publisher    = {{Oxford University Press}},
  series       = {{Monthly Notices of the Royal Astronomical Society}},
  title        = {{Thin, thick and dark discs in ΛcDM}},
  url          = {{http://dx.doi.org/10.1111/j.1365-2966.2008.13643.x}},
  doi          = {{10.1111/j.1365-2966.2008.13643.x}},
  volume       = {{389}},
  year         = {{2008}},
}