Thin, thick and dark discs in LCDM

TL;DR

This study investigates how satellite accretion in a $\\Lambda$CDM universe shapes the Milky Way's thin and thick discs and the emergence of a dark matter disc. It combines cosmological N-body simulations to quantify the frequency and geometry of near-disc-plane and high-inclination mergers with collisionless satellite-merger runs to map the final state of accreted material and disc response. Key results show typical MW-sized halos experience about 1 near-disc-plane merger with $v_{\\max}>80$ km s$^{-1}$, 2–3 with $v_{\\max}>60$ and ~5 with $v_{\\max}>40$, with high-inclination events being twice as common; the accreted stars form a thick disc that is under-massive by a factor of ~2–10 unless heated by the most massive mergers, while a dark disc can reach $0.25$–$1$ times the halo density at the solar position. High-inclination encounters reproduce inner/outer halo–like structures and induce long-lived warps and flares; the thick disc may include 50–90% heated thin-disc material, while a dark disc impacts direct detection by altering the local kinematics.

Abstract

In a LCDM 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 3 subhalos with vmax>80km/s; 7 with vmax>60km/s; and 15 with vmax>40km/s merge at redshift z>1. Assuming isotropic accretion, a third of these merge at an impact angle <20 degrees 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 0.25-1 times the halo density 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 (>20 degrees) are twice as likely as low inclination ones. These lead to structures that closely resemble the recently discovered inner/outer stellar halos. 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>80km/s) 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% of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z=1 [abridged].

Figures (14)

• Figure 1: Left: cumulative maximum circular speed $v_\mathrm{max}$ functions for accreted (solid lines) and surviving (dashed lines) subhalos. We include only halos with greater than 50 particles. For the accreted halos, we consider only halos that at $z=0$ have 0.1 of their peak $v_\mathrm{max}$ considered over all times, and which pass within $r_\mathrm{merge}=50$ kpc of the disc after $z=4.35$. Four host halos are shown, taken from the concordance cosmology simulation described in § \ref{['sec:cosmology']} (1 black, 2 red, 3 green, 4 blue); their $v_\mathrm{max}^\mathrm{host}$ are marked in km/s. The thick solid line shows a power-law fit to the surviving subhalo $v_\mathrm{max}$ function taken from 2004MNRAS.355..819G and 2008arXiv0801.1127L. A fit to the accreted subhalos is also shown (thick dotted line; see text for details). The vertical dotted lines mark $v_\mathrm{max}=40,60,80$ km/s assuming $v_\mathrm{max}^\mathrm{host} = 200$ km/s. Middle & Right: evolutionary tracks in $v_\mathrm{max}$ and radius $r$ for the six most massive subhalos with $v_\mathrm{max}>40$ km/s that are accreted by $z=0$ (solid, dotted, short dashed, dot dashed, triple-dot-dashed and long dashed lines). The different colours correspond to the same four host halos as in the left panel. For halo 1 (black lines) the $v_\mathrm{max}$ are marked on the right panel in km/s. Redshift $z=1$ ($\sim 8$ Gyrs) is marked by the vertical solid lines. Note that the pericentres are likely over-estimated due to the limited number of simulation outputs.
• Figure 2: Subhalos fall in inside loosely bound larger groups. Shown here are logarithmic density contours of a spherical region of radius 100 kpc selected around the most massive subhalo in halo 2 at $z=2.4$, before this subhalo falls into the host halo. These same particles are tagged and plotted at later times, $z=1.9$, $z=1.6$, $z=1.4$, and $z=1.2$. The peak circular velocity of this subhalo is 75 km/s, while its spherical overdensity mass and radius at $z=2.4$ are $M_\mathrm{SO}=2.55\times 10^{10}$M$_\odot$ and $r_\mathrm{SO} = 15.8$ kpc, respectively. The much larger region extending to 100 kpc and enclosing $10^{11}$ M$_\odot$ (and some other smaller subhalos) largely co-moves with this subhalo.
• Figure 3: Rotation curves for the different galaxy models. The upper red line is for MWB. The dotted lines show the dark matter halo contribution. The data points show the mean of HI measurements from 1984ApJS...54..513B, 1974AAS...17..251W, 1973AAS....8....1W, 1995ApJ...448..138M and 1986AAS...66..373K. The blue dashed line shows the rotation curve from one of the subhalos taken from the cosmological simulations of § \ref{['sec:cosmology']}.
• Figure 4: The distribution of $v_z$ velocities at the solar neighbourhood, $8 < R < 9$ kpc; $|z|<0.35$ kpc, for LMC-10$^\mathrm{o}$, LMC-20$^\mathrm{o}$ and LLMC-10$^\mathrm{o}$. The black line shows the Milky Way disc stars, the red the accreted satellite material. The distributions are normalised to peak at 1 and do not represent the mass in each component. Notice the prominent wings in the thin disc distribution for LMC-10$^\mathrm{o}$ and LMC-20$^\mathrm{o}$: some of the stars originating in the thin disc show 'thick disc' kinematics. For the more massive LLMC-10$^\mathrm{o}$ merger, the wings are now very prominent: the heated thin disc distribution looks like the accreted distribution in LMC-10$^\mathrm{o}$ and little thin disc component remains.
• Figure 5: The effect of increasing impact angle: simulations: LMC-10$^\mathrm{o}$, LMC-20$^\mathrm{o}$, LMC-40$^\mathrm{o}$ and LMC-60$^\mathrm{o}$. From left to right, the panels show: (a) logarithmic density contours, viewed from side, in units of M $_\odot$ pc$^{-2}$; (b) surface density as a function of $R$ in a slice $|z|<1.1$ kpc (the red dotted line [offset] for simulation LMC-10$^\mathrm{o}$ shows an exponential fit with scale length $R_0 = R_{1/2}/1.68 = 4.3$ kpc); (c) density as function of $z$ in a slice $8<R<9$ kpc; (d) the rotation curve ($v_\phi(R)$ for $|z|<0.35$ kpc); and (e) the $R$ (solid), $\phi$ (dotted) and $z$ (dashed) components of the stellar velocity dispersion as a function of projected radius. In all cases, we show the Milky Way stars (black), accreted satellite stars (red), Milky Way dark matter (blue dotted) and satellite accreted dark matter (blue). The black dashed lines (left three panels) and green lines (right panel) show the Milky Way disc initial conditions. The solid dots mark the half mass scale lengths for each component. The black dotted lines mark the solar position. Notice that a thick disc of stars forms for impact angles $\hbox{$\; \buildrel < \over \sim \;$} 20^\mathrm{o}$, with a corresponding thick disc of accreted dark matter. Higher impact angles give more boxy stellar and dark matter distributions that rotate more slowly and are hotter. They also do increasing damage to the Milky Way thin disc, producing a flared outer disc that for LMC-40$^\mathrm{o}$ and LMC-60$^\mathrm{o}$ is better described as a warp.